Initial support for exchangable VisaLibrary

This commit splits the VisaLibrary object into VisaLibraryBase which contains
wrapper agnostic code and NIVisaLibrary (which inherits from VisaLibraryBase)
and is the ctypes wrapper to the NI-VISA library.

It adds a open_visa_library helper function to be used by the ResourceManager.
The function takes an iterable of strings to specify the libraries to try to open.
The argument format is 'option@wrapper'.

There is only one register wrapper (`ni`) by default. If another <wrapper name> is
provided, it will be looked in a package named `pyvisa-<wrapper name>`. The package
must declare a WRAPPER_CLASS variable that points to a VisaLibraryBase derived class.

Backwards incompatible changes:
- VisaLibrary directly should not be used anymore

1 files changed, 1291 insertions, 150 deletions

diff --git a/pyvisa/highlevel.py b/pyvisa/highlevel.py
index 69e2631..d3ff5a3 100644
--- a/pyvisa/highlevel.py
+++ b/pyvisa/highlevel.py
@@ -13,66 +13,28 @@
 
 from __future__ import division, unicode_literals, print_function, absolute_import
 
+import abc
 import contextlib
+import collections
+import pkgutil
 import warnings
 from collections import defaultdict
 
 from . import logger
 from . import constants
-from . import ctwrapper
 from . import errors
-from .util import get_library_paths
-from .compat import integer_types
+from . import util
 
+#: Resource extended information
+ResourceInfo = collections.namedtuple('ResourceInfo',
+                                      'interface_type interface_board_number '
+                                      'resource_class resource_name alias')
 
-def _args_to_str(args):
-    out = []
-    for arg in args:
-        try:
-            out.append(str(arg._obj))
-        except Exception:
-            out.append(arg)
-    return tuple(out)
-
+class VisaLibraryBase(object):
+    """Base class for VISA library wrappers.
 
-def add_visa_methods(wrapper_module):
-    """Decorator factory to add methods in `wrapper_module.visa_functions`
-    iterable to a class.
-
-    :param wrapper_module: the python module/package that wraps the visa library.
-    """
-    def _internal(aclass):
-        aclass._wrapper_module = wrapper_module
-        methods = wrapper_module.visa_functions
-        for method in methods:
-            if hasattr(aclass, method):
-                setattr(aclass, '_' + method, getattr(wrapper_module, method))
-            else:
-                setattr(aclass, method, getattr(wrapper_module, method))
-        return aclass
-    return _internal
-
-
-@add_visa_methods(ctwrapper)
-class VisaLibrary(object):
-    """High level VISA Library wrapper.
-
-    The easiest way to instantiate the library is to let `pyvisa` find the
-    right one for you. This looks first in your configuration file (~/.pyvisarc).
-    If it fails, it uses `ctypes.util.find_library` to try to locate a library
-    in a way similar to what the compiler does:
-
-       >>> visa_library = VisaLibrary()
-
-    But you can also specify the path:
-
-        >>> visa_library = VisaLibrary('/my/path/visa.so')
-
-    Or use the `from_paths` constructor if you want to try multiple paths:
-
-        >>> visa_library = VisaLibrary.from_paths(['/my/path/visa.so', '/maybe/this/visa.so'])
-
-    :param library_path: path of the VISA library.
+    Derived classes must have a constructor that accept a single element
+    following the syntax of :open_visa_library:
     """
 
     #: Default ResourceManager instance for this library.
@@ -87,67 +49,28 @@ class VisaLibrary(object):
     #: Maps session handle to warnings to ignore
     _ignore_warning_in_session = defaultdict(set)
 
-    @classmethod
-    def from_paths(cls, *paths):
-        """Helper constructor that tries to instantiate VisaLibrary from an
-        iterable of possible library paths.
-        """
-        errs = []
-        for path in paths:
-            try:
-                return cls(path)
-            except OSError as e:
-                logger.debug('Could not open VISA library %s: %s', path, str(e))
-                errs.append(str(e))
-        else:
-            raise OSError('Could not open VISA library:\n' + '\n'.join(errs))
-
-    def __new__(cls, library_path=None):
-        if library_path is None:
-            paths = get_library_paths(cls._wrapper_module)
-            if not paths:
-                raise OSError('Could not found VISA library. '
-                              'Please install VISA or pass its location as an argument.')
-            return cls.from_paths(*paths)
-        else:
-            if library_path in cls._registry:
-                return cls._registry[library_path]
-
-            cls._registry[library_path] = obj = super(VisaLibrary, cls).__new__(cls)
+    def __init__(self, library_path):
+        super(VisaLibraryBase, self).__init__()
 
-        try:
-            obj.lib = cls._wrapper_module.Library(library_path)
-        except OSError as exc:
-            raise errors.LibraryError.from_exception(exc, library_path)
+        self.library_path = library_path
 
-        obj.library_path = library_path
+        self._registry[library_path] = self
 
         logger.debug('Created library wrapper for %s', library_path)
 
-        # Set the argtypes, restype and errcheck for each function
-        # of the visa library. Additionally store in `_functions` the
-        # name of the functions.
-        cls._wrapper_module.set_signatures(obj.lib, errcheck=obj._return_handler)
-
-        # Set the library functions as attributes of the object.
-        for method_name in getattr(obj.lib, '_functions', []):
-            setattr(obj, method_name, getattr(obj.lib, method_name))
-
         #: Error codes on which to issue a warning.
-        obj.issue_warning_on = set(errors.default_warnings)
+        self.issue_warning_on = set(errors.default_warnings)
 
         #: Contains all installed event handlers.
         #: Its elements are tuples with three elements: The handler itself (a Python
         #: callable), the user handle (as a ct object) and the handler again, this
         #: time as a ct object created with CFUNCTYPE.
-        obj.handlers = defaultdict(list)
+        self.handlers = defaultdict(list)
 
         #: Last return value of the library.
-        obj._last_status = 0
+        self._last_status = 0
 
-        obj._logging_extra = {'library_path': obj.library_path}
-
-        return obj
+        self._logging_extra = {'library_path': self.library_path}
 
     def __str__(self):
         return 'Visa Library at %s' % self.library_path
@@ -179,52 +102,6 @@ class VisaLibrary(object):
             self._resource_manager = ResourceManager(self)
         return self._resource_manager
 
-    def _return_handler(self, ret_value, func, arguments):
-        """Check return values for errors and warnings.
-        """
-
-        logger.debug('%s%s -> %r',
-                     func.__name__, _args_to_str(arguments), ret_value,
-                     extra=self._logging_extra)
-
-        try:
-            ret_value = constants.StatusCode(ret_value)
-        except ValueError:
-            pass
-
-        self._last_status = ret_value
-
-        # The first argument of almost all registered visa functions is a session.
-        # We store the error code per session
-        session = None
-        if func.__name__ not in ('viFindNext', ):
-            try:
-                session = arguments[0]
-            except KeyError:
-                raise Exception('Function %r does not seem to be a valid '
-                                'visa function (len args %d)' % (func, len(arguments)))
-
-            # Functions that use the first parameter to get a session value.
-            if func.__name__ in ('viOpenDefaultRM', ):
-                session = session._obj.value
-
-            if isinstance(session, integer_types):
-                self._last_status_in_session[session] = ret_value
-            else:
-                # Functions that might or might have a session in the first argument.
-                if func.__name__ not in ('viClose', 'viGetAttribute', 'viSetAttribute', 'viStatusDesc'):
-                    raise Exception('Function %r does not seem to be a valid '
-                                    'visa function (type args[0] %r)' % (func, type(session)))
-
-        if ret_value < 0:
-            raise errors.VisaIOError(ret_value)
-
-        if ret_value in self.issue_warning_on:
-            if session and ret_value not in self._ignore_warning_in_session[session]:
-                warnings.warn(errors.VisaIOWarning(ret_value), stacklevel=2)
-
-        return ret_value
-
     @contextlib.contextmanager
     def ignore_warning(self, session, *warnings_constants):
         """A session dependent context for ignoring warnings
@@ -236,7 +113,7 @@ class VisaLibrary(object):
         yield
         self._ignore_warning_in_session[session].difference_update(warnings_constants)
 
-    def install_handler(self, session, event_type, handler, user_handle=None):
+    def install_visa_handler(self, session, event_type, handler, user_handle=None):
         """Installs handlers for event callbacks.
 
         :param session: Unique logical identifier to a session.
@@ -247,14 +124,14 @@ class VisaLibrary(object):
         :returns: user handle (a ctypes object)
         """
         try:
-            new_handler = self._install_handler(self.lib, session, event_type, handler, user_handle)
+            new_handler = self.install_handler(session, event_type, handler, user_handle)
         except TypeError as e:
             raise errors.VisaTypeError(str(e))
 
         self.handlers[session].append(new_handler)
         return new_handler[1]
 
-    def uninstall_handler(self, session, event_type, handler, user_handle=None):
+    def uninstall_visa_handler(self, session, event_type, handler, user_handle=None):
         """Uninstalls handlers for events.
 
         :param session: Unique logical identifier to a session.
@@ -269,7 +146,1266 @@ class VisaLibrary(object):
                 break
         else:
             raise errors.UnknownHandler(event_type, handler, user_handle)
-        self._uninstall_handler(self.lib, session, event_type, handler, user_handle)
+        self.uninstall_handler(session, event_type, handler, user_handle)
+
+    def read_memory(self, session, space, offset, width, extended=False):
+        """Reads in an 8-bit, 16-bit, 32-bit, or 64-bit value from the specified memory space and offset.
+
+        Corresponds to viIn* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param width: Number of bits to read.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: Data read from memory, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        if width == 8:
+            return self.in_8(session, space, offset, extended)
+        elif width == 16:
+            return self.in_16(session, space, offset, extended)
+        elif width == 32:
+            return self.in_32(session, space, offset, extended)
+        elif width == 64:
+            return self.in_64(session, space, offset, extended)
+
+        raise ValueError('%s is not a valid size. Valid values are 8, 16, 32 or 64' % width)
+
+    def write_memory(self, session, space, offset, data, width, extended=False):
+        """Write in an 8-bit, 16-bit, 32-bit, value to the specified memory space and offset.
+
+        Corresponds to viOut* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param data: Data to write to bus.
+        :param width: Number of bits to read.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        if width == 8:
+            return self.out_8(session, space, offset, data, extended)
+        elif width == 16:
+            return self.out_16(session, space, offset, data, extended)
+        elif width == 32:
+            return self.out_32(session, space, offset, data, extended)
+
+        raise ValueError('%s is not a valid size. Valid values are 8, 16 or 32' % width)
+
+    def move_in(self, session, space, offset, length, width, extended=False):
+        """Moves a block of data to local memory from the specified address space and offset.
+
+        Corresponds to viMoveIn* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param width: Number of bits to read per element.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: Data read from the bus, return value of the library call.
+        :rtype: list, VISAStatus
+        """
+        if width == 8:
+            return self.move_in_8(session, space, offset, length, extended)
+        elif width == 16:
+            return self.move_in_16(session, space, offset, length, extended)
+        elif width == 32:
+            return self.move_in_32(session, space, offset, length, extended)
+        elif width == 64:
+            return self.move_in_64(session, space, offset, length, extended)
+
+        raise ValueError('%s is not a valid size. Valid values are 8, 16, 32 or 64' % width)
+
+    def move_out(self, session, space, offset, length, data, width, extended=False):
+        """Moves a block of data from local memory to the specified address space and offset.
+
+        Corresponds to viMoveOut* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param data: Data to write to bus.
+        :param width: Number of bits to read per element.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        if width == 8:
+            return self.move_out_8(session, space, offset, length, data, extended)
+        elif width == 16:
+            return self.move_out_16(session, space, offset, length, data, extended)
+        elif width == 32:
+            return self.move_out_32(session, space, offset, length, data, extended)
+        elif width == 64:
+            return self.move_out_64(session, space, offset, length, data, extended)
+
+        raise ValueError('%s is not a valid size. Valid values are 8, 16, 32 or 64' % width)
+
+    def peek(self, session, address, width):
+        """Read an 8, 16 or 32-bit value from the specified address.
+
+        Corresponds to viPeek* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param address: Source address to read the value.
+        :param width: Number of bits to read.
+        :return: Data read from bus, return value of the library call.
+        :rtype: bytes, VISAStatus
+        """
+
+        if width == 8:
+            return self.peek_8(session, address)
+        elif width == 16:
+            return self.peek_16(session, address)
+        elif width == 32:
+            return self.peek_32(session, address)
+        elif width == 64:
+            return self.peek_64(session, address)
+
+        raise ValueError('%s is not a valid size. Valid values are 8, 16, 32 or 64' % width)
+
+    def poke(self, session, address, width, data):
+        """Writes an 8, 16 or 32-bit value from the specified address.
+
+        Corresponds to viPoke* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param address: Source address to read the value.
+        :param width: Number of bits to read.
+        :param data: Data to be written to the bus.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+
+        if width == 8:
+            return self.poke_8(session, address, data)
+        elif width == 16:
+            return self.poke_16(session, address, data)
+        elif width == 32:
+            return self.poke_32(session, address, data)
+
+        raise ValueError('%s is not a valid size. Valid values are 8, 16 or 32' % width)
+
+    # Methods that VISA Library implementations must implement
+
+    def assert_interrupt_signal(self, session, mode, status_id):
+        """Asserts the specified interrupt or signal.
+
+        Corresponds to viAssertIntrSignal function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param mode: How to assert the interrupt. (Constants.ASSERT*)
+        :param status_id: This is the status value to be presented during an interrupt acknowledge cycle.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def assert_trigger(self, session, protocol):
+        """Asserts software or hardware trigger.
+
+        Corresponds to viAssertTrigger function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param protocol: Trigger protocol to use during assertion. (Constants.PROT*)
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def assert_utility_signal(self, session, line):
+        """Asserts or deasserts the specified utility bus signal.
+
+        Corresponds to viAssertUtilSignal function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param line: specifies the utility bus signal to assert. (Constants.UTIL_ASSERT*)
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def buffer_read(self, session, count):
+        """Reads data from device or interface through the use of a formatted I/O read buffer.
+
+        Corresponds to viBufRead function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param count: Number of bytes to be read.
+        :return: data read, return value of the library call.
+        :rtype: bytes, VISAStatus
+        """
+        raise NotImplementedError
+
+    def buffer_write(self, session, data):
+        """Writes data to a formatted I/O write buffer synchronously.
+
+        Corresponds to viBufWrite function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param data: data to be written.
+        :type data: bytes
+        :return: number of written bytes, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def clear(self, session):
+        """Clears a device.
+
+        Corresponds to viClear function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def close(self, session):
+        """Closes the specified session, event, or find list.
+
+        Corresponds to viClose function of the VISA library.
+
+        :param session: Unique logical identifier to a session, event, or find list.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def disable_event(self, session, event_type, mechanism):
+        """Disables notification of the specified event type(s) via the specified mechanism(s).
+
+        Corresponds to viDisableEvent function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param event_type: Logical event identifier.
+        :param mechanism: Specifies event handling mechanisms to be disabled.
+                          (Constants.QUEUE, .Handler, .SUSPEND_HNDLR, .ALL_MECH)
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def discard_events(self, session, event_type, mechanism):
+        """Discards event occurrences for specified event types and mechanisms in a session.
+
+        Corresponds to viDiscardEvents function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param event_type: Logical event identifier.
+        :param mechanism: Specifies event handling mechanisms to be disabled.
+                          (Constants.QUEUE, .Handler, .SUSPEND_HNDLR, .ALL_MECH)
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def enable_event(self, session, event_type, mechanism, context=None):
+        """Enable event occurrences for specified event types and mechanisms in a session.
+
+        Corresponds to viEnableEvent function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param event_type: Logical event identifier.
+        :param mechanism: Specifies event handling mechanisms to be disabled.
+                          (Constants.QUEUE, .Handler, .SUSPEND_HNDLR)
+        :param context:
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def find_next(self, find_list):
+        """Returns the next resource from the list of resources found during a previous call to find_resources().
+
+        Corresponds to viFindNext function of the VISA library.
+
+        :param find_list: Describes a find list. This parameter must be created by find_resources().
+        :return: Returns a string identifying the location of a device, return value of the library call.
+        :rtype: unicode (Py2) or str (Py3), VISAStatus
+        """
+        raise NotImplementedError
+
+    def find_resources(self, session, query):
+        """Queries a VISA system to locate the resources associated with a specified interface.
+
+        Corresponds to viFindRsrc function of the VISA library.
+
+        :param session: Unique logical identifier to a session (unused, just to uniform signatures).
+        :param query: A regular expression followed by an optional logical expression. Use '?*' for all.
+        :return: find_list, return_counter, instrument_description, return value of the library call.
+        :rtype: ViFindList, int, unicode (Py2) or str (Py3), VISAStatus
+        """
+        raise NotImplementedError
+
+    def flush(self, session, mask):
+        """Manually flushes the specified buffers associated with formatted I/O operations and/or serial communication.
+
+        Corresponds to viFlush function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param mask: Specifies the action to be taken with flushing the buffer.
+                     (Constants.READ*, .WRITE*, .IO*)
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def get_attribute(self, session, attribute):
+        """Retrieves the state of an attribute.
+
+        Corresponds to viGetAttribute function of the VISA library.
+
+        :param session: Unique logical identifier to a session, event, or find list.
+        :param attribute: Resource attribute for which the state query is made (see Attributes.*)
+        :return: The state of the queried attribute for a specified resource, return value of the library call.
+        :rtype: unicode (Py2) or str (Py3), list or other type, VISAStatus
+        """
+        raise NotImplementedError
+
+    def gpib_command(self, session, data):
+        """Write GPIB command bytes on the bus.
+
+        Corresponds to viGpibCommand function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param data: data tor write.
+        :type data: bytes
+        :return: Number of written bytes, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def gpib_control_atn(self, session, mode):
+        """Specifies the state of the ATN line and the local active controller state.
+
+        Corresponds to viGpibControlATN function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param mode: Specifies the state of the ATN line and optionally the local active controller state.
+                     (Constants.GPIB_ATN*)
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def gpib_control_ren(self, session, mode):
+        """Controls the state of the GPIB Remote Enable (REN) interface line, and optionally the remote/local
+        state of the device.
+
+        Corresponds to viGpibControlREN function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param mode: Specifies the state of the REN line and optionally the device remote/local state.
+                     (Constants.GPIB_REN*)
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def gpib_pass_control(self, session, primary_address, secondary_address):
+        """Tell the GPIB device at the specified address to become controller in charge (CIC).
+
+        Corresponds to viGpibPassControl function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param primary_address: Primary address of the GPIB device to which you want to pass control.
+        :param secondary_address: Secondary address of the targeted GPIB device.
+                                  If the targeted device does not have a secondary address,
+                                  this parameter should contain the value Constants.NO_SEC_ADDR.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def gpib_send_ifc(self, session):
+        """Pulse the interface clear line (IFC) for at least 100 microseconds.
+
+        Corresponds to viGpibSendIFC function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def in_8(self, session, space, offset, extended=False):
+        """Reads in an 8-bit value from the specified memory space and offset.
+
+        Corresponds to viIn8* function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: Data read from memory, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def in_16(self, session, space, offset, extended=False):
+        """Reads in an 16-bit value from the specified memory space and offset.
+
+        Corresponds to viIn16* function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: Data read from memory, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def in_32(self, session, space, offset, extended=False):
+        """Reads in an 32-bit value from the specified memory space and offset.
+
+        Corresponds to viIn32* function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: Data read from memory, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def in_64(self, session, space, offset, extended=False):
+        """Reads in an 64-bit value from the specified memory space and offset.
+
+        Corresponds to viIn64* function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: Data read from memory, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def install_handler(self, session, event_type, handler, user_handle):
+        """Installs handlers for event callbacks.
+
+        Corresponds to viInstallHandler function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param event_type: Logical event identifier.
+        :param handler: Interpreted as a valid reference to a handler to be installed by a client application.
+        :param user_handle: A value specified by an application that can be used for identifying handlers
+                            uniquely for an event type.
+        :returns: a handler descriptor which consists of three elements:
+                 - handler (a python callable)
+                 - user handle (a ctypes object)
+                 - ctypes handler (ctypes object wrapping handler)
+                 and return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def lock(self, session, lock_type, timeout, requested_key=None):
+        """Establishes an access mode to the specified resources.
+
+        Corresponds to viLock function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param lock_type: Specifies the type of lock requested, either Constants.EXCLUSIVE_LOCK or Constants.SHARED_LOCK.
+        :param timeout: Absolute time period (in milliseconds) that a resource waits to get unlocked by the
+                        locking session before returning an error.
+        :param requested_key: This parameter is not used and should be set to VI_NULL when lockType is VI_EXCLUSIVE_LOCK.
+        :return: access_key that can then be passed to other sessions to share the lock, return value of the library call.
+        :rtype: str, VISAStatus
+        """
+
+
+    def map_address(self, session, map_space, map_base, map_size,
+                    access=False, suggested=None):
+        """Maps the specified memory space into the process's address space.
+
+        Corresponds to viMapAddress function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param map_space: Specifies the address space to map. (Constants.*SPACE*)
+        :param map_base: Offset (in bytes) of the memory to be mapped.
+        :param map_size: Amount of memory to map (in bytes).
+        :param access:
+        :param suggested: If not Constants.NULL (0), the operating system attempts to map the memory to the address
+                          specified in suggested. There is no guarantee, however, that the memory will be mapped to
+                          that address. This operation may map the memory into an address region different from
+                          suggested.
+
+        :return: address in your process space where the memory was mapped, return value of the library call.
+        :rtype: address, VISAStatus
+        """
+        raise NotImplementedError
+
+    def map_trigger(self, session, trigger_source, trigger_destination, mode):
+        """Map the specified trigger source line to the specified destination line.
+
+        Corresponds to viMapTrigger function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param trigger_source: Source line from which to map. (Constants.TRIG*)
+        :param trigger_destination: Destination line to which to map. (Constants.TRIG*)
+        :param mode:
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def memory_allocation(self, session, size, extended=False):
+        """Allocates memory from a resource's memory region.
+
+        Corresponds to viMemAlloc* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param size: Specifies the size of the allocation.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: offset of the allocated memory, return value of the library call.
+        :rtype: offset, VISAStatus
+        """
+        raise NotImplementedError
+
+    def memory_free(self, session, offset, extended=False):
+        """Frees memory previously allocated using the memory_allocation() operation.
+
+        Corresponds to viMemFree* function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param offset: Offset of the memory to free.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def move(self, session, source_space, source_offset, source_width, destination_space,
+             destination_offset, destination_width, length):
+        """Moves a block of data.
+
+        Corresponds to viMove function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param source_space: Specifies the address space of the source.
+        :param source_offset: Offset of the starting address or register from which to read.
+        :param source_width: Specifies the data width of the source.
+        :param destination_space: Specifies the address space of the destination.
+        :param destination_offset: Offset of the starting address or register to which to write.
+        :param destination_width: Specifies the data width of the destination.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def move_asynchronously(self, session, source_space, source_offset, source_width,
+                            destination_space, destination_offset,
+                            destination_width, length):
+        """Moves a block of data asynchronously.
+
+        Corresponds to viMoveAsync function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param source_space: Specifies the address space of the source.
+        :param source_offset: Offset of the starting address or register from which to read.
+        :param source_width: Specifies the data width of the source.
+        :param destination_space: Specifies the address space of the destination.
+        :param destination_offset: Offset of the starting address or register to which to write.
+        :param destination_width: Specifies the data width of the destination.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :return: Job identifier of this asynchronous move operation, return value of the library call.
+        :rtype: jobid, VISAStatus
+        """
+        raise NotImplementedError
+
+    def move_in_8(self, session, space, offset, length, extended=False):
+        """Moves an 8-bit block of data from the specified address space and offset to local memory.
+
+        Corresponds to viMoveIn8* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: Data read from the bus, return value of the library call.
+        :rtype: list, VISAStatus
+        """
+        raise NotImplementedError
+
+    def move_in_16(self, session, space, offset, length, extended=False):
+        """Moves an 16-bit block of data from the specified address space and offset to local memory.
+
+        Corresponds to viMoveIn16* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: Data read from the bus, return value of the library call.
+        :rtype: list, VISAStatus
+        """
+        raise NotImplementedError
+
+    def move_in_32(self, session, space, offset, length, extended=False):
+        """Moves an 32-bit block of data from the specified address space and offset to local memory.
+
+        Corresponds to viMoveIn32* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: Data read from the bus, return value of the library call.
+        :rtype: list, VISAStatus
+        """
+        raise NotImplementedError
+
+    def move_in_64(self, session, space, offset, length, extended=False):
+        """Moves an 64-bit block of data from the specified address space and offset to local memory.
+
+        Corresponds to viMoveIn64* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: Data read from the bus, return value of the library call.
+        :rtype: list, VISAStatus
+        """
+        raise NotImplementedError
+
+    def move_out(self, session, space, offset, length, data, width, extended=False):
+        """Moves a block of data from local memory to the specified address space and offset.
+
+        Corresponds to viMoveOut* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param data: Data to write to bus.
+        :param width: Number of bits to read per element.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def move_out_8(self, session, space, offset, length, data, extended=False):
+        """Moves an 8-bit block of data from local memory to the specified address space and offset.
+
+        Corresponds to viMoveOut8* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param data: Data to write to bus.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+
+        Corresponds to viMoveOut8 function of the VISA library.
+        """
+        raise NotImplementedError
+
+    def move_out_16(self, session, space, offset, length, data, extended=False):
+        """Moves an 16-bit block of data from local memory to the specified address space and offset.
+
+        Corresponds to viMoveOut16* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param data: Data to write to bus.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def move_out_32(self, session, space, offset, length, data, extended=False):
+        """Moves an 32-bit block of data from local memory to the specified address space and offset.
+
+        Corresponds to viMoveOut32* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param data: Data to write to bus.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def move_out_64(self, session, space, offset, length, data, extended=False):
+        """Moves an 64-bit block of data from local memory to the specified address space and offset.
+
+        Corresponds to viMoveOut64* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param length: Number of elements to transfer, where the data width of the elements to transfer
+                       is identical to the source data width.
+        :param data: Data to write to bus.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def open(self, session, resource_name,
+             access_mode=constants.AccessModes.no_lock, open_timeout=constants.VI_TMO_IMMEDIATE):
+        """Opens a session to the specified resource.
+
+        Corresponds to viOpen function of the VISA library.
+
+        :param session: Resource Manager session (should always be a session returned from open_default_resource_manager()).
+        :param resource_name: Unique symbolic name of a resource.
+        :param access_mode: Specifies the mode by which the resource is to be accessed. (Constants.NULL or Constants.*LOCK*)
+        :param open_timeout: Specifies the maximum time period (in milliseconds) that this operation waits
+                             before returning an error.
+        :return: Unique logical identifier reference to a session, return value of the library call.
+        :rtype: session, VISAStatus
+        """
+        raise NotImplementedError
+
+    def open_default_resource_manager(self):
+        """This function returns a session to the Default Resource Manager resource.
+
+        Corresponds to viOpenDefaultRM function of the VISA library.
+
+        :return: Unique logical identifier to a Default Resource Manager session, return value of the library call.
+        :rtype: session, VISAStatus
+        """
+        raise NotImplementedError
+
+    def out_8(self, session, space, offset, data, extended=False):
+        """Write in an 8-bit value from the specified memory space and offset.
+
+        Corresponds to viOut8* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param data: Data to write to bus.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def out_16(self, session, space, offset, data, extended=False):
+        """Write in an 16-bit value from the specified memory space and offset.
+
+        Corresponds to viOut16* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param data: Data to write to bus.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def out_32(self, session, space, offset, data, extended=False):
+        """Write in an 32-bit value from the specified memory space and offset.
+
+        Corresponds to viOut32* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param data: Data to write to bus.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def out_64(self, session, space, offset, data, extended=False):
+        """Write in an 64-bit value from the specified memory space and offset.
+
+        Corresponds to viOut64* functions of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param space: Specifies the address space. (Constants.*SPACE*)
+        :param offset: Offset (in bytes) of the address or register from which to read.
+        :param data: Data to write to bus.
+        :param extended: Use 64 bits offset independent of the platform.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def parse_resource(self, session, resource_name):
+        """Parse a resource string to get the interface information.
+
+        Corresponds to viParseRsrc function of the VISA library.
+
+        :param session: Resource Manager session (should always be the Default Resource Manager for VISA
+                        returned from open_default_resource_manager()).
+        :param resource_name: Unique symbolic name of a resource.
+        :return: Resource information with interface type and board number, return value of the library call.
+        :rtype: :class:ResourceInfo, VISAStatus
+        """
+        raise NotImplementedError
+
+    def parse_resource_extended(self, session, resource_name):
+        """Parse a resource string to get extended interface information.
+
+        Corresponds to viParseRsrcEx function of the VISA library.
+
+        :param session: Resource Manager session (should always be the Default Resource Manager for VISA
+                        returned from open_default_resource_manager()).
+        :param resource_name: Unique symbolic name of a resource.
+        :return: Resource information, return value of the library call.
+        :rtype: :class:ResourceInfo, VISAStatus
+        """
+        raise NotImplementedError
+
+    def peek_8(self, session, address):
+        """Read an 8-bit value from the specified address.
+
+        Corresponds to viPeek8 function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param address: Source address to read the value.
+        :return: Data read from bus, return value of the library call.
+        :rtype: bytes, VISAStatus
+        """
+        raise NotImplementedError
+
+    def peek_16(self, session, address):
+        """Read an 16-bit value from the specified address.
+
+        Corresponds to viPeek16 function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param address: Source address to read the value.
+        :return: Data read from bus, return value of the library call.
+        :rtype: bytes, VISAStatus
+        """
+        raise NotImplementedError
+
+    def peek_32(self, session, address):
+        """Read an 32-bit value from the specified address.
+
+        Corresponds to viPeek32 function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param address: Source address to read the value.
+        :return: Data read from bus, return value of the library call.
+        :rtype: bytes, VISAStatus
+        """
+        raise NotImplementedError
+
+    def peek_64(self, session, address):
+        """Read an 64-bit value from the specified address.
+
+        Corresponds to viPeek64 function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param address: Source address to read the value.
+        :return: Data read from bus, return value of the library call.
+        :rtype: bytes, VISAStatus
+        """
+        raise NotImplementedError
+
+    def poke_8(self, session, address, data):
+        """Write an 8-bit value from the specified address.
+
+        Corresponds to viPoke8 function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param address: Source address to read the value.
+        :param data: value to be written to the bus.
+        :return: Data read from bus.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def poke_16(self, session, address, data):
+        """Write an 16-bit value from the specified address.
+
+        Corresponds to viPoke16 function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param address: Source address to read the value.
+        :param data: value to be written to the bus.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def poke_32(self, session, address, data):
+        """Write an 32-bit value from the specified address.
+
+        Corresponds to viPoke32 function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param address: Source address to read the value.
+        :param data: value to be written to the bus.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def poke_64(self, session, address, data):
+        """Write an 64-bit value from the specified address.
+
+        Corresponds to viPoke64 function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param address: Source address to read the value.
+        :param data: value to be written to the bus.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def read(self, session, count):
+        """Reads data from device or interface synchronously.
+
+        Corresponds to viRead function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param count: Number of bytes to be read.
+        :return: data read, return value of the library call.
+        :rtype: bytes, VISAStatus
+        """
+        raise NotImplementedError
+
+    def read_asynchronously(self, session, count):
+        """Reads data from device or interface asynchronously.
+
+        Corresponds to viReadAsync function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param count: Number of bytes to be read.
+        :return: result, jobid, return value of the library call.
+        :rtype: ctypes buffer, jobid, VISAStatus
+        """
+        raise NotImplementedError
+
+    def read_stb(self, session):
+        """Reads a status byte of the service request.
+
+        Corresponds to viReadSTB function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :return: Service request status byte, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def read_to_file(self, session, filename, count):
+        """Read data synchronously, and store the transferred data in a file.
+
+        Corresponds to viReadToFile function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param filename: Name of file to which data will be written.
+        :param count: Number of bytes to be read.
+        :return: Number of bytes actually transferred, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def set_attribute(self, session, attribute, attribute_state):
+        """Sets the state of an attribute.
+
+        Corresponds to viSetAttribute function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param attribute: Attribute for which the state is to be modified. (Attributes.*)
+        :param attribute_state: The state of the attribute to be set for the specified object.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def set_buffer(self, session, mask, size):
+        """Sets the size for the formatted I/O and/or low-level I/O communication buffer(s).
+
+        Corresponds to viSetBuf function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param mask: Specifies the type of buffer. (Constants.READ_BUF, .WRITE_BUF, .IO_IN_BUF, .IO_OUT_BUF)
+        :param size: The size to be set for the specified buffer(s).
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def status_description(self, session, status):
+        """Returns a user-readable description of the status code passed to the operation.
+
+        Corresponds to viStatusDesc function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param status: Status code to interpret.
+        :return: - The user-readable string interpretation of the status code passed to the operation,
+                 - return value of the library call.
+        :rtype: - unicode (Py2) or str (Py3)
+                - VISAStatus
+        """
+        raise NotImplementedError
+
+    def terminate(self, session, degree, job_id):
+        """Requests a VISA session to terminate normal execution of an operation.
+
+        Corresponds to viTerminate function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param degree: Constants.NULL
+        :param job_id: Specifies an operation identifier.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def uninstall_handler(self, session, event_type, handler, user_handle=None):
+        """Uninstalls handlers for events.
+
+        Corresponds to viUninstallHandler function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param event_type: Logical event identifier.
+        :param handler: Interpreted as a valid reference to a handler to be uninstalled by a client application.
+        :param user_handle: A value specified by an application that can be used for identifying handlers
+                            uniquely in a session for an event.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def unlock(self, session):
+        """Relinquishes a lock for the specified resource.
+
+        Corresponds to viUnlock function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def unmap_address(self, session):
+        """Unmaps memory space previously mapped by map_address().
+
+        Corresponds to viUnmapAddress function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def unmap_trigger(self, session, trigger_source, trigger_destination):
+        """Undo a previous map from the specified trigger source line to the specified destination line.
+
+        Corresponds to viUnmapTrigger function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param trigger_source: Source line used in previous map. (Constants.TRIG*)
+        :param trigger_destination: Destination line used in previous map. (Constants.TRIG*)
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+        raise NotImplementedError
+
+    def usb_control_in(self, session, request_type_bitmap_field, request_id, request_value,
+                       index, length=0):
+        """Performs a USB control pipe transfer from the device.
+
+        Corresponds to viUsbControlIn function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param request_type_bitmap_field: bmRequestType parameter of the setup stage of a USB control transfer.
+        :param request_id: bRequest parameter of the setup stage of a USB control transfer.
+        :param request_value: wValue parameter of the setup stage of a USB control transfer.
+        :param index: wIndex parameter of the setup stage of a USB control transfer.
+                      This is usually the index of the interface or endpoint.
+        :param length: wLength parameter of the setup stage of a USB control transfer.
+                       This value also specifies the size of the data buffer to receive the data from the
+                       optional data stage of the control transfer.
+        :return: - The data buffer that receives the data from the optional data stage of the control transfer
+                 - return value of the library call.
+        :rtype: - bytes
+                - VISAStatus
+        """
+        raise NotImplementedError
+
+    def usb_control_out(self, session, request_type_bitmap_field, request_id, request_value,
+                        index, data=""):
+        """Performs a USB control pipe transfer to the device.
+
+        Corresponds to viUsbControlOut function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param request_type_bitmap_field: bmRequestType parameter of the setup stage of a USB control transfer.
+        :param request_id: bRequest parameter of the setup stage of a USB control transfer.
+        :param request_value: wValue parameter of the setup stage of a USB control transfer.
+        :param index: wIndex parameter of the setup stage of a USB control transfer.
+                      This is usually the index of the interface or endpoint.
+        :param data: The data buffer that sends the data in the optional data stage of the control transfer.
+        :return: return value of the library call.
+        :rtype: VISAStatus
+        """
+
+    def vxi_command_query(self, session, mode, command):
+        """Sends the device a miscellaneous command or query and/or retrieves the response to a previous query.
+
+        Corresponds to viVxiCommandQuery function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param mode: Specifies whether to issue a command and/or retrieve a response. (Constants.VXI_CMD*, .VXI_RESP*)
+        :param command: The miscellaneous command to send.
+        :return: The response retrieved from the device, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def wait_on_event(self, session, in_event_type, timeout):
+        """Waits for an occurrence of the specified event for a given session.
+
+        Corresponds to viWaitOnEvent function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param in_event_type: Logical identifier of the event(s) to wait for.
+        :param timeout: Absolute time period in time units that the resource shall wait for a specified event to
+                        occur before returning the time elapsed error. The time unit is in milliseconds.
+        :return: - Logical identifier of the event actually received
+                 - A handle specifying the unique occurrence of an event
+                 - return value of the library call.
+        :rtype: - eventtype
+                - event
+                - VISAStatus
+        """
+        raise NotImplementedError
+
+    def write(self, session, data):
+        """Writes data to device or interface synchronously.
+
+        Corresponds to viWrite function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param data: data to be written.
+        :type data: str
+        :return: Number of bytes actually transferred, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+    def write_asynchronously(self, session, data):
+        """Writes data to device or interface asynchronously.
+
+        Corresponds to viWriteAsync function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param data: data to be written.
+        :return: Job ID of this asynchronous write operation, return value of the library call.
+        :rtype: jobid, VISAStatus
+        """
+        raise NotImplementedError
+
+    def write_from_file(self, session, filename, count):
+        """Take data from a file and write it out synchronously.
+
+        Corresponds to viWriteFromFile function of the VISA library.
+
+        :param session: Unique logical identifier to a session.
+        :param filename: Name of file from which data will be read.
+        :param count: Number of bytes to be written.
+        :return: Number of bytes actually transferred, return value of the library call.
+        :rtype: int, VISAStatus
+        """
+        raise NotImplementedError
+
+def list_wrappers():
+    return [name for (loader, name, ispkg) in pkgutil.iter_modules()
+            if name.startswith('pyvisa-')]
+
+_WRAPPERS = {}
+def get_wrapper(wrapper_name):
+    try:
+        return _WRAPPERS[wrapper_name]
+    except KeyError:
+        if wrapper_name == 'ni':
+            from .ctwrapper import NIVisaLibrary
+            _WRAPPERS['ni'] = NIVisaLibrary
+            return NIVisaLibrary
+
+    for pkgname in list_wrappers():
+        if pkgname.endswith('-' + wrapper_name):
+            pkg = __import__(pkgname)
+            _WRAPPERS[wrapper_name] = cls = pkg.WRAPPER_CLASS
+            return cls
+    else:
+        raise ValueError('No package named pyvisa-%s' % wrapper_name)
+
+
+def open_visa_library(*paths):
+    """Helper function to create a VISA library wrapper.
+
+    In general, you should not use the function directly. The VISA library
+    wrapper will be created automatically when you create a ResourceManager object.
+
+    The function takes
+    """
+    if not paths:
+        return open_visa_library(*util.get_library_paths())
+
+    errs = []
+    for path in paths:
+        try:
+            path, wrapper = path.split('@')
+        except ValueError:
+            wrapper = 'ni'
+
+            if not path:
+                return open_visa_library(*util.get_library_paths())
+
+        cls = get_wrapper(wrapper)
+        try:
+            return cls(path)
+        except OSError as e:
+            logger.debug('Could not open VISA library %s: %s', path, str(e))
+            errs.append(str(e))
+        except Exception as e:
+            errs.append(str(e))
+    else:
+        raise OSError('Could not open VISA library:\n' + '\n'.join(errs))
 
 
 class ResourceManager(object):
@@ -288,9 +1424,13 @@ class ResourceManager(object):
     #: Session handler for the resource manager.
     _session = None
 
-    def __new__(cls, visa_library=None):
-        if visa_library is None or isinstance(visa_library, str):
-            visa_library = VisaLibrary(visa_library)
+    def __new__(cls, visa_library=()):
+        if isinstance(visa_library, str):
+            visa_library = open_visa_library(*(visa_library,))
+        elif visa_library is None:
+            visa_library = open_visa_library()
+        else:
+            visa_library = open_visa_library(*visa_library)
 
         if visa_library in cls._registry:
             return cls._registry[visa_library]
@@ -331,6 +1471,7 @@ class ResourceManager(object):
             logger.debug('Closing ResourceManager (session: %s)', self.session)
             self.visalib.close(self.session)
             self.session = None
+            del self._registry[self.visalib]
         except errors.InvalidSession:
             pass