Migrating from PyVISA < 1.5

You don’t need to change anything in your code if you only use the instrument constructor; and attributes and methods of the resulting object. For example, this code will run unchanged in modern versions of PyVISA:

import visa
keithley = visa.instrument("GPIB::12")
print(keithley.ask("*IDN?"))

This covers almost every single program that I have seen on the internet. However, if you use other parts of PyVISA or you are interested in the design decisions behind the new version you might want to read on.

Some of these decisions were inspired by the visalib package as a part of Lantz

Short summary

PyVISA 1.5 has full compatibility with previous versions of PyVISA using the legacy module (changing some of the underlying implementation). But you are encouraged to do a few things differently if you want to keep up with the latest developments and be compatible with PyVISA > 1.5.

If you are doing:

>>> import visa
>>> keithley = visa.instrument("GPIB::12")
>>> print(keithley.ask("*IDN?"))

change it to:

>>> import visa
>>> rm = visa.ResourceManager()
>>> keithley = rm.get_instrument("GPIB::12")
>>> print(keithley.ask("*IDN?"))

If you are doing:

>>> print(visa.get_instruments_list())

change it to:

>>> print(rm.list_resources())

If you are doing:

>>> import pyvisa.vpp43 as vpp43
>>> vpp43.visa_library.load_library("/path/to/my/libvisa.so.7")

change it to:

>>> import visa
>>> lib = visa.VisaLibrary("/path/to/my/libvisa.so.7")

If you are doing::

>>> vpp43.lock(session)

change it to:

>>> lib.lock(session)

If you are doing::

>>> inst.term_chars = '\r'

change it to:

>>> inst.read_termination = '\r'
>>> inst.write_termination = '\r'

As you see, most of the code shown above is making a few things explict. It adds 1 line of code (instantiating the VisaLibrary or ResourceManager object) which is not a big deal but it makes things cleaner.

If you were using printf, queryf, scanf, sprintf or sscanf of vpp43, rewrite as pure python code (see below).

If you were using Instrument.delay, change your code or use Instrument.ask_delay (see below).

A more detailed description

Dropped support for string related functions

The VISA library includes functions to search and manipulate strings such as printf, queryf, scanf, sprintf and sscanf. This makes sense as visa involves a lot of string handling operations. The original PyVISA implementation wrapped these functions. But these operations are easily expressed in pure python and therefore were rarely used.

PyVISA 1.5 keeps these functions for backwards compatibility but it will be removed in 1.6.

We suggest that you replace such functions by a pure python version.

Isolated low-level wrapping module

In the original PyVISA implementation, the low level implementation (vpp43) was mixed with higher level constructs such as VisaLibrary, VisaException and error messages. The VISA library was wrapped using ctypes.

In 1.5, we refactored it as ctwrapper, also a ctypes wrapper module but it only depends on the constants definitions (constants.py). This allows us to test the foreign function calls by isolating them from higher level abstractions. More importantly, it also allows us to build new low level modules that can be used as drop in replacements for ctwrapper in high level modules.

We have two modules planned:

• a Mock module that allows you to test a PyVISA program even if you do not have VISA installed.
• a CFFI based wrapper. CFFI is new python package that allows easier and more robust wrapping of foreign libraries. It might be part of Python in the future.

PyVISA 1.5 keeps vpp43 in the legacy subpackage (reimplemented on top of ctwrapper) to help with the migration but it will be removed in the future.

All functions that were present in vpp43 are now present in ctwrapper but they take an additional first parameter: the foreign library wrapper.

We suggest that you replace vpp43 by using the new VisaLibrary object which provides all foreign functions as bound methods (see below).

No singleton objects

The original PyVISA implementation relied on a singleton, global objects for the library wrapper (named visa_library, an instance of the old pyvisa.vpp43.VisaLibrary) and the resource manager (named resource_manager, and instance of the old pyvisa.visa.ResourceManager). These were instantiated on import and the user could rebind to a different library using the load_library method. Calling this method however did not affect resource_manager and might lead to an inconsistent state.

In 1.5, there is a new VisaLibrary class and a new ResourceManager class (they are both in pyvisa.highlevel). The new classes are not singletons, at least not in the strict sense. Multiple instances of VisaLibrary and ResourceManager are possible, but only if they refer to different foreign libraries. In code, this means:

>>> lib1 = visa.VisaLibrary("/path/to/my/libvisa.so.7")
>>> lib2 = visa.VisaLibrary("/path/to/my/libvisa.so.7")
>>> lib3 = visa.VisaLibrary("/path/to/my/libvisa.so.8")
>>> lib1 is lib2
True
>>> lib1 is lib3
False

Most of the time, you will not need access to a VisaLibrary object but to a ResourceManager. You can do:

>>> lib = visa.VisaLibrary("/path/to/my/libvisa.so.7")
>>> rm = lib.resource_manager

or equivalently:

>>> rm = visa.ResourceManager("/path/to/my/libvisa.so.7")

Note

If the path for the library is not given, the path is obtained from the user settings file (if exists) or guessed from the OS.

You can still access the legacy classes and global objects:

>>> from pyvisa.legacy import vpp43
>>> from pyvisa.legacy import visa_library, resource_manager

In 1.5, visa_library and resource_manager, instances of the legacy classes, will be instantiated on import.

VisaLibrary methods as way to call Visa functions

In the original PyVISA implementation, the VisaLibrary class was just having a reference to the ctypes library and a few functions.

In 1.5, we introduced a new VisaLibrary class (pyvisa.highlevel) which has every single low level function defined in ctwrapper as bound methods. In code, this means that you can do:

>>> import visa
>>> lib = visa.VisaLibrary("/path/to/my/libvisa.so.7")
>>> print(lib.read_stb(session))

It also has every single VISA foreign function in the underlying library as static method. In code, this means that you can do:

>>> lib = visa.VisaLibrary("/path/to/my/libvisa.so.7")
>>> status = ctypes.c_ushort()
>>> ret library.viReadSTB(session, ctypes.byref(status))
>>> print(ret.value)

Removal of Instrument.delay and added Instrument.ask_delay

In the original PyVISA implementation, Instrument takes a delay argument that adds a pause after each write operation (This also can be changed using the delay attribute).

In PyVISA 1.5, delay is removed. Delays after write operations must be added to the application code. Instead, a new attribute and argument ask_delay is available. This allows you to pause between write and read operations inside ask. Additionally, ask takes an optional argument called delay allowing you to change it for each method call.

Deprecated term_chars and automatic removal of CR + LF

In the original PyVISA implementation, Instrument takes a term_chars argument to change at the read and write termination characters. If this argument is None, CR + LF is appended to each outgoing message and not expected for incoming messages (although removed if present).

In PyVISA 1.5, term_chars is replaced by read_termination and write_termination. In this way, you can set independently the termination for each operation. term_chars is still present in 1.5 (but will be removed) and sets both at the same time. Automatic removal of CR + LF is still present in 1.5 but will be removed in 1.6.