Migrating from PyVISA < 1.5


if you want PyVISA 1.4 compatibility use PyVISA 1.5 that provides Python 3 support, better visa library detection heuristics, Windows, Linux and OS X support, and no singleton object. PyVISA 1.6+ introduces a few compatibility breaks.

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.

Indeed PyVISA 1.6 breaks compatibility to bring across a few good things.

If you are doing:

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

change it to:

>>> import pyvisa
>>> rm = pyvisa.ResourceManager()
>>> keithley = rm.open_resource("GPIB::12")
>>> print(keithley.query("*IDN?"))

If you are doing:

>>> print(pyvisa.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 pyvisa
>>> rm = pyvisa.ResourceManager("/path/to/my/libvisa.so.7")
>>> lib = rm.visalib

If you are doing::

>>> vpp43.lock(session)

change it to:

>>> lib.lock(session)

or better:

>>> resource.lock()

If you are doing::

>>> inst.term_chars = '\r'

change it to:

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

If you are doing::

>>> print(lib.status)

change it to:

>>> print(lib.last_status)

or even better, do it per resource:

>>> print(rm.last_status) # for the resource manager
>>> print(inst.last_status) # for a specific instrument

If you are doing::

>>> inst.timeout = 1  # Seconds

change it to:

>>> inst.timeout = 1000  # Milliseconds

As you see, most of the code shown above is making a few things explict. It adds 1 line of code (instantiating the 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.query_delay (see below).

A few alias has been created to ease the transition:

  • ask -> query
  • ask_delay -> query_delay
  • get_instrument -> open_resource

A more detailed description

Isolated low-level wrapping module

In the original PyVISA implementation, the low level implementation (vpp43) was mixed with higher level constructs. The VISA library was wrapped using ctypes.

In 1.5, we refactored it as ctwrapper. 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.

In 1.6, we made the ResourceManager the object exposed to the user. The type of the VisaLibrary can selected depending of the library_path and obtained from a plugin package.

We have two of such packages 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. This module is gone in 1.6.

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 accessing the VisaLibrary object under the attribute visalib of the resource manager 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.

There were additionally a few global structures such a status which stored the last status returned by the library and the warning context to prevent unwanted warnings.

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

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

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

or equivalently:

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


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.

In 1.6, the state returned by the library is stored per resource. Additionally, warnings can be silenced by resource as well. You can access with the last_status property.

All together, these changes makes PyVISA thread safe.

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 pyvisa
>>> rm = pyvisa.ResourceManager("/path/to/my/libvisa.so.7")
>>> lib = rm.visalib
>>> print(lib.read_stb(session))

(But it is very likely that you do not have to do it as the resource should have the function you need)

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

>>> status = ctypes.c_ushort()
>>> ret lib.viReadSTB(session, ctypes.byref(status))
>>> print(ret.value)

Ask vs. query

Historically, the method ask has been used in PyVISA to do a write followed by a read. But in many other programs this operation is called query. Thereby we have decided to switch the name, keeping an alias to help with the transition.

However, ask_for_values has not been aliased to query_values because the API is different. ask_for_values still uses the old formatting API which is limited and broken. We suggest that you migrate everything to query_values

Seconds to milliseconds

The timeout is now in milliseconds (not in seconds as it was before). The reason behind this change is to make it coherent with all other VISA implementations out there. The C-API, LabVIEW, .NET: all use milliseconds. Using the same units not only makes it easy to migrate to PyVISA but also allows to profit from all other VISA docs out there without extra cognitive effort.

Removal of Instrument.delay and added Instrument.query_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 1.6, delay is removed. Delays after write operations must be added to the application code. Instead, a new attribute and argument query_delay is available. This allows you to pause between write` and ``read operations inside query. Additionally, query takes an optional argument called query 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 1.6, term_chars is replaced by read_termination` and ``write_termination. In this way, you can set independently the termination for each operation. Automatic removal of CR + LF is also gone in 1.6.