Migrating from PyVISA < 1.5¶
Note
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 visa
>>> keithley = visa.instrument("GPIB::12")
>>> print(keithley.ask("*IDN?"))
change it to:
>>> import visa
>>> rm = visa.ResourceManager()
>>> keithley = rm.open_resource("GPIB::12")
>>> print(keithley.query("*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
>>> rm = visa.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 = 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.
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 visa
>>> rm = visa.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.