How to compare version numbers in a Python condition

Written by Claudio Kuenzler - 0 comments

Published on February 14th 2025 - Listed in Python Coding

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To fix a bug in the check_esxi_hardware monitoring plugin, I needed to find a way to do a "version check" in a condition. If the detected version (in semantic versioning format "X.Y.Z") is newer than a certain version number, then the executed code should be different. Luckily there's a fairly easy way to do this in Python.

Allowing backward compatibility

Older versions of the Python pywbem module used the internal functions pywbem.cim_operations and pywbem.cim_http, which could be used as exception handlers and detect errors. For example authentication errors could be caught using pywbem.cim_http.AuthError.

But starting with pywbem 1.0.0, these internal functions were renamed and were prefixed with an underscore:

pywbem < 1.0.0	pywbem >= 1.0.0
pywbem.cim_operations	pywbem._cim_operations
pywbem.cim_http.AuthError	pywbem._cim_http.AuthError

Unfortunately this renaming causes a conflict now.

The original code would throw a Python traceback when executed with pywbem 1.0.0 or newer:

Traceback (most recent call last):
  File "./check_esxi_hardware.py", line 757, in <module>
    except pywbem.cim_operations.CIMError as args:
           ^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'pywbem' has no attribute 'cim_operations'. Did you mean: '_cim_operations'?

On the other hand, if the newer pywbem functions (with the prefixed underscore) is used on older pywbem versions (prior to 1.0.0), the following error shows up:

 Traceback (most recent call last):
  File "./check_esxi_hardware.py", line 796, in <module>
    except pywbem._cim_operations.CIMError as args:
AttributeError: module 'pywbem' has no attribute '_cim_operations'

As, IMHO, monitoring plugins should be backward compatible as much as possible and support ongoing LTS versions (EL 8 and EL 9 distributions ship pywbem packages older than 1.0.0), I have to fix this and execute the relevant pywbem functions based on the detected pywbem version.

Version comparison with packaging module

The most efficient (and easiest to implement) way to compare two versions is by using the packaging module, which contains a version sub-module. This sub-module is made for that specific purpose (handling versioning, doing comparisons, etc).

Here's an easy example:

ck@mint ~ $ cat /tmp/versioncomparison.py
#!/usr/bin/python3
import pywbem
from packaging.version import Version

pywbemversion = pywbem.__version__
print("Found pywbem version {}".format(pywbemversion))

if Version(pywbemversion) > Version("1.0.0"):
  print("pywbem version {} is newer than 1.0.0".format(pywbemversion))
else:
  print("pywbem version {} is older than 1.0.0".format(pywbemversion))

Code explained:

• The Python modules pywbem and the version sub-module from the packaging module are imported
• The version of the detected pywbem module is detected and saved under the variable pywbemversion
• The Version() function comes from the imported packaging.version module and handles the given version. The version stored in the variable pywbemversion is compared to the fixed version string "1.0.0".
  
• Depending on true or false returns of the condition, a different string is shown at the output

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