[Python] Filtering based on custom warning categories

In addition to pre-existing warning categories, users can define their own warning classes, such as in the code below:

$ cat mwe.py
#!/usr/bin/env python3.5

import warnings
import pprint

class ObnoxiousWarning(UserWarning):
pass

for i in range(3):
print(i)
warnings.warn("I don't like this.", ObnoxiousWarning)


When invoking Python, the -W flag controls how to filter warnings. But when I try to get it to ignore my freshly minted warning category, I'm told the filter is ignored:

$ python3.5 -W ignore::ObnoxiousWarning ./mwe.py
Invalid -W option ignored: unknown warning category: 'ObnoxiousWarning'
0
./mwe.py:11: ObnoxiousWarning: I don't like this.
warnings.warn("I don't like this.", ObnoxiousWarning)
1
2


How can I use the commandline to insert a filter for custom warning categories (as opposed to all UserWarnings or filtering based on the warning message, which I can do)?

Edit 2018-11-29: See Issue 66733: -W option cannot use nonstandard categories, still open as of 2025-08-18.

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