Should I use "camel case" or underscores in python?

Function names should be lowercase, with words separated by underscores as necessary to improve readability. mixedCase is allowed only in contexts where that's already the prevailing style

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PEP 8 advises the first form for readability. You can find it here.

Function names should be lowercase, with words separated by underscores as necessary to improve readability.

for everything related to Python's style guide: i'd recommend you read PEP8.

To answer your question:

Function names should be lowercase, with words separated by underscores as necessary to improve readability.