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I disagree. Purpose of __doc__ is to explain functionality all at once.
This command idiom is useful:
$ python -c 'from a_module import thing; help(thing)'
The doctest module is a lightweight nicety that helps verify that which
is suitable. The sufficiently simple algorithms of my code have doc
strings that are the complete test and explanation. For others I
provide both docstring and unit tests. But with many I explain the
arguments and output, possibly the algorithm in a doc string. Tests and
use case examples reside in the module's unit test.
I'm among the "Choose correct tool for the job. python comes with full
tool bag." group.