Mockito is a spying framework originally based on the Java library with the same name. (Actually we invented the strict stubbing mode back in 2009.)

pip install mockito

90% use case is that you want to stub out a side effect.

from mockito import when, mock, unstub

when(os.path).exists('/foo').thenReturn(True)

# or:
import requests  # the famous library
# you actually want to return a Response-like obj, we'll fake it
response = mock({'status_code': 200, 'text': 'Ok'})
when(requests).get(...).thenReturn(response)

# use it
requests.get('http://google.com/')

# clean up
unstub()

http://mockito-python.readthedocs.io/en/latest/

I use rye, and if you do too: you just clone this repo to your computer, then run rye sync in the root directory. Finally, activate the virtualenv. (rye shell gives you a hint on how to do that.)

pytest