python mock and libraries that are not installed

Question

I am working on software for a robot, which is normally run on the Raspberry Pi. Let\'s consider the imports of two files:

motor.py (runs the motors):

Answer 1

You can use patch.dict() to patch sys.modules and mock RPi module as showed in pointed documentation.

Use follow code at the top of your test module:

>>> from mock import MagicMock, patch
>>> mymodule = MagicMock()
>>> patch.dict("sys.modules", RPi=mymodule).start()
>>> from RPi import GPIO as gpio
>>> gpio
<MagicMock name='mock.GPIO' id='139664555819920'>
>>> import os
>>> os
<module 'os' from '/usr/lib/python2.7/os.pyc'>

In Python3 you have same behavior.

---

In your specific case use patch.dict is little bit overkill; maybe you aren't interested in patch context and original state recover. So you can simplify it by set sys.modules["RPi"] directly:

>>> from unittest.mock import MagicMock
>>> mymodule = MagicMock()
>>> import sys
>>> sys.modules["RPi"] = mymodule
>>> from RPi import GPIO as gpio
>>> gpio
<MagicMock name='mock.GPIO' id='140511459454648'>
>>> import os
>>> os
<module 'os' from '/usr/lib/python3.4/os.py'>