MagicMock基于输入的返回值

Question

I am trying to refactor my tests from flexmock to mock. Given the following syntax from flexmock:

flexmock(subprocess).should_receive('check_output').with_args('ls /').and_return(output)

How can I rewrite this using Mock? In particular, how do I pin a return value to a spesific input using Mock?

Answer 1

You can use side_effect patch attribute to do it:

>>> from unittest.mock import *
>>> root = '''bin   cdrom  etc   initrd.img  lib32  libx32      media  opt   root  sbin  sys  usr  vmlinuz
... boot  dev    home  lib         lib64  lost+found  mnt    proc  run   srv   tmp  var'''
>>> answer = {'ls /': root}
>>> import subprocess

>>> with patch('subprocess.check_output', side_effect=lambda arg, *args, **kwargs: answer[arg]) as mock_check_output :
...     assert root == subprocess.check_output('ls /')
...     mock_check_output.assert_called_with('ls /')
...     subprocess.check_output('something else')
... 
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
  File "/usr/lib/python3.4/unittest/mock.py", line 896, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr/lib/python3.4/unittest/mock.py", line 962, in _mock_call
    ret_val = effect(*args, **kwargs)
  File "<stdin>", line 1, in <lambda>
KeyError: 'something else'
>>> 

I'm quite sure that you can find it little bit harder compared to flexmock 's syntax, but what you need is more a stub than a mock. If you can design your test to be isolated and configure your mock/stub in some setup stage maybe you will find this syntax good and all mock asserts and patch options very powerful.