mock file open in python

python unit-testing mocking magicmock
TzurEl picture TzurEl · Jul 19, 2016 · Viewed 21k times · Source

I'm trying to mock file open, and all of the examples show that I need to 

@patch('open', create=True)

but I keep getting 

Need a valid target to patch. You supplied: 'open'

I know patch needs the full dotted path of open, but I have no idea what it is. As a matter of fact, I'm not even sure that's the problem.

Answer

Martijn Pieters picture Martijn Pieters · Jul 19, 2016

You need to include a module name; if you are testing in a script, the name of the module is __main__:

@patch('__main__.open')

otherwise use the name of the module that contains the code you are testing:

@patch('module_under_test.open')

so that any code that uses the open() built-in will find the patched global instead.

Note that the mock module comes with a mock_open() utility that'll let you build a suitable open() call with file data:

@patch('__main__.open', mock_open(read_data='foo\nbar\nbaz\n'))

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