Mocking a function inside a function in Python Unit Testing using Mock

Question

I am using Mock for Python Unit Testing. I am testing a function foo which contains a function bar that doesn't return anything, but fills a variable x (type:io.StringIO) as a side effect. I have mocked bar using MagicMock, but I don't know how to assign to x from a test script.

I have the following situation:

def foo():
    x = io.StringIO()
    bar(x)                 # x is filled with some string by bar method
    here some operation on x

To write a Unit Test case for foo, I have mocked bar with MagicMock (return value=None) but how to assign to x which is needed by foo.

Is the variable passed to the nested function, or is it just relying on the closure? — Daniel Roseman
– Daniel Roseman, Commented Dec 6, 2013 at 9:09
You're lucky that's an external call, which can be mocked. For dirty local tricks, see here. — Cees Timmerman
– Cees Timmerman, Commented Mar 22 at 16:20

Simeon Visser · Accepted Answer · 2013-12-06 13:57:11Z

5

You need to mock io.StringIO and you can then replace it with something else:

@mock.patch('mymodule.io.StringIO')
def test_foo(self, mock_stringio):
    mock_stringio.return_value = mock.Mock()

Note the use of return_value which means you're mocking the instance that's returned from the StringIO() call.

answered Dec 6, 2013 at 13:57

Simeon Visser

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