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Mock

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Reference:docs.python.org

Usage ​

Import from unittest.mock, or from mock if installed separately.

Set value ​

python
@mock.patch('calc.Calculator.sum')
def test_sum_return_value(self, sum):
    sum.return_value = 9

Side effect ​

python
def sum(a, b):
    if a > b:
        return 5
    return 10

@mock.patch('mymodule.sum')
def test_sum_side_effect(self, mock_sum):
    mock_sum.side_effect = sum

@mock.patch('mymodule.sum', side_effect=Exception())
def test_sum_side_effect(self, mock_sum):
    mymodule.sum()  # Will raise Exception()

Assert call ​

python
@mock.patch('calc.Calculator.sum')
def test_call_count(self, sum):
    sum.assert_called_with(1)  # Asserts the last call's parameters
    sum.assert_any_call(1)     # Asserts any call's parameters
    assert sum.call_count == 3
    sum.call_args
    sum.call_args_list

Mock file ​

python
def test_open_file(self):
    mock_open = mock.mock_open(read_data='file_content')
    with mock.patch('builtins.open', mock_open):
        with open('/some/path', 'r') as text_file:
            text_file.read()  # Returns 'file_content'
    mock_open.assert_any_call('/some/path', 'r')
    mock_open().write.call_args.args

Mock module ​

python
import sys
sys.modules['some_module'] = mock.MagicMock()

Tips ​

Mocking can sometimes be tricky. Understanding how it works can help saving some time.

Mock your own module ​

Mock your own module instead the original module. Let's say your module is at a.b and it imports x.y.z. You want to mock z.

The result of mocking may be like this:

python
@mock.patch('x.y.z')  # Does not work...
@mock.patch('a.b.z')  # Works!

Reloading the module ​

Mocking a module that is already loaded won't work. This often happens when you load a module in a import statement at the top of the script, and create a mock for it in runtime.

To solve this, reload the module after creating the mock:

python
from importlib import reload  # Python >= 3.4 only, else unneeded
reload(x.y.z)