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Python | Unit Test Objects Patching | Set-1
  • Last Updated : 12 Jun, 2019

The problem is writing unit tests and need to apply patches to selected objects in order to make assertions about how they were used in the test (e.g., assertions about being called with certain parameters, access to selected attributes, etc.).

To do so, the unittest.mock.patch() function can be used to help with this problem. It’s a little unusual, but patch() can be used as a decorator, a context manager, or stand-alone.

Code #1: Using unittest.mock.patch as a decorator

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from unittest.mock import patch
import example
@patch('example.func')
  
def test1(x, mock_func):
    # Uses patched example.func
    example.func(x) 
    mock_func.assert_called_with(x)

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Code #2: Using unittest.mock.patch as a decorator

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with patch('example.func') as mock_func:
    example.func(x) 
    mock_func.assert_called_with(x)

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Code #3: Using unittest.mock.patch to patch things manually.



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p = patch('example.func')
mock_func = p.start()
example.func(x)
mock_func.assert_called_with(x)
p.stop()

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Code #4: Stacking decorators and context managers to patch multiple objects

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@patch('example.func1')
@patch('example.func2')
@patch('example.func3')
  
def test1(mock1, mock2, mock3):
    ...
def test2():
    with patch('example.patch1') as mock1, \
    patch('example.patch2') as mock2, \
    patch('example.patch3') as mock3:
    ...

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patch() works by taking an existing object with the fully qualified name that you provide and replacing it with a new value. The original value is then restored after the completion of the decorated function or context manager. By default, values are replaced with MagicMock instances.
 
Code #5 : Example

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x = 42
with patch('__main__.x'):
    print(x)
  
print (x)

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Output :

<MagicMock name='x' id='4314230032'>
42

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