Python Import Mocker

The Python Import Mocker provides an easy way to import a module and mock its dependencies in an isolated way. The mocking of the dependencies will apply only to the current import and will not affect the global module namespace.

Quick Start

Install

pip install import-mocker

Mock imports

from import_mocker import ImportMocker

modules_to_mock = ['B', 'C']
imocker = ImportMocker(modules_to_mock)
A = imocker.import_module('A')

Verify behavior on mocked modules

mocks = imocker.get_mocks()
b_mock = mocks['B']
b_mock.some_method.assert_called()

Reset mocked modules

imocker.reset_mock('B')
imocker.reset_mocks()

Execute code within a mocked module context

This is useful when the code to execute will perform an inline import.

imocker.execute(lambda: function_that_calls_inline_import(x, y, z=4))

Rationale

When unit testing in Python we couldn't find a way to easily mock imports without affecting the global scope and without having to carefully mock and de-mock the imported modules.

This was a problem for us because we needed to test some files, and then mock those same files when testing other files, and we can't control the order in which the tests are executed. Here is an example:

# ***** SOURCE CODE *****
# FILE: A.py
import B
import C
import D
...

# FILE: B.py
import C
import D
...


# ***** TESTS *****
# FILE: test_a.py
# We need to mock B, and C, and the real version of D
from importlib import reload
from unittest import mock
sys.modules["B"] = mock.Mock()
sys.modules["C"] = mock.Mock()

import D
D = reload(D) # Make sure we get the real module if D was mocked before

import A # this line recursively imports B, C, and D
A = reload(A) # Make sure the correct mocks are used if A was mocked before

...

# FILE: test_b.py
# We need to mock only D, and need the real version of C
from importlib import reload
from unittest import mock
sys.modules["D"] = mock.Mock()

import C # Make sure the correct mocks are used if C was mocked before
C = reload(C)
import B # Make sure the correct mocks are used if B was mocked before
B = reload(B)

As it can be seen, this can get very verbose, especially when dependencies start to grow and we need different configurations for mocking.

This is why we created Python Import Mocker, to greately simplify this process without having to reinvent the wheel every time. We hope you find this as useful as we did 😀.

Example

Following the example given in the previous section, here is how the Python Import Mocker would be used:

# ***** SOURCE CODE *****
# FILE: A.py
import B
import C
import D
...

# FILE: B.py
import C
import D
...


# ***** TESTS USING PYTHON IMPORT MOCKER *****
# FILE: test_a.py
# We need to mock B and C
from unittest import mock
from import_mocker import ImportMocker

modules_to_mock = ['B', 'C']
imocker = ImportMocker(modules_to_mock)
A = imocker.import_module('A')
...

def my_test_01():
    # Do something and verify B and C
    imocker.reset_mocks()
    execute_code()
    mocks = imocker.get_mocks()
    b_mock = mocks['B']
    b_mock.some_method.assert_called()
    ...
    
    # Do something else and verify C
    imocker.reset_mock('C')
    execute_mode_code()
    c_mock = imocker.get_mock('C')
    c_mock.some_method.assert_called_once()
    ...
...

# FILE: test_b.py
# We need to mock only D, and need the real version of C
from import_mocker import ImportMocker

modules_to_mock = ['D']
imocker = ImportMocker(modules_to_mock)
B = imocker.import_module('B')
...

Note: You can find more practical examples in the test files.

API

These are the functions provided by the ImportMocker class.

import_module(module_to_import: str)

Imports module_to_import inside a context that that returns the mocked modules when they are imported, all other imports will work normally.

If module_to_import was previously imported, then it's reloaded so that its imported modules can be mocked again.

import_modules(modules_to_import: List[str])

Uses the same logic of import_module but receives a list of module names to import and returns a list with the imported modules in the same order.

execute(function, *args, **kwargs)

Executes a function inside a context that returns the mocked modules when they are imported, all other imports will work normally. *args and **kwargs are the arguments to pass down to function.

This is useful when you are testing code that has import statements inside a function, and you want to mock those imports.

IMPORTANT: If a module has been previously imported outside the current instance of the ImportMocker, it will not be re imported when executing the function.

get_mocks()

Gets a copy of the dictionary containing all the mocked modules.

get_mock(mock_name: str)

Gets the specified mocked module.

reset_mocks()

Resets all the mocked modules to their original state.

reset_mock(mock_name: str)

Resets the specified mocked module to its original state.

Contributing

This project welcomes contributions and suggestions. Most contributions require you to agree to a Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us the rights to use your contribution. For details, visit https://cla.opensource.microsoft.com.

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This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.

Trademarks

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