pytest-mock-generator 1.0.0

A pytest fixture wrapper for https://pypi.org/project/mock-generator

pytest-mock-generator

A pytest fixture wrapper for https://pypi.org/project/mock-generator

Installation

pip install pytest-mock-generator

or install with poetry:

poetry add pytest-mock-generator

Usage

This pytest plugin adds the mg fixture which helps when writing tests that use python mocks.

Let's start with an easy example. Assume you have a module named tested_module.py which holds a function to process a string sent to it and then add it to a zip file:

import zipfile

def process_and_zip(zip_path, file_name, contents):
    processed_contents = "processed " + contents  # some complex logic
    with zipfile.ZipFile(zip_path, 'w') as zip_container:
        zip_container.writestr(file_name, processed_contents)

This is the unit under test, or UUT.

Although this is a very short function, writing the test code takes a lot of time. It's the fact that the function uses a context manager makes the testing more complex than it should be. If you don't believe me, try to manually write mocks and asserts which verify that zip_container.writestr was called with the right parameters.

In any case, you start with a test skeleton:

from tests.sample.code.tested_module import process_and_zip

def test_process_and_zip(mocker, mg):
    # Arrange: todo  
    
    # Act: invoking the tested code
    process_and_zip('/path/to.zip', 'in_zip.txt', 'foo bar')
    
    # Assert: todo

Now it's time to use Mock Generator instead of manually writing the 'Arrange' and 'Assert' sections.

Generating the 'Arrange' section

To generate the 'Arrange' section, simply put this code at the beginning of your test function skeleton and run it (make sure to add the mg fixture to your test function):

mg.generate_uut_mocks(process_and_zip)

This will generate the 'Arrange' section for you:

# mocked dependencies
mock_ZipFile = mocker.MagicMock(name='ZipFile')
mocker.patch('tests.sample.code.tested_module.zipfile.ZipFile', new=mock_ZipFile)

The generated code is returned, printed to the console and also copied to the clipboard for your convenience. Just paste it (as simple as ctrl+V) at the start of your test function:

from tests.sample.code.tested_module import process_and_zip

def test_process_and_zip(mocker, mg):
    # mocked dependencies
    mock_ZipFile = mocker.MagicMock(name='ZipFile')
    mocker.patch('tests.sample.code.tested_module.zipfile.ZipFile', new=mock_ZipFile)
    
    # Act: invoking the tested code
    process_and_zip('/path/to.zip', 'in_zip.txt', 'foo bar')
    
    # Assert: todo

Excellent! Arrange section is ready.

Generating the Assert section

Now it's time to add the asserts. Add the following code at the 'Assert' step:

mg.generate_asserts(mock_ZipFile)

The mock_ZipFile is the mock object you generated earlier. Now execute the test function to get the assert section:

assert 1 == mock_ZipFile.call_count
mock_ZipFile.assert_called_once_with('/path/to.zip', 'w')
mock_ZipFile.return_value.__enter__.assert_called_once_with()
mock_ZipFile.return_value.__enter__.return_value.writestr.assert_called_once_with('in_zip.txt', 'processed foo bar')
mock_ZipFile.return_value.__exit__.assert_called_once_with(None, None, None)

Wow, that's a handful of asserts! Some are very useful:

• Checking that we opened the zip file with the right parameters.
• Checking that we wrote the correct data to the proper file.
• Finally, ensuring that __enter__ and __exit__ are called, so there are no open file handles which could cause problems.

You can remove any generated line which you find unnecessary.

Paste that code right after the act phase, and you're done!

The complete test function:

from tests.sample.code.tested_module import process_and_zip

def test_process_and_zip(mocker):
    # mocked dependencies
    mock_ZipFile = mocker.MagicMock(name='ZipFile')
    mocker.patch('tests.sample.code.tested_module.zipfile.ZipFile', new=mock_ZipFile)
    
    # Act: invoking the tested code
    process_and_zip('/path/to.zip', 'in_zip.txt', 'foo bar')
    
    assert 1 == mock_ZipFile.call_count
    mock_ZipFile.assert_called_once_with('/path/to.zip', 'w')
    mock_ZipFile.return_value.__enter__.assert_called_once_with()
    mock_ZipFile.return_value.__enter__.return_value.writestr.assert_called_once_with('in_zip.txt', 'processed foo bar')
    mock_ZipFile.return_value.__exit__.assert_called_once_with(None, None, None)

Can you imagine the time it would have taken you to code this on your own?

What's Next

Asserting Existing Mocks

At times, you may be editing a test code already containing mocks, or you choose to write the mocks yourself, to gain some extra control.

Mock Generator can generate the assert section for standard Python mocks, even if they were not created using the Mock Generator.

Put this after the 'Act' (replace mock_obj with your mock object name):

mg.generate_asserts(mock_obj)

Take the generated code and paste it at the 'Assert' section.

Generating the 'Arrange' and 'Assert' sections in one call

You can make the generate_uut_mocks_with_asserts call create the generate_asserts code for you (instead of having to call generate_uut_mocks):

mg.generate_uut_mocks_with_asserts(function_under_test)

More information

Additional documentation can be found in the mock-generator pypi.

📈 Releases

You can see the list of available releases on the GitHub Releases page.

We follow Semantic Versions specification.

🛡 License

This project is licensed under the terms of the MIT license. See LICENSE for more details.

📃 Citation

@misc{pytest-mock-generator,
  author = {Peter Kogan},
  title = {A pytest fixture wrapper for https://pypi.org/project/mock-generator},
  year = {2021},
  publisher = {GitHub},
  journal = {GitHub repository},
  howpublished = {\url{https://github.com/pksol/pytest-mock-generator}}
}

Credits

This project was generated with python-package-template