mock-generator

Mock Generator

A tool to generate the basic mocks and asserts for faster unit testing.

🎉 New: you can now use pytest-mock-generator, for more fluid pytest development. See documentation later in this doc.

Introduction

A typical unit test looks like this (AAA pattern):

• Arrange – Setup and prepare the various objects and prerequisites.
• Act – Invoke the tested code by calling the tested function.
• Assert – Verify the outcome. This can be the return value of the tested function and/or some side effects.

When using mocks, much time is wasted on the wiring. The Arrange and Assert sections are notorious in that regard. Only a fraction of the time is spent on the actual logic of the test.

This tool is meant to save you time, by generating the Arrange and Assert sections for you. The generated code can then be used as is, or altered according to your needs.

Usage

Note: All examples assume usage of the pytest-mock which is a fixture for pytest.

Getting Started

Let's 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:

import mock_autogen
from tests.sample.code.tested_module import process_and_zip

def test_process_and_zip(mocker):
    # 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:

mock_autogen.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:

import mock_autogen
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: 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:

mock_autogen.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

Using pytest-mock-generator

You can now use pytest-mock-generator to generate mocks:

pip install pytest-mock-generator

Then use mg fixture in your tests, instead of having to import mock_autogen, like so:

from tests.sample.code.tested_module import process_and_zip

def test_process_and_zip(mocker, mg):
    # Arrange: todo 
    # replace with the output of the below command   
    mg.generate_uut_mocks(process_and_zip)

    # Act: invoking the tested code
    process_and_zip('/path/to.zip', 'in_zip.txt', 'foo bar')

    # Assert: todo

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):

import mock_autogen
mock_autogen.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):

import mock_autogen
mock_autogen.generate_uut_mocks_with_asserts(function_under_test)

Mocking Everything

So far you have seen examples of mocking the dependencies of a single function. Mock Generator can generate mocks for objects, classes and entire modules!

A great way to learn about those capabilities is to see them in action:

from mock_autogen import PytestMocker
PytestMocker(a_module_or_a_class_you_test_and_want_to_mock_its_dependencies).mock_everything().generate() 

PytestMocker class has many options to produce different kind of mocks. See its documentation for further details.

Troubleshooting

No output is printed to the console when running Pytest

Be sure to run Pytest with appropriate flags to print the output: pytest -rA.

No output is copied to the clipboard

Mock Generator uses pyperclip for clipboard manipulations, look for errors / warnings printed in the console, similar to this one:

WARNING mock_autogen.utils:utils.py:28 
Could not copy func results to clipboard Traceback (most recent call last): 
    File "/home/username/Documents/code/test_mock_gen/.envtest/lib/python3.6/site-packages/mock_autogen/utils.py", line 25, in to_clipboard 
        pyperclip.copy(result) 
    File "/home/username/Documents/code/test_mock_gen/.envtest/lib/python3.6/site-packages/pyperclip/__init__.py", line 659, in lazy_load_stub_copy 
        return copy(text) 
    File "/home/username/Documents/code/test_mock_gen/.envtest/lib/python3.6/site-packages/pyperclip/__init__.py", line 336, in __call__
        raise PyperclipException(EXCEPT_MSG) pyperclip.PyperclipException: 

    Pyperclip could not find a copy/paste mechanism for your system. 
    For more information, please visit https://pyperclip.readthedocs.io/en/latest/index.html#not-implemented-error

It's possible that your system lacks some dependency, visit https://pyperclip.readthedocs.io/en/latest/index.html#not-implemented-error for additional troubleshooting steps.

Wrapping up

I hope that you were convinced that this tool can save you a lot of time.

If you have improvement suggestions, bug reports, or would like to contribute pull requests, let me know.