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https://github.com/boxed/mutmut

Mutation testing system
https://github.com/boxed/mutmut

mutation-testing python testing

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Mutation testing system

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README 

        
mutmut - python mutation tester

===============================



.. image:: https://github.com/boxed/mutmut/actions/workflows/tests.yml/badge.svg

    :target: https://github.com/boxed/mutmut/actions/workflows/tests.yml



.. image:: https://readthedocs.org/projects/mutmut/badge/?version=latest

    :target: https://mutmut.readthedocs.io/en/latest/?badge=latest

    :alt: Documentation Status



Mutmut is a mutation testing system for Python, with a strong focus on ease

of use. If you don't know what mutation testing is try starting with

`this article `_.



Some highlight features:



- Found mutants can be applied on disk with a simple command making it very

  easy to work with the results

- Remembers work that has been done, so you can work incrementally

- Knows which tests to execute, speeding up mutation testing

- Interactive terminal based UI

- Parallel and fast execution



.. image:: browse_screenshot.png



If you want to mutate code outside of functions, you can try using mutmut 2,

which has a different execution model than mutmut 3+.



Requirements

------------



Mutmut must be run on a system with `fork` support. This means that if you want

to run on windows, you must run inside WSL.



Install and run

---------------



You can get started with a simple:



.. code-block:: console



    pip install mutmut

    mutmut run



This will by run pytest on tests in the "tests" or "test" folder and

it will try to figure out where the code to mutate is.



You can stop the mutation run at any time and mutmut will restart where you

left off. It will continue where it left off, and re-test functions that were

modified since last run.



To work with the results, use `mutmut browse` where you can see the mutants,

retest them when you've updated your tests.



You can also write a mutant to disk from the `browse` interface, or via

`mutmut apply `. You should **REALLY** have the file you mutate under

source code control and committed before you apply a mutant!



Configuration

-------------



In `setup.cfg` in the root of your project you can configure mutmut if you need to:



.. code-block:: ini



    [mutmut]

    paths_to_mutate=src/

    tests_dir=tests/



See below for more options for configuring mutmut.



Wildcards for testing mutants

-----------------------------



Unix filename pattern matching style on mutants is supported. Example:



.. code-block:: console



    mutmut run "my_module*"

    mutmut run "my_module.my_function*"



In the `browse` TUI you can press `f` to retest a function, and `m` to retest

an entire module.



"also copy" files

-----------------



To run the full test suite some files are often needed above the tests and the

source. You can configure to copy extra files that you need by adding

directories and files to `also_copy` in your `setup.cfg`:



.. code-block:: ini



    also_copy=

        iommi/snapshots/

        conftest.py



Limit stack depth

-----------------



In big code bases some functions are called incidentally by huge swaths of the

codebase, but you really don't want tests that hit those executions to count

for mutation testing purposes. Incidentally tested functions lead to slow

mutation testing as hundreds of tests can be checked for things that should

have clean and fast unit tests, and it leads to bad test suites as any

introduced bug in those base functions will lead to many tests that fail which

are hard to understand how they relate to the function with the change.



You can configure mutmut to only count a test as being relevant for a function

if the stack depth from the test to the function is below some limit. In your

`setup.cfg` add:



.. code-block:: ini



    max_stack_depth=8



A lower value will increase mutation speed and lead to more localized tests,

but will also lead to more surviving mutants that would otherwise have been

caught.



Exclude files from mutation

---------------------------



You can exclude files from mutation in `setup.cfg`:



.. code-block::



    do_not_mutate=

        *__tests.py



Whitelisting

------------



You can mark lines like this:



.. code-block:: python



    some_code_here()  # pragma: no mutate



to stop mutation on those lines. Some cases we've found where you need to

whitelist lines are:



- The version string on your library. You really shouldn't have a test for this :P

- Optimizing break instead of continue. The code runs fine when mutating break

  to continue, but it's slower.



Example mutations

-----------------



- Integer literals are changed by adding 1. So 0 becomes 1, 5 becomes 6, etc.

- `<` is changed to `<=`

- break is changed to continue and vice versa



In general the idea is that the mutations should be as subtle as possible.

See `__init__.py` for the full list.



Workflow

--------



This section describes how to work with mutmut to enhance your test suite.



1. Run mutmut with `mutmut run`. A full run is preferred but if you're just

   getting started you can exit in the middle and start working with what you

   have found so far.

2. Show the mutants with `mutmut browse`

3. Find a mutant you want to work on and write a test to try to kill it.

4. Press `r` to rerun the mutant and see if you successfully managed to kill it.



Mutmut keeps the data of what it has done and the mutants in the `mutants/`

directory.If  you want to make sure you run a full mutmut run you can delete

this directory to start from scratch.