Skip to main content

Add the mutation testing feature to pytest

Project description


Mutagen is a plugin to pytest that makes it easy to do mutation testing. Mutation testing is a method of testing your tests. Mutagen helps you to define "mutant" versions of your code---code which is intentionally buggy---then you run your test suite on these mutants and verify that your tests actually catch the bugs. Mutation testing helps you to gauge test coverage and verify that your tests are good enough to exercise interesting behaviors in your code.

For Property-Based Testing

If you are a user of a property-based testing framework such as Hypothesis, mutation testing can also be used to test your input generators. It is relatively easy to write a generator that cannot generate a certain kind of input. Mutation testing can be used to find those gaps.


python3 -m pip install pytest-mutagen


Python import

import pytest_mutagen as mg

Declare a mutant

  • Mutant function
    To mutate a whole function you have to write the new version of the function, decorated with @mg.mutant_of(function_qual_name, mutant_name, file (optional), description (optional)). If the mutations affect an object (function or class) you have to be sure that this object exists in the __globals__ symbols table of either the test functions or the mutated functions. For this purpose you can simply write from [your_module] import [target_object] in the test file or in the mutation file. Example :

     def  inc(x):
     	return x + 1
     @mg.mutant_of("inc", "INC_OBO", description="Increment is off by one.")
     def  inc_mut(x):
     	return x + 2
  • Mutant expression
    If you don't want to change the whole function but only one line, you must decorate the function with @mg.has_mutant(mutant_name, file (optional), description (optional)). Then you have two ways to do it :

    • By replacing the expression by the mg.mut(mutant_name, normal_expression, mutant_expression) function, using lambda expressions.
      Example : mg.mut("FLIP_LT", lambda: a < b, lambda: b < a)

    • Using the mg.not_mutant(mutant_name) function combined with an if statement.
      Example : k = inc(k) if mg.not_mutant("INC_OBO2") else inc(k) + 1

Mutating a class method

In fact the @mutant_of decorator doesn't require the function name but its fully qualified name. It does not change anything for top-level functions but in the case of a class method you need to write the dotted path leading to the object from the module top-level. Example :

class Foo:
	def bar(self):

	def static_bar():

@mg.mutant_of("", "")
def bar_mut(self):

@mg.mutant_of("Foo.static_bar", "")
def static_bar_mut():

Global functioning

Mutagen collects all declared mutants, stored per file names. Then it looks through all tests collected by pytest and apply the mutants to the matching files. This is handled by the optional file parameter in @has_mutant and @mutant_of which can be a file name or a list of file names where you want your mutant to be applied. You can set it to APPLY_TO_ALL (constant string declared in mutagen) if you want it to be applied to all collected files. By default, file is :

  • APPLY_TO_ALL for @has_mutant
  • the current file name for @mutant_of (the one where it is written)

Therefore you can either :

  • write your mutations and specify for each one where you want it to be applied (use the function mg.link_to_file(filename) at the beginning of your file to link the current file to the specified filename)
  • or create a file where you import all test files you want (from import *), write your mutant_of with no file specified and run pytest on

Run the tests

python3 -m pytest --mutate

The --quick-mut option will stop each mutant after its first failed test. If not specified each mutant will run the whole test suite

Cache use

Mutagen stores in the pytest cache the functions that failed during the last run, for each mutant. For the next runs it will try these functions first, in order to find failures more quickly. If you don't need this feature you can simply use the --cache-clear option that will clear the cache before running the tests.

Run only the mutations

If you don't want to run the original test suite but only the mutations you can use the pytest option --collect-only


You can find some examples in the examples folder

  • The file is a very simple example of the use of mutagen to test a merge sort function
  • The file implements the Binary Search Tree data structure, and the test suite and mutations from How to specify it! (John Hughes, 2019)
  • The subfolder separate_files is an example of the separation between the source file, the test file and the mutation file

The scripts show how to run these tests

Project details

Download files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

Source Distribution

pytest-mutagen-1.0.5.tar.gz (7.1 kB view hashes)

Uploaded source

Built Distribution

pytest_mutagen-1.0.5-py3-none-any.whl (8.4 kB view hashes)

Uploaded py3

Supported by

AWS AWS Cloud computing Datadog Datadog Monitoring Facebook / Instagram Facebook / Instagram PSF Sponsor Fastly Fastly CDN Google Google Object Storage and Download Analytics Huawei Huawei PSF Sponsor Microsoft Microsoft PSF Sponsor NVIDIA NVIDIA PSF Sponsor Pingdom Pingdom Monitoring Salesforce Salesforce PSF Sponsor Sentry Sentry Error logging StatusPage StatusPage Status page