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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.


  • Clone this repository
  • Find the folder on your system that contains python libraries :
     python3 -c "import sys; print(sys.path)"
  • Create in this folder a symbolic link to the pytest-mutagen repository previously cloned :
     ln -s /path/to/pytest-mutagen pytest_mutagen
  • Install it with pip
     python3 -m pip install -e /path/to/symbolic/link/pytest_mutagen


Python import

import pytest_mutagen as mg

Declare a mutant

  • Mutant function
    If you want to run the tests from with some mutations, you can either write the mutations in or in a new file by using from testfile import *. 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. 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, description (optional)). 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, filename (optional), description (optional)) where filename is the name of the mutation file. If you don't specify a filename it will be set to the file where has_mutant is written. 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 If you want to mutate several expressions in the same function you have to use one decorator per mutation.

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():

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


  • 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)

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