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
import pytest_mutagen as mg
Declare a mutant
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 the mutant functions. For this purpose you can simply write
from [your_module] import [target_object]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
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.
mg.mut("FLIP_LT", lambda: a < b, lambda: b < a)
mg.not_mutant(mutant_name)function combined with an
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.
class Foo: def bar(self): pass @staticmethod def static_bar(): pass @mg.mutant_of("Foo.bar", "") def bar_mut(self): pass @mg.mutant_of("Foo.static_bar", "") def static_bar_mut(): pass
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
@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
- 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 mutations.py file where you import all test files you want (
from testfile.py import *), write your
mutant_ofwith no file specified and run pytest on mutation.py.
Run the tests
python3 -m pytest --mutate
--quick-mut option will stop each mutant after its first failed test. If not specified each mutant will run the whole test suite
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
Selective run of mutants
--select option expects a comma-separated list of mutants (no spaces) and will run these ones exclusively.
python3 -m pytest --mutate --select INC_OBO,FLIP_LT
--mutagen-stats option adds a section to the terminal summary, which displays the number of tests that caught each mutant.
Add trivial mutations
To find holes in a test suite with mutagen, we often try trivial mutations on some functions (like
replacing them with pass) to see whether a lot of tests catch them or not.
For this purpose the
trivial_mutations(functions, obj=None, file=APPLY_TO_ALL) function with a
list of functions as input adds all mutants corresponding to replacing them by an empty function.
There are two ways to use it:
from module import sort, invert, ExampleClass # With a list of top-level functions mg.trivial_mutations([sort, invert]) # With a list of method names and the corresponding object mg.trivial_mutations(["sort", "clear"], ExampleClass)
This is equivalent to doing this:
from module import sort, invert, ExampleClass mg.link_to_file(mg.APPLY_TO_ALL) @mg.mutant_of("sort", "SORT_NOTHING") def sort_mut(*args, **kwargs): pass @mg.mutant_of("invert", "INVERT_NOTHING") def invert_mut(*args, **kwargs): pass @mg.mutant_of("ExampleClass.sort", "EXAMPLECLASS.SORT_NOTHING") def sort_mut(*args, **kwargs): pass @mg.mutant_of("ExampleClass.clear", "EXAMPLECLASS.CLEAR_NOTHING") def clear_mut(*args, **kwargs): pass
trivial_mutations has an optional file parameter to specify the test file where the mutations
should be applied, which is by default set to APPLY_TO_ALL.
trivial_mutations_all(object, file=APPLY_TO_ALL) applies this process to each
method of the class (or list of classes) given as a parameter.
from module import ExampleClass mg.trivial_mutations_all(ExampleClass)
You can find some examples in the examples folder
- The file short_example.py is a very simple example of the use of mutagen to test a merge sort function
- The file BST_mutations.py 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 run-tests.py scripts show how to run these tests
Writing mutations by hand can be very long and we are aware that it can discourage a lot of programmers from using pytest-mutagen, that is initially a manual mutation-testing tool. To fix this problem while keeping the ability to manually edit the mutants we've added the possibility to interactively generate a bunch of mutants following a set of rules.
python3 -m pytest_mutagen [-h] [-o OUTPUT_PATH] [-m MODULE_PATH] input_path
This command will browse the provided
input_path (that can be a file or a directory) and
interactively propose several mutants. You can accept them by pressing ENTER and refuse them by
typing 'n' then ENTER. The purpose of this is to avoid false positives and equivalent mutants, that
are among the main problems of mutation testing. Finally all accepted mutants are written in the
mutagen syntax (ready to be use with
pytest --mutate) in mutation.py or the file/directory
specified with the
-o command-line option.
For more details on its use you can use
python3 -m pytest_mutagen --help.
- Integers are incremented
- Operators are switched to a different (but close) operator
- In assignments, the right value is replaced with None
- The return statement is removed
- The condition of if statements are replaced with not (condition)