mutmut

• Fix the trampoline dropping a generator's return value, so yield from on a mutated generator no longer yields None instead of its result

• Fix async generators, so that aclose()/athrow() are handled by the except/finally blocks of the mutated function instead of being swallowed by the trampoline

• Fix max_stack_depth when a test changes the working directory (for example via a tmpdir fixture)

• Fix type_check_command reporting type errors in copied files that have no mutants

• Per-function source hashing for incremental cache invalidation — only re-test mutants in functions that changed

• Cross-call dependency tracking — invalidate mutants in callers when a called function changes

• Use git to detect non-Python dependency file changes; falls back to a curated file list when git is unavailable

• Add cache_invalidation_exclude config to suppress noisy files from change detection

• Add use_git_change_detection config (default true) to opt out of git-based detection

• Add uv.lock, poetry.lock, Pipfile.lock, and pdm.lock to the list of files copied by default

• Invalidate cached results automatically when result-affecting config fields change

• Make mutmut browse, show and apply much faster and use far less memory, by storing where each mutant is in the mutated file instead of parsing the whole file to find it

• Add # pragma: no mutate block and # pragma: no mutate start/end comments

• Add do_not_mutate_patterns to disable mutations with a regex

• Add only_mutate config to select which files get mutated

• Also mutate methods decorated with (only) @staticmethod or @classmethod

• Add use_setproctitle option to disable process renaming (automatically disabled on MacOS)

• Add (unstable) timeout_multiplier and timeout_constant options

• Rename paths_to_mutate to source_paths

• Deprecate tests_dir in favor of pytest_add_cli_args_test_selection

• Change max_stack_depth to only consider functions inside source_paths

• Disable mutation of enums and @staticmethod/@classmethod methods when type_check_command is set, as these mutations break type checking

• Fix mutation for enum class methods

• Fix mutate_only_covered_lines when project uses custom coverage.py config

• Fix execution when running mutmut via python -m mutmut run

• Fix mutation of class _SomePrivateClass class methods

• Fix mutation of default args

• Fix mutmut browse crash when no file is selected yet

• Fix timeout checker looking up the wrong mutant's expected test time, which could cause mutants to hang

• Compare stats paths after resolving symlinks

• Warn when mutmut cannot match mutants with the collected stats

• Filter mutants based on type annotations. This feature needs to be turned on with the type_check_command config.

• Show error message when running on Windows

• Save mutation stats for use with CI/CD pipeline (#460)

• Async functions fixed

• Fix for functions with positional only arguments

• Support signatures with PEP 649 deferred annotations

• Add action to view tests for mutant

• Add basic description for all results in mutmut browse

• Add description for timeout mutants

• Early exit when stats find no tests for any mutant

• Support python 3.14

• Performance improvements

• Fix mutate_only_covered_lines when files are excluded from test run

• Add pytest_add_cli_args and pytest_add_cli_args_test_selection configs

• mutate_only_covered_lines added to config to control whether coverage.py is used to filter down mutations

• Filter out identical string mutants with different values

• Handle more exit codes

• Disable common order-randomising pytest plugins, as that can seriously deteriorate mutation testing performance

• Fixed packaging issue

• Increased threshold for mutant timeouts

• Added tests_dir config. Accepts a single entry or a list of directories.

• Async generators fixes

• Fixed bad mutations for certain string escape sequences

• Performance fixes

• Various internal bug fixes

• Python 3.13 compatibility!

• New argument --show-killed for mutmut browse

• Fix to avoid accidentally importing the un-mutated original code

• Handle segfault for mutant subprocesses

• Added mutations for string literals

• Added mutations for common string methods

• Faster mutant generation via subprocesses

• Fix self parameter for mutated class methods

• Fix trampoline generation for function calls with 'orig' or 'mutants' as argument names.

• Copy full source directory before creating mutants

• Improved error message when forced fail test fails

• Fixed issue with spaces in the python executable path

• Do not mutate __new__

• Annotate mutant dicts (and fixes compatibility with Pydantic)

• Replaced parso with LibCST

• Crash with error message on invalid imports for src module

• Autodetect simpler project configurations with test_*.py in the dir directly

• Handle filenames (as opposed to dirnames) in paths_to_mutate

• Also copy setup.cfg and pyproject.toml by default

• Handle single line paths_to_mutate

• Fixed crash when running mutmut results

• Read paths_to_mutate from config file

• Mutate break to return to avoid timeouts

• Added debug mode. Enable with debug=True in setup.cfg under [mutmut]

• Fixed new test detection. The old code incorrectly detected new tests when there were none, creating a much slower interaction loop for fixing mutants.

• And many more fixes

• Timeouts for mutants implemented.

• Browser: syntax highlighting for diff view

• More fixes for generators.

• Fix for src-style layout of projects.

• Fixed bug where mutmut would recollect all tests on every run, slowing down startup.

• Correctly handle mutation for generator functions (yield).

• Fixed so that from __future__ lines are always first.

• If no stats are collected exit directly, as that is a breaking error for mutation testing.

• Changed name mangling to make mutants less likely to trigger name-based python magic, like in pytest where functions named test_* have special meaning.

• Another attempt to get the pypi package to work

• Another attempt to get the pypi package to work

• Fixed missing requirement in install package

• Fixed missing file from the install package

• Fixed bad entrypoint definition

• Ignore files that can't be parsed by parso

• Missed a file in distribution, so browse command was broken.

• Execution model switched to mutation schemata, which enabled parallel execution.

• New terminal UI

• Pytest only, which enabled better integration, cutting execution time significantly.

• New execution model. This should result in some modest speed improvements when using pytest.

• A special execution mode when using the hammett test runner. This is MUCH MUCH faster. Please try it!

• Dropped support for python < 3.7. If you need to use mutmut on older versions of python, please use mutmut 1.9.0

• Some other speed improvements.

• mutmut run 7 will always rerun the mutant 7

• mutmut show <filename> to show all mutants for that file

• mutmut run <filename> to run mutation testing on that file

• New experimental plugin system: create a file mutmut_config.py in your base directory. In it you can have an init() function, and a pre_mutation(context) function. You can set context.skip = True to skip a mutant, and you can modify context.config.runner, this is useful to limit the tests. Check out the Context class for what information you get.

• Better display of mutmut show/mutmut result

• Fixed a spurious mutant on assigning a local variable with type annotations

• mutmut now will rerun tests without mutation when tests have changed. This avoids a common pitfall of introducing a failing test and then having all mutants killed incorrectly

• Added mutmut html report generation.

• Bugfix for multiple assignment. Mutmut used to not handle foo = bar = baz correctly (Thanks Roxane Bellot!)

• Bugfix for incorrect mutation of "in" operator (Thanks Roxane Bellot!)

• Fixed bug where a mutant survived in the internal AST too long. This could cause mutmut to apply more than one mutant at a time.

• Vastly improved startup performance when resuming a mutation run.

• Added new experimental feature for advanced config at runtime of mutations

• Add mutmut show [path to file] command that shows all mutants for a given file

• Better error messages if .coverage file isn't usable

• Added support for windows paths in tests

• Use the same python executable as mutmut is started as if possible

• Dropped python 2 support

• Added more assignment operator mutations

• Bugfixes

• New mutation: None -> ''

• Display all diffs for surviving mutants for a specific file with mutmut show all path/to/file.py

• Display all diffs for surviving mutants with mutmut show all

• Fixed a bug with grouping of the results for mutmut results

• Fixed bug where mutmut show X sometimes showed no diff

• Fixed bug where mutmut apply X sometimes didn't apply a mutation

• Improved error message when trying to find the code

• Fixed incorrect help message

• New setting: --test-time-base=15.0. This flag can be used to avoid issues with timing.

• Post and pre hooks for the mutation step: --pre-mutation=command and --post-mutation=command if you want to run some command before and after a mutation testing round.

• Fixed a bug with mutation of imports.

• Fixed missing newline at end of the output of mutmut.

• Support for mutating only lines specified by a patch file: --use-patch-file=foo.patch.

• Fixed mutation of arguments in function call.

• Looser heuristics for finding the source to mutate. This should mean more projects will just work out of the box.

• Fixed mutation of arguments in function call for python 2.7.

• Fixed a bug where if mutmut couldn't find the test code it thought the tests hadn't changed. Now mutmut treats this situation as the tests always being changed.

• Fixed bug where the function body was skipped for mutation if a return type annotation existed.

• Fixed a bug where mutmut crashed if a file contained exactly zero bytes.

• Fixed incorrect loading of coverage data when using the --use-coverage flag.

• Fixed a bug when updating the cache.

• Fixed incorrect handling of source files that didn't end with a newline.

• JUnit XML output: Run mutmut junitxml to output the results as a JUnit compatible XML file.

• Python 2 compatibility fixes.

• pypy compatibility fixes.

• Fixed an issue where mutmut couldn't kill the spawned test process.

• Travis tests now test much more thoroughly, both python2, 3, pypy and on windows.

• The return code of mutmut now reflects what mutmut found during execution.

• New command line option --test-time-multiplier to tweak the detection threshold for mutations that make the code slower.

• Fixed compatibility with Windows.

Thanks goes out Marcelo Da Cruz Pinto, Savo Kovačević,

• New mutant: mutate the first argument of function calls to None if it's not already None

• Totally overhauled cache system: now handles duplicates lines correctly.

• Minor UX fixes: --version command was broken, incorrect documentation shown, missing newline at the very end.

• Caching the baseline test time. This makes restarting/rechecking existing mutants much faster, with a small risk of that time being out of date.

• Totally new user interface! Should be much easier to understand and it's easier to see that something is happening

• Totally new cache handling. Mutmut will now know which mutants are already killed and not try them again, and it will know which mutants to retest if the tests change

• Infinite loop detection now works in Python < 3.3

• Added --version flag

• Nice error message when no .coverage file is found when using the --use-coverage flag

• Fixed crash when using --use-coverage flag. Thanks Daniel Hahler!

• Added mutation based on finding on tri.struct

• Make number_mutation more robust to floats (Thanks Trevin Gandhi!)

• Fixed crash when using Python 3 typing to declare a type but not assigning to that variable

• Handle annotated assignment in Python 3.6. Thanks William Orr!

• Fixed critical bug: mutmut reported killed mutants as surviving and vice versa.

• Fixed an issue where the install failed on some systems.

• Handle tests dirs spread out in the file system. This is the normal case for django projects for example.

• Fixes for supporting both python 3 and 2.

• Misc mutation fixes.

• Ability to test a single mutation.

• Feature to print the cache (--print-cache).

• Turned off error recovery mode for parso. You will now get exceptions for invalid or unsupported python code.

• Changed AST library from baron to parso

• Some usability enhancements suggested by David M. Howcraft

• Caching of mutation testing results. This is still rather primitive but can in some cases cut down on rerunning mutmut drastically.

• New mutation IDs. They are now indexed per line instead of an index for the entire file. This means you can apply your mutations in any order you see fit and the rest of the apply commands will be unaffected.

• Fixed bug where initial mutation count was wrong, which caused mutmut to miss mutants at the end of the file

• Changed mutation API to always require a Context object. This makes is much easier to pass additional data out to the caller

• Support specifying individual files to mutate (thanks Felipe Pontes!)

• Improve error message when baron crashes a bit (fixes #10)

• New mutation: right hand side of assignments

• Fixed nasty bug where applying a mutation could apply a different mutation than the one that was found during mutation testing

• Don't assume UNIX (fixes github issue #9: didn't work on windows)

• Changed default runner to add -x flag to pytest. Could radically speed up tests if you're lucky!

• New flag: --show-times

• Now warns if a mutation triggers very long test times

• Added a workaround for pytest-testmon (all tests deselected is return code 5 even though it's a success)

• Fixed bug that made mutmut crash when setup.cfg was missing

• Renamed parameter --testsdir to --tests-dir

• Refactored handling of setup.cfg file. Much cleaner solution and adds --dict-synonyms command line parameter

• Bug with dict param mutations: it mutated all parameters, this could vastly decrease the odds of finding a mutant

• New mutation: remove the body or return 0 instead of None

• Previous version had broken version on pypi

• Fixed bug where pragma didn't work for decorator mutations

• Dict literals looking like dict(a=foo) now have mutated keys. You can also declare synonyms in setup.cfg.

• Fix from x import *

• New mutation: remove decorators!

• Improved status while running. This should make it easier to handle when you hit mutants that cause infinite loops.

• Fixes failing attempts to mutate parentheses. (Thanks Hristo Georgiev!)