Memory leak when running `pytest` on macOS

[janbridley]

Description

Running pytest on macOS (both for the Pytest library itself as well as a few other libraries) leaks a small amount of memory.

Steps to reproduce

> git clone https://github.com/pytest-dev/pytest.git
> pip install .
> sudo leaks --atExit -- python -m pytest | grep bytes

Process 95884: 3 leaks for 4656 total leaked bytes.
    1 (560 bytes) ROOT LEAK: <malloc in _PyObject_Malloc 0x105d5ad10> [560]

When running pytest on other projects, I get a leak of a different size and a different source: but it only occurs when running pytest. Importing the package and calling functions from it does not leak any memory.

> sudo leaks --atExit -- python -m pytest <my_libraries> | grep bytes
   1 (16 bytes) ROOT LEAK: <non-object from libedit.3.dylib 0x127092d20> [16]

pip list

> pip list
Package            Version
------------------ ----------------------
argcomplete        3.6.2
attrs              25.3.0
certifi            2025.8.3
charset-normalizer 3.4.2
elementpath        5.0.3
hypothesis         6.137.1
idna               3.10
iniconfig          2.1.0
mock               5.2.0
packaging          25.0
pip                25.2
pluggy             1.6.0
Pygments           2.19.2
pytest             8.5.0.dev66+g5989efe3e
requests           2.32.4
setuptools         80.9.0
sortedcontainers   2.4.0
urllib3            2.5.0
xmlschema          4.1.0

I reproduced this behavior in a uv environment, and in a micromamba environment where I installed pip.

> pytest --version
pytest 8.4.1 # Built from source
> python --version
Python 3.15.5

[bluetech]

pytest is written in pure python, and I don't think it is possible for pure python code to leak PyObjects. So it's most likely that pytest is not the root cause of this.

[RonnyPfannschmidt]

actually this may be a special case as python uses libedit for python interactive shell editing on osx
its possible that the libedit/readline workarounds in pytest trigger the allocation of the object

we may need the help of someone on osx to investigate a resolution and/or mark a acceptable exception to the tools

[webknjaz]

Also, is this an official Python build or one of those uv-provided remakes?

[The-Compiler]

The workaround only consists of a import readline. Does sudo leaks --atExit -- python -c "import readline" | grep bytes show the same behavior?

[janbridley]

> sudo leaks --atExit -- python -c "import readline" | grep bytes
Process 4505: 1 leak for 16 total leaked bytes.
    1 (16 bytes) ROOT LEAK: <non-object from libedit.3.dylib 0x14df18cf0> [16]

So it looks like the 16 leaked bytes I see across a variety of packages does stem from readline. I must have been grepping for a single instance, as this is also visible in when pytesting pytest. We're still leaking an additional 560 bytes, but one mystery solved. I'll raise an issue on the uv Python source.

> sudo leaks --atExit -- python -m pytest | grep bytes  


Process 4686: 2 leaks for 576 total leaked bytes.
    1 (560 bytes) ROOT LEAK: <malloc in _PyObject_Malloc 0x1601f3560> [560]
    1 (16 bytes) ROOT LEAK: <non-object from libedit.3.dylib 0x16009edb0> [16]

As mentioned above, this is specific to UV's python distribution. I've raised an issue on their site and will follow up with the results. If I identify a resolution for those additional 560 bytes, I'll raise a PR on this repo -- thanks everyone.