Mutable default argument in _SharedMemoryTracker.__init__ causes shared state between instances

Bug report

Bug description:

_SharedMemoryTracker.__init__ uses a mutable default argument for segment_names:

Line 1297 in 28a8c18

def __init__(self, name, segment_names=[]):

Because the default list is created only once, multiple _SharedMemoryTracker instances created without explicitly passing segment_names unexpectedly share the same list.

Minimal reproducer:

from multiprocessing.managers import _SharedMemoryTracker

t1 = _SharedMemoryTracker("ctx1")
t1.register_segment("seg1")

t2 = _SharedMemoryTracker("ctx2")

print(t2.segment_names)
print(t1.segment_names is t2.segment_names)

Output:

['seg1']
True

Expected behavior:

[]
False

Right now, this bug is hidden because _SharedMemoryTracker is usually created just once for each SharedMemoryServer process, so the shared state does not usually show up. But if you create more than one tracker in the same interpreter, they end up sharing bookkeeping state by accident because of the mutable default list.

To fix this, we can make the constructor safer by using None as the default value and creating a new list for each instance if one is not provided.

CPython versions tested on:

3.15

Operating systems tested on:

Windows

Linked PRs

But if you create more than one tracker in the same interpreter

When is it the case? the class is private so you're not meant to use it directly.

it's a code smell regardless of if it happens in practice via public API surfaces as noted in the PR description's "Right now, this bug is hidden because _SharedMemoryTracker is usually created just once for each SharedMemoryServer process, so the shared state does not usually show up."

I'm happy to merge the fix.

When is it the case? the class is private so you're not meant to use it directly.

AFAIK, there is no current execution path in CPython where more than one _SharedMemoryTracker instance is created in the same interpreter. I saw this as a latent bug because if multiple instances were created, they would end up sharing segment_names by accident due to the mutable default. This does not seem to cause problems right now, but I noticed the mutable default while reading the code and suggested changing it to None to make things more correct.

I'm ok with it being fixed, but I would prefer that we don't call it a "bug". It's something that can be improved, but it's not a bug per-se (at least not to me). If Greg wants to backport the change though, feel free to do it.

the way we structure our "New Issue" button forces people to pick a category up front with Bug being the first choice. i wouldn't worry about what we call this. :) its an internal cleanup in my book.

gh-153141: Fix mutable default argument in _SharedMemoryTracker.__ini…

[3.14] gh-153141: Fix mutable default argument in _SharedMemoryTracke…

[3.13] gh-153141: Fix mutable default argument in _SharedMemoryTracke…

[3.15] gh-153141: Fix mutable default argument in _SharedMemoryTracke…

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Mutable default argument in _SharedMemoryTracker.init causes shared state between instances #153141

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Mutable default argument in _SharedMemoryTracker.__init__ causes shared state between instances #153141

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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picnixz commented on Jul 5, 2026

gpshead commented on Jul 5, 2026

whyvineet commented on Jul 5, 2026

picnixz commented on Jul 5, 2026

gpshead commented on Jul 5, 2026

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Mutable default argument in _SharedMemoryTracker.init causes shared state between instances #153141