`zipfile.Path` has only partial compatibility `io.BytesIO`

[yngvem]

Bug description:

I encountered an edge-case while using zipfile.Path with in-memory data buffers. It seems like the filename of the root is set to None, which breaks anything that tries to join the zip-filename and its internal path (including str(), ``.

I have two suggested fixes:

1. Update the initialization of the zipfile.FastLookup-class and set self.filename to ":memory:" if it is None, matching what you would write in e.g. sqlite to get an in-memory database.
2. Update all places where the code attempts do Path-operations on self.root.filename to notice if the filename is None and use ":memory:" instead.

The first is definitely the easiest fix, but the second would maybe be more backwards compatible in case someone uses Path.root.filename to notice that the zipfile.Path-object points to an in-memory zipfile. Though, I'm not sure how big of a problem that is.

I don't have any strong opinions of which to choose, and I would be happy to implement the fix.

Minimal reproducible example

import io
import zipfile

# Create a dummy Zip file
data = io.BytesIO()
with zipfile.ZipFile(data, mode="w") as zf:
    zf.writestr("hello.txt", "python\n")

# Try to iterate over its content
data.seek(0)
p = zipfile.Path(data)
print(str(p))
print(list(p.iterdir()))

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[92], line 13
     11 p = zipfile.Path(data)
---> 12 print(str(p))

File /usr/lib/python3.12/zipfile/_path/__init__.py:391, in Path.__str__(self)
    390 def __str__(self):
--> 391     return posixpath.join(self.root.filename, self.at)

File <frozen posixpath>:76, in join(a, *p)

TypeError: expected str, bytes or os.PathLike object, not NoneType

Demonstration of fix

import io
import zipfile

# Create a dummy Zip file
data = io.BytesIO()
with zipfile.ZipFile(data, mode="w") as zf:
    zf.writestr("hello.txt", "python\n")

# Try to iterate over its content
data.seek(0)
p = zipfile.Path(data)
p.root.filename = ":memory:"
print(str(p))
print(list(p.iterdir()))

:memory:/
[Path(':memory:', 'hello.txt')]

CPython versions tested on:

3.12, 3.14

Operating systems tested on:

Ubuntu 24.04 (WSL), Fedora (via the Python devcontainer on GitHub codespaces)

Linked PRs

• gh-130120: Prevent str(zipfile.Path) from raising exception on io.BytesIO-input #130381

[yngvem]

I just realised that option 2. might be preferable, because we should probably overload the parent method to prevent it from resolving to "." when the path is ":memory:" anyways?

[yngvem]

I've started working on this now. I had one idea though: How about setting the name to \0 if it represents an in-memory byte string? That way it's clear that it isn't a local path with the name ":memory:".

Update: Looking at the tests, it seems like it's supposed to fail. So I'll only update the code so __str__ doesn't fail