Bug report

Bug description:

I'm using thezipfile module to create a ZIP file with thousands of members. Each file is created by creating a correspondingzipfile.Path object first and then calling.open() on it.

The implementation ofopen() contains a check whether the file already exists when opening a file in read mode:

ifnotself.exists()andzip_mode=='r':raiseFileNotFoundError(self)

However,self.exists() is called even in write mode, because it is theand operator's first argument.
The call toself.exists() is quite slow however, because this requires the ZIP file's.namelist() to be computed, which in turn requires to compute all the implied directories. After all, this is the reason why theFastLookup optimization exists for ZIP files in read mode.

I found this issue after profiling my application and was surprised that more than half of its execution time was spent in computing implied ZIP directories.

I would propose to swap theand arguments so the check looks like this:

ifzip_mode=="r"andnotself.exists():raiseFileNotFoundError(self)

This will cause theself.exists() check to only be run in read mode, where the check is fast because ofFastLookup anyway.

I'm happy to provide a pull request with this change.

CPython versions tested on:

3.11, 3.12

Operating systems tested on:

Linux, macOS

Linked PRs

• gh-126565: Skipzipfile.Path.exists check in write mode #126576
• [3.13] gh-126565: Skipzipfile.Path.exists check in write mode (GH-126576) #126642
• [3.12] gh-126565: Skipzipfile.Path.exists check in write mode (GH-126576) #126643