gh-126565: Skip zipfile.Path.exists check in write mode

janhicken · janhicken · commit 84cb5de4e50a · 2024-11-08T11:07:05.000+01:00
When `zipfile.Path.open` is called, the implementation will check
whether the path already exists in the ZIP file. However, this check is
only required when the ZIP file is in read mode. By swapping arguments
of the `and` operator, the short-circuiting will prevent the check from
being run in write mode.

This change will improve the performance of `open()`, because checking
whether a file exists is slow in write mode, especially when the archive
has many members.
diff --git a/Lib/zipfile/_path/__init__.py b/Lib/zipfile/_path/__init__.py
@@ -339,7 +339,7 @@ def open(self, mode='r', *args, pwd=None, **kwargs):
         if self.is_dir():
             raise IsADirectoryError(self)
         zip_mode = mode[0]
-        if not self.exists() and zip_mode == 'r':
+        if zip_mode == 'r' and not self.exists():
             raise FileNotFoundError(self)
         stream = self.root.open(self.at, zip_mode, pwd=pwd)
         if 'b' in mode:
diff --git a/Misc/NEWS.d/next/Library/2024-11-08-11-06-14.gh-issue-126565.dFFO22.rst b/Misc/NEWS.d/next/Library/2024-11-08-11-06-14.gh-issue-126565.dFFO22.rst
@@ -0,0 +1,6 @@
+Calling :meth:`zipfile.Path.open` no longer checks for an existing file in
+write mode. Checking for file existence in ZIP files opened in write mode is
+particularly slow, because the list of members cannot be cached up-front. As
+the check is irrelevant for the write mode any, it will no longer be performed
+resulting in a performance improvement especially with ZIP files that have many
+members.
