Currently the zipfile library allows you to create invalid zip files. The following code is an example:

from zipfile import ZipFile
import tempfile
temporary_file = tempfile.NamedTemporaryFile()
my_zip = ZipFile(temporary_file.name, 'w')
my_zip.writestr('/some_folder/some_file.txt', 'Some content')
my_zip.close()

The generated zipfile contains "/some_folder/some_file.txt". However, according to the specification for zip files, this is invalid. See below, from the .ZIP File Format Specification version 6.3.9:

       4.4.17.1 The name of the file, with optional relative path.
       The path stored MUST NOT contain a drive or
       device letter, or a leading slash.  All slashes
       MUST be forward slashes '/' as opposed to
       backwards slashes '\' for compatibility with Amiga
       and UNIX file systems etc.  If input came from standard
       input, there is no file name field.  

This is significant because the default Windows Explorer zip file extractor cannot handle zip files that contain a leading slash, producing an error that "The compressed (zipped) folder is invalid."

I propose fixing this in documentation.

Raising an error is probably not warranted because zip files are often created on one system and used on another, so you can't raise an error based on current OS, and having a leading slash in the name is both useful and does work in MacOS and Unix. It probably works in some unzip programs on Windows as well (if anyone can test that would be great).

Therefore I propose adding the following notes to ZipInfo, ZipFile.write() and ZipFile.writestr():

Note: a leading slash in the archive / filename may lead to the archive being un-openable in some zip programs on Windows systems.

If that sounds good I can make a PR.

That addition to the documentation sounds appropriate to me. Thanks!

New changeset 7c5dab4340032eb15d3797d8b601ef11649bbab3 by andrei kulakov in branch 'main':
[doc] bpo-43066: zipfile - add note on leading slash in the filename arg (GH-26899)
https://github.com/python/cpython/commit/7c5dab4340032eb15d3797d8b601ef11649bbab3

New changeset 98f6a72ff621c2033e8f2c38df410e9bfa2b772a by Miss Islington (bot) in branch '3.10':
[doc] bpo-43066: zipfile - add note on leading slash in the filename arg (GH-26899)
https://github.com/python/cpython/commit/98f6a72ff621c2033e8f2c38df410e9bfa2b772a

New changeset 1a2c0ecfa2ef8542baee951f12b60afd165be9cf by Miss Islington (bot) in branch '3.9':
[doc] bpo-43066: zipfile - add note on leading slash in the filename arg (GH-26899) (GH-27606)
https://github.com/python/cpython/commit/1a2c0ecfa2ef8542baee951f12b60afd165be9cf

Thanks for the report, Garrison. And Andrei for the patch! ✨ 🍰 ✨