Reject main when copying an existing zipapp archive #159354

zipapp.create_archive() documents that main can only be specified for a directory source without __main__.py. Existing archive sources enter the copy path before this argument is checked, silently ignoring it and modifying the target. The same problem occurs when main is explicitly set to an empty string.

Reproducer, run with the interpreter built from the CPython checkout:

import io
import zipapp
import zipfile

source = io.BytesIO()
with zipfile.ZipFile(source, 'w') as archive:
    archive.writestr('__main__.py', '')
for main in ('pkg.mod:fn', ''):
    source.seek(0)
    target = io.BytesIO(b'original content')
    try:
        zipapp.create_archive(source, target, main=main)
    except zipapp.ZipAppError as exc:
        print(main, exc)
    else:
        print(main, 'accepted')

Expected: Raise ZipAppError for either explicitly supplied value before reading/copying the archive or changing the target. Omitting main or passing None should continue to allow copying.

Observed: Both calls succeed and copy the existing archive, ignoring main.

Environment: CPython main, Windows debug build.

Linked PRs

• gh-159354: Validate explicit main arguments in zipapp #159355