Thank you David for your benchmarks and patch. There are several backward compatibility issues with the reading from ZipFile opened for write and from closed ZipFile. This behavior is mostly undocumented (except the reading from closed ZipFile), but even our tests depend on it and changing it could break user code with good chance.

Here is a patch which preserves current behavior. Added new tests to check this behavior explicitly. Other advantage of the patch is that it doesn't change the signature of ZipExtFile constructor at all.

Benchmarks don't show stable significant difference between patched and unpatched versions.