Issue #29094: Offsets in a ZIP file created with extern file object and modes

"w" now are relative to the start of the file.

Lib/zipfile.py

@@ -772,7 +772,7 @@ class ZipFile(object):
                 # set the modified flag so central directory gets written
                 # even if no files are added to the archive
                 self._didModify = True
-                self._start_disk = self.fp.tell()
+                self._start_disk = 0
             elif key == 'a':
                 try:
                     # See if file is a zip file

Misc/NEWS

@@ -15,6 +15,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #29094: Offsets in a ZIP file created with extern file object and mode
+  "w" now are relative to the start of the file.
+
 - Issue #13051: Fixed recursion errors in large or resized
   curses.textpad.Textbox.  Based on patch by Tycho Andersen.