gh-152212: Reject a POSIX TZ footer with a missing std offset in pure-Python zoneinfo

The pure-Python _parse_tz_str defaulted a missing std offset to 0, so a POSIX TZ
footer with a bare std abbreviation and no offset (e.g. 'AAA') was silently accepted
as a fixed offset-0 zone.  POSIX requires the offset after std, and the C accelerator
already rejects it, so the pure-Python parser now raises ValueError to match.

Author: tonghuaroot

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -1142,6 +1142,11 @@ def test_extreme_tzstr(self):
     def test_invalid_tzstr(self):
         invalid_tzstrs = [
             "PST8PDT",  # DST but no transition specified
+            # gh-152212: the std offset is required (POSIX TZ grammar)
+            "AAA",
+            "A",
+            "AA",
+            "B",
             "+11",  # Unquoted alphanumeric
             "GMT,M3.2.0/2,M11.1.0/3",  # Transition rule but no DST
             "GMT0+11,M3.2.0/2,M11.1.0/3",  # Unquoted alphanumeric in DST

Lib/zoneinfo/_zoneinfo.py

@@ -672,7 +672,7 @@ def _parse_tz_str(tz_str):
         except ValueError as e:
             raise ValueError(f"Invalid STD offset in {tz_str}") from e
     else:
-        std_offset = 0
+        raise ValueError(f"Invalid STD offset in {tz_str}")
 
     if dst_abbr is not None:
         if dst_offset := m.group("dstoff"):

Misc/NEWS.d/next/Library/2026-06-25-12-00-00.gh-issue-152212.Zk7Qm2.rst

@@ -0,0 +1,6 @@
+Fixed a parity bug in the pure-Python :mod:`zoneinfo` implementation, which
+accepted a POSIX TZ footer whose ``std`` field had no offset (for example
+``AAA``), silently building a fixed offset-0 zone. Such a string is invalid per
+POSIX (the offset following ``std`` is required), and the C accelerator already
+rejects it; the pure-Python parser now raises :exc:`ValueError` to match.
+Patch by tonghuaroot.