Handle duplicate X-WR-TIMEZONE values

CONTRIBUTING.md

@@ -28,6 +28,9 @@ Contributors
 - can submit pull requests
 - can work on branches inside the repository
 
+Contributor changes should include focused tests where practical and should
+update the changelog in `README.rst` unless the change is trivial.
+
 ## Merge requirements
 
 - Passing tests

README.rst

@@ -243,6 +243,9 @@ Changelog
 
 - v3.0.0
 
+  - Handle calendars that provide multiple ``X-WR-TIMEZONE`` values.
+    If all values match, use the repeated timezone; if values conflict,
+    ignore them because the target timezone is ambiguous.
   - Use ``icalendar.is_utc()`` instead of ``is_UTC()`` to identify UTC, catching aliases like ``Etc/GMT``.
   - Remove ``is_UTC()`` from ``CalendarWalker``.
   - Test ``pytz.timezone("UTC")`` and ``datetime.timezone.utc`` as arguments to ``to_standard()``.

test/calendars/duplicate-conflicting-x-wr-timezone-values.in.ics

@@ -0,0 +1,11 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//x-wr-timezone test//
+X-WR-TIMEZONE:Europe/Berlin
+X-WR-TIMEZONE:America/New_York
+BEGIN:VEVENT
+UID:1
+DTSTART:20240101T120000Z
+DTEND:20240101T130000Z
+END:VEVENT
+END:VCALENDAR

test/calendars/duplicate-matching-x-wr-timezone-values.in.ics

@@ -0,0 +1,11 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//x-wr-timezone test//
+X-WR-TIMEZONE:Europe/Berlin
+X-WR-TIMEZONE:Europe/Berlin
+BEGIN:VEVENT
+UID:1
+DTSTART:20240101T120000Z
+DTEND:20240101T130000Z
+END:VEVENT
+END:VCALENDAR

test/test_convert_examples.py

@@ -122,3 +122,25 @@ def test_calendar_walker_is_utc_compatibility_hook():
 
     assert walker.is_UTC(utc_dt)
     assert not walker.is_UTC(floating_dt)
+
+
+def test_duplicate_matching_x_wr_timezone_values_are_used(calendars):
+    calendar = calendars["duplicate-matching-x-wr-timezone-values.in.ics"].as_icalendar()
+    output_bytes = x_wr_timezone.to_standard(calendar).to_ical()
+
+    assert_has_line(
+        output_bytes,
+        ("DTSTART", "TZID=Europe/Berlin", "20240101T130000"),
+        "Duplicate matching time zones are used.",
+    )
+
+
+def test_duplicate_conflicting_x_wr_timezone_values_are_ignored(calendars):
+    calendar = calendars["duplicate-conflicting-x-wr-timezone-values.in.ics"].as_icalendar()
+    output_bytes = x_wr_timezone.to_standard(calendar).to_ical()
+
+    assert_has_line(
+        output_bytes,
+        ("DTSTART", "20240101T120000Z"),
+        "Conflicting time zones are ignored.",
+    )

x_wr_timezone.py

@@ -24,6 +24,16 @@
 X_WR_TIMEZONE = "X-WR-TIMEZONE"
 
 
+def _as_single_timezone(timezone):
+    """Return one time zone value from calendar metadata, or None if ambiguous."""
+    if not isinstance(timezone, list):
+        return timezone
+    values = set(map(str, timezone))
+    if len(values) == 1:
+        return timezone[0]
+    return None
+
+
 def _is_utc_fallback(dt):
     """Fallback UTC check for icalendar < 7.0.0."""
     if dt.tzname() is None:
@@ -193,6 +203,7 @@ def to_standard(
     """
     if timezone is None:
         timezone = calendar.get(X_WR_TIMEZONE, None)
+        timezone = _as_single_timezone(timezone)
     if timezone is not None and not isinstance(timezone, datetime.tzinfo):
         timezone = zoneinfo.ZoneInfo(str(timezone))
     result : icalendar.Calendar = calendar