Handle transition.previous being None

Author: bryanforbes

pendulum/tz/timezone.py

@@ -105,7 +105,7 @@ def _normalize(
                 # We set the fold attribute for later
                 if dst_rule == POST_TRANSITION:
                     fold = 1
-            else:
+            elif transition.previous is not None:
                 transition = transition.previous
 
         if transition.is_ambiguous(sec):
@@ -149,7 +149,7 @@ def _convert(self, dt):  # type: (datetime) -> datetime
             transition = dt.tzinfo._lookup_transition(stamp)
             offset = transition.ttype.offset
 
-            if stamp < transition.local:
+            if stamp < transition.local and transition.previous is not None:
                 if (
                     transition.previous.is_ambiguous(stamp)
                     and getattr(dt, "fold", 1) == 0
@@ -161,7 +161,7 @@ def _convert(self, dt):  # type: (datetime) -> datetime
         stamp -= offset
 
         transition = self._lookup_transition(stamp, is_utc=True)
-        if stamp < transition.at:
+        if stamp < transition.at and transition.previous is not None:
             transition = transition.previous
 
         offset = transition.ttype.offset
@@ -254,7 +254,7 @@ def _get_transition(self, dt):  # type: (datetime) -> Transition
 
             transition = self._lookup_transition(stamp)
 
-            if stamp < transition.local:
+            if stamp < transition.local and transition.previous is not None:
                 fold = getattr(dt, "fold", 1)
                 if transition.is_ambiguous(stamp):
                     if fold == 0:
@@ -270,7 +270,7 @@ def fromutc(self, dt):  # type: (datetime) -> datetime
         stamp = timestamp(dt)
 
         transition = self._lookup_transition(stamp, is_utc=True)
-        if stamp < transition.at:
+        if stamp < transition.at and transition.previous is not None:
             transition = transition.previous
 
         stamp += transition.ttype.offset

pendulum/tz/zoneinfo/transition.py

@@ -1,5 +1,5 @@
 from datetime import timedelta
-from typing import Union
+from typing import Union, Optional
 
 from .transition_type import TransitionType
 
@@ -51,7 +51,7 @@ def ttype(self):  # type: () -> TransitionType
         return self._ttype
 
     @property
-    def previous(self):  # type: () -> Transition
+    def previous(self):  # type: () -> Optional[Transition]
         return self._previous
 
     @property
@@ -67,7 +67,10 @@ def is_missing(self, stamp):  # type: (int) -> bool
     def utcoffset(self):  # type: () -> timedelta
         return self._utcoffset
 
-    def __contains__(self, stamp):  # type: () -> bool
+    def __contains__(self, stamp):  # type: (int) -> bool
+        if self.previous is None:
+            return stamp < self.local
+
         return self.previous.local <= stamp < self.local
 
     def __repr__(self):  # type: () -> str

tests/tz/test_timezone.py

@@ -303,6 +303,12 @@ def test_utcoffset():
     assert utcoffset == timedelta(0, -18000)
 
 
+def test_utcoffset_pre_transition():
+    tz = pendulum.timezone("America/Chicago")
+    utcoffset = tz.utcoffset(datetime(1883, 11, 18))
+    assert utcoffset == timedelta(days=-1, seconds=64800)
+
+
 def test_dst():
     tz = pendulum.timezone("Europe/Amsterdam")
     dst = tz.dst(datetime(1940, 7, 1))