[3.2.x] Refs #32365 -- Allowed use of non-pytz timezone implementations.

Backport of 10d126198434810529e0220b0c6896ed64ca0e88 from master

3 files changed, 43 insertions, 28 deletions

diff --git a/django/forms/utils.py b/django/forms/utils.py
index fbe79f1142..50412f414b 100644
--- a/django/forms/utils.py
+++ b/django/forms/utils.py
@@ -161,6 +161,11 @@ def from_current_timezone(value):
     if settings.USE_TZ and value is not None and timezone.is_naive(value):
         current_timezone = timezone.get_current_timezone()
         try:
+            if (
+                not timezone._is_pytz_zone(current_timezone) and
+                timezone._datetime_ambiguous_or_imaginary(value, current_timezone)
+            ):
+                raise ValueError('Ambiguous or non-existent time.')
             return timezone.make_aware(value, current_timezone)
         except Exception as exc:
             raise ValidationError(
diff --git a/django/utils/dateformat.py b/django/utils/dateformat.py
index 9bd05a437b..38e89c47bb 100644
--- a/django/utils/dateformat.py
+++ b/django/utils/dateformat.py
@@ -20,7 +20,8 @@ from django.utils.dates import (
 )
 from django.utils.regex_helper import _lazy_re_compile
 from django.utils.timezone import (
-    get_default_timezone, is_aware, is_naive, make_aware,
+    _datetime_ambiguous_or_imaginary, get_default_timezone, is_aware, is_naive,
+    make_aware,
 )
 from django.utils.translation import gettext as _
 
@@ -160,15 +161,9 @@ class TimeFormat(Formatter):
         if not self.timezone:
             return ""
 
-        name = None
-        try:
+        if not _datetime_ambiguous_or_imaginary(self.data, self.timezone):
             name = self.timezone.tzname(self.data)
-        except Exception:
-            # pytz raises AmbiguousTimeError during the autumn DST change.
-            # This happens mainly when __init__ receives a naive datetime
-            # and sets self.timezone = get_default_timezone().
-            pass
-        if name is None:
+        else:
             name = self.format('O')
         return str(name)
 
@@ -184,16 +179,13 @@ class TimeFormat(Formatter):
 
         If timezone information is not available, return an empty string.
         """
-        if not self.timezone:
+        if (
+            not self.timezone or
+            _datetime_ambiguous_or_imaginary(self.data, self.timezone)
+        ):
             return ""
 
-        try:
-            offset = self.timezone.utcoffset(self.data)
-        except Exception:
-            # pytz raises AmbiguousTimeError during the autumn DST change.
-            # This happens mainly when __init__ receives a naive datetime
-            # and sets self.timezone = get_default_timezone().
-            return ""
+        offset = self.timezone.utcoffset(self.data)
 
         # `offset` is a datetime.timedelta. For negative values (to the west of
         # UTC) only days can be negative (days=-1) and seconds are always
@@ -232,16 +224,12 @@ class DateFormat(TimeFormat):
 
     def I(self):  # NOQA: E743, E741
         "'1' if Daylight Savings Time, '0' otherwise."
-        try:
-            if self.timezone and self.timezone.dst(self.data):
-                return '1'
-            else:
-                return '0'
-        except Exception:
-            # pytz raises AmbiguousTimeError during the autumn DST change.
-            # This happens mainly when __init__ receives a naive datetime
-            # and sets self.timezone = get_default_timezone().
+        if (
+            not self.timezone or
+            _datetime_ambiguous_or_imaginary(self.data, self.timezone)
+        ):
             return ''
+        return '1' if self.timezone.dst(self.data) else '0'
 
     def j(self):
         "Day of the month without leading zeros; i.e. '1' to '31'"
diff --git a/django/utils/timezone.py b/django/utils/timezone.py
index a87ec5fc33..cf22ec34d0 100644
--- a/django/utils/timezone.py
+++ b/django/utils/timezone.py
@@ -24,6 +24,11 @@ __all__ = [
 # UTC time zone as a tzinfo instance.
 utc = pytz.utc
 
+_PYTZ_BASE_CLASSES = (pytz.tzinfo.BaseTzInfo, pytz._FixedOffset)
+# In releases prior to 2018.4, pytz.UTC was not a subclass of BaseTzInfo
+if not isinstance(pytz.UTC, pytz._FixedOffset):
+    _PYTZ_BASE_CLASSES = _PYTZ_BASE_CLASSES + (type(pytz.UTC),)
+
 
 def get_fixed_timezone(offset):
     """Return a tzinfo instance with a fixed offset from UTC."""
@@ -68,7 +73,7 @@ def get_current_timezone_name():
 
 def _get_timezone_name(timezone):
     """Return the name of ``timezone``."""
-    return timezone.tzname(None)
+    return str(timezone)
 
 # Timezone selection functions.
 
@@ -229,7 +234,7 @@ def make_aware(value, timezone=None, is_dst=None):
     """Make a naive datetime.datetime in a given time zone aware."""
     if timezone is None:
         timezone = get_current_timezone()
-    if hasattr(timezone, 'localize'):
+    if _is_pytz_zone(timezone):
         # This method is available for pytz time zones.
         return timezone.localize(value, is_dst=is_dst)
     else:
@@ -249,3 +254,20 @@ def make_naive(value, timezone=None):
     if is_naive(value):
         raise ValueError("make_naive() cannot be applied to a naive datetime")
     return value.astimezone(timezone).replace(tzinfo=None)
+
+
+def _is_pytz_zone(tz):
+    """Checks if a zone is a pytz zone."""
+    return isinstance(tz, _PYTZ_BASE_CLASSES)
+
+
+def _datetime_ambiguous_or_imaginary(dt, tz):
+    if _is_pytz_zone(tz):
+        try:
+            tz.utcoffset(dt)
+        except (pytz.AmbiguousTimeError, pytz.NonExistentTimeError):
+            return True
+        else:
+            return False
+
+    return tz.utcoffset(dt.replace(fold=not dt.fold)) != tz.utcoffset(dt)