Fixed test failures on MySQL.

Some tests failed when the time zone definitions were loaded in MySQL
and pytz wasn't installed. This setup isn't supported.

django/db/backends/mysql/base.py

@@ -30,6 +30,11 @@ if (version < (1, 2, 1) or (version[:3] == (1, 2, 1) and
 from MySQLdb.converters import conversions, Thing2Literal
 from MySQLdb.constants import FIELD_TYPE, CLIENT
 
+try:
+    import pytz
+except ImportError:
+    pytz = None
+
 from django.conf import settings
 from django.db import utils
 from django.db.backends import *
@@ -186,6 +191,15 @@ class DatabaseFeatures(BaseDatabaseFeatures):
 
     @cached_property
     def has_zoneinfo_database(self):
+        # MySQL accepts full time zones names (eg. Africa/Nairobi) but rejects
+        # abbreviations (eg. EAT). When pytz isn't installed and the current
+        # time zone is LocalTimezone (the only sensible value in this
+        # context), the current time zone name will be an abbreviation. As a
+        # consequence, MySQL cannot perform time zone conversions reliably.
+        if pytz is None:
+            return False
+
+        # Test if the time zone definitions are installed.
         cursor = self.connection.cursor()
         cursor.execute("SELECT 1 FROM mysql.time_zone LIMIT 1")
         return cursor.fetchone() is not None

django/db/models/sql/compiler.py

@@ -1092,7 +1092,7 @@ class SQLDateTimeCompiler(SQLCompiler):
                     if datetime is None:
                         raise ValueError("Database returned an invalid value "
                                          "in QuerySet.dates(). Are time zone "
-                                         "definitions installed?")
+                                         "definitions and pytz installed?")
                     datetime = datetime.replace(tzinfo=None)
                     datetime = timezone.make_aware(datetime, self.query.tzinfo)
                 yield datetime

docs/ref/models/querysets.txt

@@ -627,7 +627,8 @@ object. If it's ``None``, Django uses the :ref:`current time zone
     - SQLite: install pytz_ — conversions are actually performed in Python.
     - PostgreSQL: no requirements (see `Time Zones`_).
     - Oracle: no requirements (see `Choosing a Time Zone File`_).
-    - MySQL: load the time zone tables with `mysql_tzinfo_to_sql`_.
+    - MySQL: install pytz_ and load the time zone tables with
+      `mysql_tzinfo_to_sql`_.
 
     .. _pytz: http://pytz.sourceforge.net/
     .. _Time Zones: http://www.postgresql.org/docs/current/static/datatype-datetime.html#DATATYPE-TIMEZONES