Python django.conf.settings 模块，USE_TZ 实例源码

def datetime_extract_sql(self, lookup_type, field_name, tzname):
        # Django 1.6
        if settings.USE_TZ:
            field_name = "CONVERT_TZ({0}, 'UTC', %s)".format(field_name)
            params = [tzname]
        else:
            params = []

        # http://dev.mysql.com/doc/mysql/en/date-and-time-functions.html
        if lookup_type == 'week_day':
            # DAYOFWEEK() returns an integer, 1-7, Sunday=1.
            # Note: WEEKDAY() returns 0-6, Monday=0.
            sql = "DAYOFWEEK({0})".format(field_name)
        else:
            sql = "EXTRACT({0} FROM {1})".format(lookup_type.upper(),
                                                 field_name)
        return sql, params

def datetime_trunc_sql(self, lookup_type, field_name, tzname):
        # Django 1.6
        if settings.USE_TZ:
            field_name = "CONVERT_TZ({0}, 'UTC', %s)".format(field_name)
            params = [tzname]
        else:
            params = []
        fields = ['year', 'month', 'day', 'hour', 'minute', 'second']
        format_ = ('%Y-', '%m', '-%d', ' %H:', '%i', ':%S')
        format_def = ('0000-', '01', '-01', ' 00:', '00', ':00')
        try:
            i = fields.index(lookup_type) + 1
        except ValueError:
            sql = field_name
        else:
            format_str = ''.join([f for f in format_[:i]] +
                                 [f for f in format_def[i:]])
            sql = "CAST(DATE_FORMAT({0}, '{1}') AS DATETIME)".format(
                field_name, format_str)
        return sql, params

def value_to_db_datetime(self, value):
        if value is None:
            return None
        # MySQL doesn't support tz-aware times
        if timezone.is_aware(value):
            if settings.USE_TZ:
                value = value.astimezone(timezone.utc).replace(tzinfo=None)
            else:
                raise ValueError(
                    "MySQL backend does not support timezone-aware times."
                )
        if not self.connection.features.supports_microsecond_precision:
            value = value.replace(microsecond=0)
        if not self.connection.use_pure:
            return datetime_to_mysql(value)
        return self.connection.converter.to_mysql(value)

def from_current_timezone(value):
    """
    When time zone support is enabled, convert naive datetimes
    entered in the current time zone to aware datetimes.
    """
    if settings.USE_TZ and value is not None and timezone.is_naive(value):
        current_timezone = timezone.get_current_timezone()
        try:
            return timezone.make_aware(value, current_timezone)
        except Exception:
            message = _(
                '%(datetime)s couldn\'t be interpreted '
                'in time zone %(current_timezone)s; it '
                'may be ambiguous or it may not exist.'
            )
            params = {'datetime': value, 'current_timezone': current_timezone}
            six.reraise(ValidationError, ValidationError(
                message,
                code='ambiguous_timezone',
                params=params,
            ), sys.exc_info()[2])
    return value

def has_key(self, key, version=None):
        key = self.make_key(key, version=version)
        self.validate_key(key)

        db = router.db_for_read(self.cache_model_class)
        connection = connections[db]
        table = connection.ops.quote_name(self._table)

        if settings.USE_TZ:
            now = datetime.utcnow()
        else:
            now = datetime.now()
        now = now.replace(microsecond=0)

        with connection.cursor() as cursor:
            cursor.execute("SELECT cache_key FROM %s "
                           "WHERE cache_key = %%s and expires > %%s" % table,
                           [key, connection.ops.adapt_datetimefield_value(now)])
            return cursor.fetchone() is not None

def adapt_datetime_warn_on_aware_datetime(value, conv):
    # Remove this function and rely on the default adapter in Django 2.0.
    if settings.USE_TZ and timezone.is_aware(value):
        warnings.warn(
            "The MySQL database adapter received an aware datetime (%s), "
            "probably from cursor.execute(). Update your code to pass a "
            "naive datetime in the database connection's time zone (UTC by "
            "default).", RemovedInDjango20Warning)
        # This doesn't account for the database connection's timezone,
        # which isn't known. (That's why this adapter is deprecated.)
        value = value.astimezone(timezone.utc).replace(tzinfo=None)
    return Thing2Literal(value.strftime("%Y-%m-%d %H:%M:%S.%f"), conv)

# MySQLdb-1.2.1 returns TIME columns as timedelta -- they are more like
# timedelta in terms of actual behavior as they are signed and include days --
# and Django expects time, so we still need to override that. We also need to
# add special handling for SafeText and SafeBytes as MySQLdb's type
# checking is too tight to catch those (see Django ticket #6052).

def adapt_datetimefield_value(self, value):
        """
        Transform a datetime value to an object compatible with what is expected
        by the backend driver for datetime columns.

        If naive datetime is passed assumes that is in UTC. Normally Django
        models.DateTimeField makes sure that if USE_TZ is True passed datetime
        is timezone aware.
        """

        if value is None:
            return None

        # cx_Oracle doesn't support tz-aware datetimes
        if timezone.is_aware(value):
            if settings.USE_TZ:
                value = timezone.make_naive(value, self.connection.timezone)
            else:
                raise ValueError("Oracle backend does not support timezone-aware datetimes when USE_TZ is False.")

        return Oracle_datetime.from_datetime(value)

def year_lookup_bounds_for_datetime_field(self, value):
        """
        Returns a two-elements list with the lower and upper bound to be used
        with a BETWEEN operator to query a DateTimeField value using a year
        lookup.

        `value` is an int, containing the looked-up year.
        """
        first = datetime.datetime(value, 1, 1)
        second = datetime.datetime(value, 12, 31, 23, 59, 59, 999999)
        if settings.USE_TZ:
            tz = timezone.get_current_timezone()
            first = timezone.make_aware(first, tz)
            second = timezone.make_aware(second, tz)
        first = self.adapt_datetimefield_value(first)
        second = self.adapt_datetimefield_value(second)
        return [first, second]

def timezone(self):
        """
        Time zone for datetimes stored as naive values in the database.

        Returns a tzinfo object or None.

        This is only needed when time zone support is enabled and the database
        doesn't support time zones. (When the database supports time zones,
        the adapter handles aware datetimes so Django doesn't need to.)
        """
        if not settings.USE_TZ:
            return None
        elif self.features.supports_timezones:
            return None
        elif self.settings_dict['TIME_ZONE'] is None:
            return timezone.utc
        else:
            # Only this branch requires pytz.
            return pytz.timezone(self.settings_dict['TIME_ZONE'])

def datetimes(self, field_name, kind, order='ASC', tzinfo=None):
        """
        Returns a list of datetime objects representing all available
        datetimes for the given field_name, scoped to 'kind'.
        """
        assert kind in ("year", "month", "day", "hour", "minute", "second"), \
            "'kind' must be one of 'year', 'month', 'day', 'hour', 'minute' or 'second'."
        assert order in ('ASC', 'DESC'), \
            "'order' must be either 'ASC' or 'DESC'."
        if settings.USE_TZ:
            if tzinfo is None:
                tzinfo = timezone.get_current_timezone()
        else:
            tzinfo = None
        return self.annotate(
            datetimefield=DateTime(field_name, kind, tzinfo),
            plain_field=F(field_name)
        ).values_list(
            'datetimefield', flat=True
        ).distinct().filter(plain_field__isnull=False).order_by(('-' if order == 'DESC' else '') + 'datetimefield')

def get_prep_value(self, value):
        value = super(DateTimeField, self).get_prep_value(value)
        value = self.to_python(value)
        if value is not None and settings.USE_TZ and timezone.is_naive(value):
            # For backwards compatibility, interpret naive datetimes in local
            # time. This won't work during DST change, but we can't do much
            # about it, so we let the exceptions percolate up the call stack.
            try:
                name = '%s.%s' % (self.model.__name__, self.name)
            except AttributeError:
                name = '(unbound)'
            warnings.warn("DateTimeField %s received a naive datetime (%s)"
                          " while time zone support is active." %
                          (name, value),
                          RuntimeWarning)
            default_timezone = timezone.get_default_timezone()
            value = timezone.make_aware(value, default_timezone)
        return value

def template_localtime(value, use_tz=None):
    """
    Checks if value is a datetime and converts it to local time if necessary.

    If use_tz is provided and is not None, that will force the value to
    be converted (or not), overriding the value of settings.USE_TZ.

    This function is designed for use by the template engine.
    """
    should_convert = (isinstance(value, datetime)
        and (settings.USE_TZ if use_tz is None else use_tz)
        and not is_naive(value)
        and getattr(value, 'convert_to_local_time', True))
    return localtime(value) if should_convert else value


# Utilities

def from_current_timezone(value):
    """
    When time zone support is enabled, convert naive datetimes
    entered in the current time zone to aware datetimes.
    """
    if settings.USE_TZ and value is not None and timezone.is_naive(value):
        current_timezone = timezone.get_current_timezone()
        try:
            return timezone.make_aware(value, current_timezone)
        except Exception:
            message = _(
                '%(datetime)s couldn\'t be interpreted '
                'in time zone %(current_timezone)s; it '
                'may be ambiguous or it may not exist.'
            )
            params = {'datetime': value, 'current_timezone': current_timezone}
            six.reraise(ValidationError, ValidationError(
                message,
                code='ambiguous_timezone',
                params=params,
            ), sys.exc_info()[2])
    return value

def has_key(self, key, version=None):
        key = self.make_key(key, version=version)
        self.validate_key(key)

        db = router.db_for_read(self.cache_model_class)
        connection = connections[db]
        table = connection.ops.quote_name(self._table)

        if settings.USE_TZ:
            now = datetime.utcnow()
        else:
            now = datetime.now()
        now = now.replace(microsecond=0)

        with connection.cursor() as cursor:
            cursor.execute("SELECT cache_key FROM %s "
                           "WHERE cache_key = %%s and expires > %%s" % table,
                           [key, connection.ops.adapt_datetimefield_value(now)])
            return cursor.fetchone() is not None

def from_current_timezone(value):
    """
    When time zone support is enabled, convert naive datetimes
    entered in the current time zone to aware datetimes.
    """
    if settings.USE_TZ and value is not None and timezone.is_naive(value):
        current_timezone = timezone.get_current_timezone()
        try:
            return timezone.make_aware(value, current_timezone)
        except Exception:
            message = _(
                '%(datetime)s couldn\'t be interpreted '
                'in time zone %(current_timezone)s; it '
                'may be ambiguous or it may not exist.'
            )
            params = {'datetime': value, 'current_timezone': current_timezone}
            six.reraise(ValidationError, ValidationError(
                message,
                code='ambiguous_timezone',
                params=params,
            ), sys.exc_info()[2])
    return value

def has_key(self, key, version=None):
        key = self.make_key(key, version=version)
        self.validate_key(key)

        db = router.db_for_read(self.cache_model_class)
        connection = connections[db]
        table = connection.ops.quote_name(self._table)

        if settings.USE_TZ:
            now = datetime.utcnow()
        else:
            now = datetime.now()
        now = now.replace(microsecond=0)

        with connection.cursor() as cursor:
            cursor.execute("SELECT cache_key FROM %s "
                           "WHERE cache_key = %%s and expires > %%s" % table,
                           [key, connection.ops.adapt_datetimefield_value(now)])
            return cursor.fetchone() is not None

def datetime_extract_sql(self, lookup_type, field_name, tzname):
        # Django 1.6
        if settings.USE_TZ:
            field_name = "CONVERT_TZ({0}, 'UTC', %s)".format(field_name)
            params = [tzname]
        else:
            params = []

        # http://dev.mysql.com/doc/mysql/en/date-and-time-functions.html
        if lookup_type == 'week_day':
            # DAYOFWEEK() returns an integer, 1-7, Sunday=1.
            # Note: WEEKDAY() returns 0-6, Monday=0.
            sql = "DAYOFWEEK({0})".format(field_name)
        else:
            sql = "EXTRACT({0} FROM {1})".format(lookup_type.upper(),
                                                 field_name)
        return sql, params

def datetime_trunc_sql(self, lookup_type, field_name, tzname):
        # Django 1.6
        if settings.USE_TZ:
            field_name = "CONVERT_TZ({0}, 'UTC', %s)".format(field_name)
            params = [tzname]
        else:
            params = []
        fields = ['year', 'month', 'day', 'hour', 'minute', 'second']
        format_ = ('%Y-', '%m', '-%d', ' %H:', '%i', ':%S')
        format_def = ('0000-', '01', '-01', ' 00:', '00', ':00')
        try:
            i = fields.index(lookup_type) + 1
        except ValueError:
            sql = field_name
        else:
            format_str = ''.join([f for f in format_[:i]] +
                                 [f for f in format_def[i:]])
            sql = "CAST(DATE_FORMAT({0}, '{1}') AS DATETIME)".format(
                field_name, format_str)
        return sql, params

def value_to_db_datetime(self, value):
        if value is None:
            return None
        # MySQL doesn't support tz-aware times
        if timezone.is_aware(value):
            if settings.USE_TZ:
                value = value.astimezone(timezone.utc).replace(tzinfo=None)
            else:
                raise ValueError(
                    "MySQL backend does not support timezone-aware times."
                )
        if not self.connection.features.supports_microsecond_precision:
            value = value.replace(microsecond=0)
        if not self.connection.use_pure:
            return datetime_to_mysql(value)
        return self.connection.converter.to_mysql(value)

def _DATETIME_to_python(self, value, dsc=None):
        """Connector/Python always returns naive datetime.datetime

        Connector/Python always returns naive timestamps since MySQL has
        no time zone support. Since Django needs non-naive, we need to add
        the UTC time zone.

        Returns datetime.datetime()
        """
        if not value:
            return None
        dt = MySQLConverter._DATETIME_to_python(self, value)
        if dt is None:
            return None
        if settings.USE_TZ and timezone.is_naive(dt):
            dt = dt.replace(tzinfo=timezone.utc)
        return dt

def from_current_timezone(value):
    """
    When time zone support is enabled, convert naive datetimes
    entered in the current time zone to aware datetimes.
    """
    if settings.USE_TZ and value is not None and timezone.is_naive(value):
        current_timezone = timezone.get_current_timezone()
        try:
            return timezone.make_aware(value, current_timezone)
        except Exception:
            message = _(
                '%(datetime)s couldn\'t be interpreted '
                'in time zone %(current_timezone)s; it '
                'may be ambiguous or it may not exist.'
            )
            params = {'datetime': value, 'current_timezone': current_timezone}
            six.reraise(ValidationError, ValidationError(
                message,
                code='ambiguous_timezone',
                params=params,
            ), sys.exc_info()[2])
    return value

def has_key(self, key, version=None):
        key = self.make_key(key, version=version)
        self.validate_key(key)

        db = router.db_for_read(self.cache_model_class)
        connection = connections[db]
        table = connection.ops.quote_name(self._table)

        if settings.USE_TZ:
            now = datetime.utcnow()
        else:
            now = datetime.now()
        now = now.replace(microsecond=0)

        with connection.cursor() as cursor:
            cursor.execute("SELECT cache_key FROM %s "
                           "WHERE cache_key = %%s and expires > %%s" % table,
                           [key, connection.ops.adapt_datetimefield_value(now)])
            return cursor.fetchone() is not None

def datetime_extract_sql(self, lookup_type, field_name, tzname):
        # Django 1.6
        if settings.USE_TZ:
            field_name = "CONVERT_TZ({0}, 'UTC', %s)".format(field_name)
            params = [tzname]
        else:
            params = []

        # http://dev.mysql.com/doc/mysql/en/date-and-time-functions.html
        if lookup_type == 'week_day':
            # DAYOFWEEK() returns an integer, 1-7, Sunday=1.
            # Note: WEEKDAY() returns 0-6, Monday=0.
            sql = "DAYOFWEEK({0})".format(field_name)
        else:
            sql = "EXTRACT({0} FROM {1})".format(lookup_type.upper(),
                                                 field_name)
        return sql, params

def datetime_trunc_sql(self, lookup_type, field_name, tzname):
        # Django 1.6
        if settings.USE_TZ:
            field_name = "CONVERT_TZ({0}, 'UTC', %s)".format(field_name)
            params = [tzname]
        else:
            params = []
        fields = ['year', 'month', 'day', 'hour', 'minute', 'second']
        format_ = ('%Y-', '%m', '-%d', ' %H:', '%i', ':%S')
        format_def = ('0000-', '01', '-01', ' 00:', '00', ':00')
        try:
            i = fields.index(lookup_type) + 1
        except ValueError:
            sql = field_name
        else:
            format_str = ''.join([f for f in format_[:i]] +
                                 [f for f in format_def[i:]])
            sql = "CAST(DATE_FORMAT({0}, '{1}') AS DATETIME)".format(
                field_name, format_str)
        return sql, params

def value_to_db_datetime(self, value):
        if value is None:
            return None
        # MySQL doesn't support tz-aware times
        if timezone.is_aware(value):
            if settings.USE_TZ:
                value = value.astimezone(timezone.utc).replace(tzinfo=None)
            else:
                raise ValueError(
                    "MySQL backend does not support timezone-aware times."
                )
        if not self.connection.features.supports_microsecond_precision:
            value = value.replace(microsecond=0)
        if not self.connection.use_pure:
            return datetime_to_mysql(value)
        return self.connection.converter.to_mysql(value)

def _DATETIME_to_python(self, value, dsc=None):
        """Connector/Python always returns naive datetime.datetime

        Connector/Python always returns naive timestamps since MySQL has
        no time zone support. Since Django needs non-naive, we need to add
        the UTC time zone.

        Returns datetime.datetime()
        """
        if not value:
            return None
        dt = MySQLConverter._DATETIME_to_python(self, value)
        if dt is None:
            return None
        if settings.USE_TZ and timezone.is_naive(dt):
            dt = dt.replace(tzinfo=timezone.utc)
        return dt

def _format_parameters( self, parameters ):
        parameters = list( parameters )
        for index in range( len( parameters ) ):
            # With raw SQL queries, datetimes can reach this function
            # without being converted by DateTimeField.get_db_prep_value.
            if settings.USE_TZ and isinstance( parameters[index], datetime.datetime ):
                param = parameters[index]
                if timezone.is_naive( param ):
                    warnings.warn(u"Received a naive datetime (%s)"
                              u" while time zone support is active." % param,
                              RuntimeWarning)
                    default_timezone = timezone.get_default_timezone()
                    param = timezone.make_aware( param, default_timezone )
                param = param.astimezone(timezone.utc).replace(tzinfo=None)
                parameters[index] = param
        return tuple( parameters )

    # Over-riding this method to modify SQLs which contains format parameter to qmark.

def datetime_extract_sql(self, lookup_type, field_name, tzname):
        if settings.USE_TZ:
            hr, min = self._get_utcoffset(tzname)
            if hr < 0:
                field_name = "%s - %s HOURS - %s MINUTES" % (field_name, -hr, -min)
            else:
                field_name = "%s + %s HOURS + %s MINUTES" % (field_name, hr, min)

        if lookup_type.upper() == 'WEEK_DAY':
            return " DAYOFWEEK(%s) " % ( field_name ), []
        else:
            return " %s(%s) " % ( lookup_type.upper(), field_name ), []

    # Truncating the date value on the basic of lookup type.
    # e.g If input is 2008-12-04 and month then output will be 2008-12-01 00:00:00
    # Reference: http://www.ibm.com/developerworks/data/library/samples/db2/0205udfs/index.html

def datetime_trunc_sql( self, lookup_type, field_name, tzname ):
        sql = "TIMESTAMP(SUBSTR(CHAR(%s), 1, %d) || '%s')"
        if settings.USE_TZ:
            hr, min = self._get_utcoffset(tzname)
            if hr < 0:
                field_name = "%s - %s HOURS - %s MINUTES" % (field_name, -hr, -min)
            else:
                field_name = "%s + %s HOURS + %s MINUTES" % (field_name, hr, min)
        if lookup_type.upper() == 'SECOND':
            sql = sql % ( field_name, 19, '.000000' )
        if lookup_type.upper() == 'MINUTE':
            sql = sql % ( field_name, 16, '.00.000000' )
        elif lookup_type.upper() == 'HOUR':
            sql = sql % ( field_name, 13, '.00.00.000000' )
        elif lookup_type.upper() == 'DAY':
            sql = sql % ( field_name, 10, '-00.00.00.000000' )
        elif lookup_type.upper() == 'MONTH':
            sql = sql % ( field_name, 7, '-01-00.00.00.000000' )
        elif lookup_type.upper() == 'YEAR':
            sql = sql % ( field_name, 4, '-01-01-00.00.00.000000' )
        return sql, []

def value_to_db_datetime( self, value ):
        if value is None:
            return None

        if( djangoVersion[0:2] <= ( 1, 3 ) ):
            #DB2 doesn't support time zone aware datetime
            if ( value.tzinfo is not None ):
                raise ValueError( "Timezone aware datetime not supported" )
            else:
                return value
        else:
            if is_aware(value):
                if settings.USE_TZ:
                    value = value.astimezone( utc ).replace( tzinfo=None )
                else:
                    raise ValueError( "Timezone aware datetime not supported" )
            return unicode( value )

def from_current_timezone(value):
    """
    When time zone support is enabled, convert naive datetimes
    entered in the current time zone to aware datetimes.
    """
    if settings.USE_TZ and value is not None and timezone.is_naive(value):
        current_timezone = timezone.get_current_timezone()
        try:
            return timezone.make_aware(value, current_timezone)
        except Exception:
            message = _(
                '%(datetime)s couldn\'t be interpreted '
                'in time zone %(current_timezone)s; it '
                'may be ambiguous or it may not exist.'
            )
            params = {'datetime': value, 'current_timezone': current_timezone}
            six.reraise(ValidationError, ValidationError(
                message,
                code='ambiguous_timezone',
                params=params,
            ), sys.exc_info()[2])
    return value

def has_key(self, key, version=None):
        key = self.make_key(key, version=version)
        self.validate_key(key)

        db = router.db_for_read(self.cache_model_class)
        connection = connections[db]
        table = connection.ops.quote_name(self._table)

        if settings.USE_TZ:
            now = datetime.utcnow()
        else:
            now = datetime.now()
        now = now.replace(microsecond=0)

        with connection.cursor() as cursor:
            cursor.execute("SELECT cache_key FROM %s "
                           "WHERE cache_key = %%s and expires > %%s" % table,
                           [key, connection.ops.adapt_datetimefield_value(now)])
            return cursor.fetchone() is not None

def from_current_timezone(value):
    """
    When time zone support is enabled, convert naive datetimes
    entered in the current time zone to aware datetimes.
    """
    if settings.USE_TZ and value is not None and timezone.is_naive(value):
        current_timezone = timezone.get_current_timezone()
        try:
            return timezone.make_aware(value, current_timezone)
        except Exception:
            message = _(
                '%(datetime)s couldn\'t be interpreted '
                'in time zone %(current_timezone)s; it '
                'may be ambiguous or it may not exist.'
            )
            params = {'datetime': value, 'current_timezone': current_timezone}
            six.reraise(ValidationError, ValidationError(
                message,
                code='ambiguous_timezone',
                params=params,
            ), sys.exc_info()[2])
    return value

def has_key(self, key, version=None):
        key = self.make_key(key, version=version)
        self.validate_key(key)

        db = router.db_for_read(self.cache_model_class)
        table = connections[db].ops.quote_name(self._table)

        if settings.USE_TZ:
            now = datetime.utcnow()
        else:
            now = datetime.now()
        now = now.replace(microsecond=0)

        with connections[db].cursor() as cursor:
            cursor.execute("SELECT cache_key FROM %s "
                           "WHERE cache_key = %%s and expires > %%s" % table,
                           [key, connections[db].ops.value_to_db_datetime(now)])
            return cursor.fetchone() is not None

def _cull(self, db, cursor, now):
        if self._cull_frequency == 0:
            self.clear()
        else:
            # When USE_TZ is True, 'now' will be an aware datetime in UTC.
            now = now.replace(tzinfo=None)
            table = connections[db].ops.quote_name(self._table)
            cursor.execute("DELETE FROM %s WHERE expires < %%s" % table,
                           [connections[db].ops.value_to_db_datetime(now)])
            cursor.execute("SELECT COUNT(*) FROM %s" % table)
            num = cursor.fetchone()[0]
            if num > self._max_entries:
                cull_num = num // self._cull_frequency
                cursor.execute(
                    connections[db].ops.cache_key_culling_sql() % table,
                    [cull_num])
                cursor.execute("DELETE FROM %s "
                               "WHERE cache_key < %%s" % table,
                               [cursor.fetchone()[0]])

def datetime_trunc_sql(self, lookup_type, field_name, tzname):
        if settings.USE_TZ:
            field_name = "CONVERT_TZ(%s, 'UTC', %%s)" % field_name
            params = [tzname]
        else:
            params = []
        fields = ['year', 'month', 'day', 'hour', 'minute', 'second']
        format = ('%%Y-', '%%m', '-%%d', ' %%H:', '%%i', ':%%s')  # Use double percents to escape.
        format_def = ('0000-', '01', '-01', ' 00:', '00', ':00')
        try:
            i = fields.index(lookup_type) + 1
        except ValueError:
            sql = field_name
        else:
            format_str = ''.join([f for f in format[:i]] + [f for f in format_def[i:]])
            sql = "CAST(DATE_FORMAT(%s, '%s') AS DATETIME)" % (field_name, format_str)
        return sql, params

def adapt_datetime_with_timezone_support(value, conv):
    # Equivalent to DateTimeField.get_db_prep_value. Used only by raw SQL.
    if settings.USE_TZ:
        if timezone.is_naive(value):
            warnings.warn("MySQL received a naive datetime (%s)"
                          " while time zone support is active." % value,
                          RuntimeWarning)
            default_timezone = timezone.get_default_timezone()
            value = timezone.make_aware(value, default_timezone)
        value = value.astimezone(timezone.utc).replace(tzinfo=None)
    return Thing2Literal(value.strftime("%Y-%m-%d %H:%M:%S.%f"), conv)

# MySQLdb-1.2.1 returns TIME columns as timedelta -- they are more like
# timedelta in terms of actual behavior as they are signed and include days --
# and Django expects time, so we still need to override that. We also need to
# add special handling for SafeText and SafeBytes as MySQLdb's type
# checking is too tight to catch those (see Django ticket #6052).
# Finally, MySQLdb always returns naive datetime objects. However, when
# timezone support is active, Django expects timezone-aware datetime objects.

def value_to_db_datetime(self, value):
        """
        Transform a datetime value to an object compatible with what is expected
        by the backend driver for datetime columns.

        If naive datetime is passed assumes that is in UTC. Normally Django
        models.DateTimeField makes sure that if USE_TZ is True passed datetime
        is timezone aware.
        """

        if value is None:
            return None

        # cx_Oracle doesn't support tz-aware datetimes
        if timezone.is_aware(value):
            if settings.USE_TZ:
                value = value.astimezone(timezone.utc).replace(tzinfo=None)
            else:
                raise ValueError("Oracle backend does not support timezone-aware datetimes when USE_TZ is False.")

        return Oracle_datetime.from_datetime(value)

def from_current_timezone(value):
    """
    When time zone support is enabled, convert naive datetimes
    entered in the current time zone to aware datetimes.
    """
    if settings.USE_TZ and value is not None and timezone.is_naive(value):
        current_timezone = timezone.get_current_timezone()
        try:
            return timezone.make_aware(value, current_timezone)
        except Exception:
            message = _(
                '%(datetime)s couldn\'t be interpreted '
                'in time zone %(current_timezone)s; it '
                'may be ambiguous or it may not exist.'
            )
            params = {'datetime': value, 'current_timezone': current_timezone}
            six.reraise(ValidationError, ValidationError(
                message,
                code='ambiguous_timezone',
                params=params,
            ), sys.exc_info()[2])
    return value

def has_key(self, key, version=None):
        key = self.make_key(key, version=version)
        self.validate_key(key)

        db = router.db_for_read(self.cache_model_class)
        connection = connections[db]
        table = connection.ops.quote_name(self._table)

        if settings.USE_TZ:
            now = datetime.utcnow()
        else:
            now = datetime.now()
        now = now.replace(microsecond=0)

        with connection.cursor() as cursor:
            cursor.execute("SELECT cache_key FROM %s "
                           "WHERE cache_key = %%s and expires > %%s" % table,
                           [key, connection.ops.adapt_datetimefield_value(now)])
            return cursor.fetchone() is not None

def from_current_timezone(value):
    """
    When time zone support is enabled, convert naive datetimes
    entered in the current time zone to aware datetimes.
    """
    if settings.USE_TZ and value is not None and timezone.is_naive(value):
        current_timezone = timezone.get_current_timezone()
        try:
            return timezone.make_aware(value, current_timezone)
        except Exception:
            message = _(
                '%(datetime)s couldn\'t be interpreted '
                'in time zone %(current_timezone)s; it '
                'may be ambiguous or it may not exist.'
            )
            params = {'datetime': value, 'current_timezone': current_timezone}
            six.reraise(ValidationError, ValidationError(
                message,
                code='ambiguous_timezone',
                params=params,
            ), sys.exc_info()[2])
    return value

def has_key(self, key, version=None):
        key = self.make_key(key, version=version)
        self.validate_key(key)

        db = router.db_for_read(self.cache_model_class)
        connection = connections[db]
        table = connection.ops.quote_name(self._table)

        if settings.USE_TZ:
            now = datetime.utcnow()
        else:
            now = datetime.now()
        now = now.replace(microsecond=0)

        with connection.cursor() as cursor:
            cursor.execute("SELECT cache_key FROM %s "
                           "WHERE cache_key = %%s and expires > %%s" % table,
                           [key, connection.ops.adapt_datetimefield_value(now)])
            return cursor.fetchone() is not None

def from_current_timezone(value):
    """
    When time zone support is enabled, convert naive datetimes
    entered in the current time zone to aware datetimes.
    """
    if settings.USE_TZ and value is not None and timezone.is_naive(value):
        current_timezone = timezone.get_current_timezone()
        try:
            return timezone.make_aware(value, current_timezone)
        except Exception:
            message = _(
                '%(datetime)s couldn\'t be interpreted '
                'in time zone %(current_timezone)s; it '
                'may be ambiguous or it may not exist.'
            )
            params = {'datetime': value, 'current_timezone': current_timezone}
            six.reraise(ValidationError, ValidationError(
                message,
                code='ambiguous_timezone',
                params=params,
            ), sys.exc_info()[2])
    return value

def has_key(self, key, version=None):
        key = self.make_key(key, version=version)
        self.validate_key(key)

        db = router.db_for_read(self.cache_model_class)
        table = connections[db].ops.quote_name(self._table)

        if settings.USE_TZ:
            now = datetime.utcnow()
        else:
            now = datetime.now()
        now = now.replace(microsecond=0)

        with connections[db].cursor() as cursor:
            cursor.execute("SELECT cache_key FROM %s "
                           "WHERE cache_key = %%s and expires > %%s" % table,
                           [key, connections[db].ops.value_to_db_datetime(now)])
            return cursor.fetchone() is not None

def _cull(self, db, cursor, now):
        if self._cull_frequency == 0:
            self.clear()
        else:
            # When USE_TZ is True, 'now' will be an aware datetime in UTC.
            now = now.replace(tzinfo=None)
            table = connections[db].ops.quote_name(self._table)
            cursor.execute("DELETE FROM %s WHERE expires < %%s" % table,
                           [connections[db].ops.value_to_db_datetime(now)])
            cursor.execute("SELECT COUNT(*) FROM %s" % table)
            num = cursor.fetchone()[0]
            if num > self._max_entries:
                cull_num = num // self._cull_frequency
                cursor.execute(
                    connections[db].ops.cache_key_culling_sql() % table,
                    [cull_num])
                cursor.execute("DELETE FROM %s "
                               "WHERE cache_key < %%s" % table,
                               [cursor.fetchone()[0]])

def datetime_trunc_sql(self, lookup_type, field_name, tzname):
        if settings.USE_TZ:
            field_name = "CONVERT_TZ(%s, 'UTC', %%s)" % field_name
            params = [tzname]
        else:
            params = []
        fields = ['year', 'month', 'day', 'hour', 'minute', 'second']
        format = ('%%Y-', '%%m', '-%%d', ' %%H:', '%%i', ':%%s')  # Use double percents to escape.
        format_def = ('0000-', '01', '-01', ' 00:', '00', ':00')
        try:
            i = fields.index(lookup_type) + 1
        except ValueError:
            sql = field_name
        else:
            format_str = ''.join([f for f in format[:i]] + [f for f in format_def[i:]])
            sql = "CAST(DATE_FORMAT(%s, '%s') AS DATETIME)" % (field_name, format_str)
        return sql, params

def adapt_datetime_with_timezone_support(value, conv):
    # Equivalent to DateTimeField.get_db_prep_value. Used only by raw SQL.
    if settings.USE_TZ:
        if timezone.is_naive(value):
            warnings.warn("MySQL received a naive datetime (%s)"
                          " while time zone support is active." % value,
                          RuntimeWarning)
            default_timezone = timezone.get_default_timezone()
            value = timezone.make_aware(value, default_timezone)
        value = value.astimezone(timezone.utc).replace(tzinfo=None)
    return Thing2Literal(value.strftime("%Y-%m-%d %H:%M:%S.%f"), conv)

# MySQLdb-1.2.1 returns TIME columns as timedelta -- they are more like
# timedelta in terms of actual behavior as they are signed and include days --
# and Django expects time, so we still need to override that. We also need to
# add special handling for SafeText and SafeBytes as MySQLdb's type
# checking is too tight to catch those (see Django ticket #6052).
# Finally, MySQLdb always returns naive datetime objects. However, when
# timezone support is active, Django expects timezone-aware datetime objects.

def from_current_timezone(value):
    """
    When time zone support is enabled, convert naive datetimes
    entered in the current time zone to aware datetimes.
    """
    if settings.USE_TZ and value is not None and timezone.is_naive(value):
        current_timezone = timezone.get_current_timezone()
        try:
            return timezone.make_aware(value, current_timezone)
        except Exception:
            message = _(
                '%(datetime)s couldn\'t be interpreted '
                'in time zone %(current_timezone)s; it '
                'may be ambiguous or it may not exist.'
            )
            params = {'datetime': value, 'current_timezone': current_timezone}
            six.reraise(ValidationError, ValidationError(
                message,
                code='ambiguous_timezone',
                params=params,
            ), sys.exc_info()[2])
    return value

def has_key(self, key, version=None):
        key = self.make_key(key, version=version)
        self.validate_key(key)

        db = router.db_for_read(self.cache_model_class)
        table = connections[db].ops.quote_name(self._table)

        if settings.USE_TZ:
            now = datetime.utcnow()
        else:
            now = datetime.now()
        now = now.replace(microsecond=0)

        with connections[db].cursor() as cursor:
            cursor.execute("SELECT cache_key FROM %s "
                           "WHERE cache_key = %%s and expires > %%s" % table,
                           [key, connections[db].ops.value_to_db_datetime(now)])
            return cursor.fetchone() is not None

def _cull(self, db, cursor, now):
        if self._cull_frequency == 0:
            self.clear()
        else:
            # When USE_TZ is True, 'now' will be an aware datetime in UTC.
            now = now.replace(tzinfo=None)
            table = connections[db].ops.quote_name(self._table)
            cursor.execute("DELETE FROM %s WHERE expires < %%s" % table,
                           [connections[db].ops.value_to_db_datetime(now)])
            cursor.execute("SELECT COUNT(*) FROM %s" % table)
            num = cursor.fetchone()[0]
            if num > self._max_entries:
                cull_num = num // self._cull_frequency
                cursor.execute(
                    connections[db].ops.cache_key_culling_sql() % table,
                    [cull_num])
                cursor.execute("DELETE FROM %s "
                               "WHERE cache_key < %%s" % table,
                               [cursor.fetchone()[0]])