# Django 2.2.9.dev20191202104323 文档

# django.utils.timezone 源代码

    """
    Timezone-related classes and functions.
    """

    import functools
    import warnings
    from contextlib import ContextDecorator
    from datetime import datetime, timedelta, timezone, tzinfo
    from threading import local

    import pytz

    from django.conf import settings
    from django.utils.deprecation import RemovedInDjango31Warning

    __all__ = [
        'utc', 'get_fixed_timezone',
        'get_default_timezone', 'get_default_timezone_name',
        'get_current_timezone', 'get_current_timezone_name',
        'activate', 'deactivate', 'override',
        'localtime', 'now',
        'is_aware', 'is_naive', 'make_aware', 'make_naive',
    ]

    # UTC and local time zones

    ZERO = timedelta(0)

    class FixedOffset(tzinfo):
        """
        Fixed offset in minutes east from UTC. Taken from Python's docs.

        Kept as close as possible to the reference version. __init__ was changed
        to make its arguments optional, according to Python's requirement that
        tzinfo subclasses can be instantiated without arguments.
        """

        def __init__(self, offset=None, name=None):
            warnings.warn(
                'FixedOffset is deprecated in favor of datetime.timezone',
                RemovedInDjango31Warning, stacklevel=2,
            )
            if offset is not None:
                self.__offset = timedelta(minutes=offset)
            if name is not None:
                self.__name = name

        def utcoffset(self, dt):
            return self.__offset

        def tzname(self, dt):
            return self.__name

        def dst(self, dt):
            return ZERO

    # UTC time zone as a tzinfo instance.
    utc = pytz.utc

    def get_fixed_timezone(offset):
        """Return a tzinfo instance with a fixed offset from UTC."""
        if isinstance(offset, timedelta):
            offset = offset.total_seconds() // 60
        sign = '-' if offset < 0 else '+'
        hhmm = '%02d%02d' % divmod(abs(offset), 60)
        name = sign + hhmm
        return timezone(timedelta(minutes=offset), name)

    # In order to avoid accessing settings at compile time,
    # wrap the logic in a function and cache the result.
    @functools.lru_cache()
    def get_default_timezone():
        """
        Return the default time zone as a tzinfo instance.

        This is the time zone defined by settings.TIME_ZONE.
        """
        return pytz.timezone(settings.TIME_ZONE)

    # This function exists for consistency with get_current_timezone_name
    def get_default_timezone_name():
        """Return the name of the default time zone."""
        return _get_timezone_name(get_default_timezone())

    _active = local()

    def get_current_timezone():
        """Return the currently active time zone as a tzinfo instance."""
        return getattr(_active, "value", get_default_timezone())

    def get_current_timezone_name():
        """Return the name of the currently active time zone."""
        return _get_timezone_name(get_current_timezone())

    def _get_timezone_name(timezone):
        """Return the name of ``timezone``."""
        return timezone.tzname(None)

    # Timezone selection functions.

    # These functions don't change os.environ['TZ'] and call time.tzset()
    # because it isn't thread safe.

    def activate(timezone):
        """
        Set the time zone for the current thread.

        The ``timezone`` argument must be an instance of a tzinfo subclass or a
        time zone name.
        """
        if isinstance(timezone, tzinfo):
            _active.value = timezone
        elif isinstance(timezone, str):
            _active.value = pytz.timezone(timezone)
        else:
            raise ValueError("Invalid timezone: %r" % timezone)

    def deactivate():
        """
        Unset the time zone for the current thread.

        Django will then use the time zone defined by settings.TIME_ZONE.
        """
        if hasattr(_active, "value"):
            del _active.value

    class override(ContextDecorator):
        """
        Temporarily set the time zone for the current thread.

        This is a context manager that uses django.utils.timezone.activate()
        to set the timezone on entry and restores the previously active timezone
        on exit.

        The ``timezone`` argument must be an instance of a ``tzinfo`` subclass, a
        time zone name, or ``None``. If it is ``None``, Django enables the default
        time zone.
        """
        def __init__(self, timezone):
            self.timezone = timezone

        def __enter__(self):
            self.old_timezone = getattr(_active, 'value', None)
            if self.timezone is None:
                deactivate()
            else:
                activate(self.timezone)

        def __exit__(self, exc_type, exc_value, traceback):
            if self.old_timezone is None:
                deactivate()
            else:
                _active.value = self.old_timezone

    # Templates

    def template_localtime(value, use_tz=None):
        """
        Check 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 localtime(value=None, timezone=None):
        """
        Convert an aware datetime.datetime to local time.

        Only aware datetimes are allowed. When value is omitted, it defaults to
        now().

        Local time is defined by the current time zone, unless another time zone
        is specified.
        """
        if value is None:
            value = now()
        if timezone is None:
            timezone = get_current_timezone()
        # Emulate the behavior of astimezone() on Python < 3.6.
        if is_naive(value):
            raise ValueError("localtime() cannot be applied to a naive datetime")
        return value.astimezone(timezone)

    def localdate(value=None, timezone=None):
        """
        Convert an aware datetime to local time and return the value's date.

        Only aware datetimes are allowed. When value is omitted, it defaults to
        now().

        Local time is defined by the current time zone, unless another time zone is
        specified.
        """
        return localtime(value, timezone).date()

    def now():
        """
        Return an aware or naive datetime.datetime, depending on settings.USE_TZ.
        """
        if settings.USE_TZ:
            # timeit shows that datetime.now(tz=utc) is 24% slower
            return datetime.utcnow().replace(tzinfo=utc)
        else:
            return datetime.now()

    # By design, these four functions don't perform any checks on their arguments.
    # The caller should ensure that they don't receive an invalid value like None.

    def is_aware(value):
        """
        Determine if a given datetime.datetime is aware.

        The concept is defined in Python's docs:
        https://docs.python.org/library/datetime.html#datetime.tzinfo

        Assuming value.tzinfo is either None or a proper datetime.tzinfo,
        value.utcoffset() implements the appropriate logic.
        """
        return value.utcoffset() is not None

    def is_naive(value):
        """
        Determine if a given datetime.datetime is naive.

        The concept is defined in Python's docs:
        https://docs.python.org/library/datetime.html#datetime.tzinfo

        Assuming value.tzinfo is either None or a proper datetime.tzinfo,
        value.utcoffset() implements the appropriate logic.
        """
        return value.utcoffset() is None

    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'):
            # This method is available for pytz time zones.
            return timezone.localize(value, is_dst=is_dst)
        else:
            # Check that we won't overwrite the timezone of an aware datetime.
            if is_aware(value):
                raise ValueError(
                    "make_aware expects a naive datetime, got %s" % value)
            # This may be wrong around DST changes!
            return value.replace(tzinfo=timezone)

    def make_naive(value, timezone=None):
        """Make an aware datetime.datetime naive in a given time zone."""
        if timezone is None:
            timezone = get_current_timezone()
        # Emulate the behavior of astimezone() on Python < 3.6.
        if is_naive(value):
            raise ValueError("make_naive() cannot be applied to a naive datetime")
        return value.astimezone(timezone).replace(tzinfo=None)