_common.py 文件源码

def _fromutc(self, dt):
        """
        Given a timezone-aware datetime in a given timezone, calculates a
        timezone-aware datetime in a new timezone.

        Since this is the one time that we *know* we have an unambiguous
        datetime object, we take this opportunity to determine whether the
        datetime is ambiguous and in a "fold" state (e.g. if it's the first
        occurence, chronologically, of the ambiguous datetime).

        :param dt:
            A timezone-aware :class:`datetime.dateime` object.
        """

        # Re-implement the algorithm from Python's datetime.py
        if not isinstance(dt, datetime):
            raise TypeError("fromutc() requires a datetime argument")
        if dt.tzinfo is not self:
            raise ValueError("dt.tzinfo is not self")

        dtoff = dt.utcoffset()
        if dtoff is None:
            raise ValueError("fromutc() requires a non-None utcoffset() "
                             "result")

        # The original datetime.py code assumes that `dst()` defaults to
        # zero during ambiguous times. PEP 495 inverts this presumption, so
        # for pre-PEP 495 versions of python, we need to tweak the algorithm.
        dtdst = dt.dst()
        if dtdst is None:
            raise ValueError("fromutc() requires a non-None dst() result")
        delta = dtoff - dtdst
        if delta:
            dt += delta
            # Set fold=1 so we can default to being in the fold for
            # ambiguous dates.
            dtdst = enfold(dt, fold=1).dst()
            if dtdst is None:
                raise ValueError("fromutc(): dt.dst gave inconsistent "
                                 "results; cannot convert")
        return dt + dtdst