Class TZContextBase

tests/_common.py:49–109

Base class for a context manager which allows changing of time zones. Subclasses may define a guard variable to either block or or allow time zone changes by redefining ``_guard_var_name`` and ``_guard_allows_change``. The default is that the guard variable must be affirmatively se

class TZContextBase(object):
    """
    Base class for a context manager which allows changing of time zones.

    Subclasses may define a guard variable to either block or or allow time
    zone changes by redefining ``_guard_var_name`` and ``_guard_allows_change``.
    The default is that the guard variable must be affirmatively set.

    Subclasses must define ``get_current_tz`` and ``set_current_tz``.
    """
    _guard_var_name = "DATEUTIL_MAY_CHANGE_TZ"
    _guard_allows_change = True

    def __init__(self, tzval):
        self.tzval = tzval
        self._old_tz = None

    @classmethod
    def tz_change_allowed(cls):
        """
        Class method used to query whether or not this class allows time zone
        changes.
        """
        guard = bool(os.environ.get(cls._guard_var_name, False))

        # _guard_allows_change gives the "default" behavior - if True, the
        # guard is overcoming a block. If false, the guard is causing a block.
        # Whether tz_change is allowed is therefore the XNOR of the two.
        return guard == cls._guard_allows_change

    @classmethod
    def tz_change_disallowed_message(cls):
        """ Generate instructions on how to allow tz changes """
        msg = ('Changing time zone not allowed. Set {envar} to {gval} '
               'if you would like to allow this behavior')

        return msg.format(envar=cls._guard_var_name,
                          gval=cls._guard_allows_change)

    def __enter__(self):
        if not self.tz_change_allowed():
            msg = self.tz_change_disallowed_message()
            pytest.skip(msg)

            # If this is used outside of a test suite, we still want an error.
            raise ValueError(msg)  # pragma: no cover

        self._old_tz = self.get_current_tz()
        self.set_current_tz(self.tzval)

    def __exit__(self, type, value, traceback):
        if self._old_tz is not None:
            self.set_current_tz(self._old_tz)

        self._old_tz = None

    def get_current_tz(self):
        raise NotImplementedError