def coerce(cls, string):
        """Create a :class:`Requirement` object from a given package spec."""
        match = re.match(cls.spec_regex, string)
        if not match:
            raise InvalidRequirementSpecError("could not parse requirement")
        package = match.group(1)
        if all(match.group(2, 3)):
            version = "".join(match.group(2, 3))
        else:
            version = None
        return cls(package, version)
def update(self, *args, **kwargs):
        chain = []
        for arg in args:
            if isinstance(arg, dict):
                iterator = arg.items()
            else:
                iterator = arg
            chain = itertools.chain(chain, iterator)
        if kwargs:
            chain = itertools.chain(chain, kwargs.items())
        for key, value in iterator:
            self._collection[key] = self._from_python(key, value)