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 create_node(self, *args, **kwargs):
        nbparam = len(args) + len(kwargs)
        assert nbparam in (0, len(Fields)), \
            "Bad argument number for {}: {}, expecting {}".\
            format(Name, nbparam, len(Fields))
        self._fields = Fields
        self._attributes = Attributes
        for argname, argval in zip(self._fields, args):
            setattr(self, argname, argval)
        for argname, argval in kwargs.items():
            assert argname in Fields, \
                    "Invalid Keyword argument for {}: {}".format(Name, argname)
            setattr(self, argname, argval)