def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def _from_python(self, key, value):
        new_path = self._path + (key,)
        if isinstance(value, dict):
            value = self.custom_classes.get(new_path, ConfigDict)(value, new_path)
        elif isinstance(value, list):
            value = self.custom_classes.get(new_path, ConfigList)(value, new_path)
        elif isinstance(value, str):
            match = self.PROXY_VAR_RE.match(value)
            if match:
                value = self._make_proxy(key, match)
        return value