def load(self, filename, create_if_missing=True):
        filename = Path(filename)
        if not filename.exists() and create_if_missing:
            self.load_pip_requirements()
            with filename.open("w") as f:
                f.write(yaml.dump(self.serialized, default_flow_style=False,
                                  encoding=None))
            self.filename = filename
            return self.save(filename)
        with filename.open() as f:
            for group, requirements in yaml.load(f.read()).items():
                for requirement in requirements:
                    self[group].add(Requirement.coerce(requirement))
        self.filename = filename
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