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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def from_envvar(self, variable_name: str, silent: bool=False) -> None: """Load the configuration from a location specified in the environment. This will load a cfg file using :meth:`from_pyfile` from the location specified in the environment, for example the two blocks below are equivalent. .. code-block:: python app.config.from_envvar('CONFIG') .. code-block:: python filename = os.environ['CONFIG'] app.config.from_pyfile(filename) """

        value = os.environ.get(variable_name)
        if value is None and not silent:
            raise RuntimeError(
                f"Environment variable {variable_name} is not present, cannot load config",
            )
        return self.from_pyfile(value)