code: def from_envvar(self, variable_name: str, silent: bool=False) -> None:
          
          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)
docs: 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)