code: def from_string(key, password='notasecret'):
          
          parsed_pem_key = _helpers._parse_pem_key(_helpers._to_bytes(key))
          if parsed_pem_key:
              pkey = RSA.importKey(parsed_pem_key)
          else:
              raise NotImplementedError(
                  'No key in PEM format was detected. This implementation '
                  'can only use the PyCrypto library for keys in PEM '
                  'format.')
          return PyCryptoSigner(pkey)
docs: Construct a Signer instance from a string.
  
          Args:
              key: string, private key in PEM format.
              password: string, password for private key file. Unused for PEM
                        files.
  
          Returns:
              Signer instance.
  
          Raises:
              NotImplementedError if the key isn't in PEM format.