code: def from_string(cls, key, password='notasecret'):
          
          key = _helpers._from_bytes(key)  # pem expects str in Py3
          marker_id, key_bytes = pem.readPemBlocksFromFile(
              six.StringIO(key), _PKCS1_MARKER, _PKCS8_MARKER)
  
          if marker_id == 0:
              pkey = rsa.key.PrivateKey.load_pkcs1(key_bytes,
                                                   format='DER')
          elif marker_id == 1:
              key_info, remaining = decoder.decode(
                  key_bytes, asn1Spec=_PKCS8_SPEC)
              if remaining != b'':
                  raise ValueError('Unused bytes', remaining)
              pkey_info = key_info.getComponentByName('privateKey')
              pkey = rsa.key.PrivateKey.load_pkcs1(pkey_info.asOctets(),
                                                   format='DER')
          else:
              raise ValueError('No key could be detected.')
  
          return cls(pkey)
docs: Construct an RsaSigner instance from a string.
  
          Args:
              key: string, private key in PEM format.
              password: string, password for private key file. Unused for PEM
                        files.
  
          Returns:
              RsaSigner instance.
  
          Raises:
              ValueError if the key cannot be parsed as PKCS#1 or PKCS#8 in
              PEM format.