import rsa only when needed

Author: daniel-sanche

google/auth/crypt/rsa.py

@@ -24,7 +24,6 @@
 
 from google.auth import _helpers
 from google.auth.crypt import _cryptography_rsa
-from google.auth.crypt import _python_rsa
 from google.auth.crypt import base
 
 RSA_KEY_MODULE_PREFIX = "rsa.key"
@@ -43,6 +42,7 @@ class RSAVerifier(base.Verifier):
         public_key (Union[rsa.key.PublicKey, cryptography.hazmat.primitives.asymmetric.rsa.RSAPublicKey]):
             The public key used to verify signatures.
     Raises:
+        ImportError: if called with an rsa.key.PublicKey, when the rsa library is not installed
         ValueError: if an unrecognized public key is provided
     """
 
@@ -51,6 +51,7 @@ def __init__(self, public_key):
         if isinstance(public_key, RSAPublicKey):
             impl_lib = _cryptography_rsa
         elif module_str.startswith(RSA_KEY_MODULE_PREFIX):
+            from google.auth.crypt import _python_rsa
             impl_lib = _python_rsa
         else:
             raise ValueError(f"unrecognized public key type: {type(public_key)}")
@@ -97,6 +98,7 @@ class RSASigner(base.Signer, base.FromServiceAccountMixin):
             public key or certificate.
 
     Raises:
+        ImportError: if called with an rsa.key.PublicKey, when the rsa library is not installed
         ValueError: if an unrecognized public key is provided
     """
 
@@ -105,6 +107,7 @@ def __init__(self, private_key, key_id=None):
         if isinstance(private_key, RSAPrivateKey):
             impl_lib = _cryptography_rsa
         elif module_str.startswith(RSA_KEY_MODULE_PREFIX):
+            from google.auth.crypt import _python_rsa
             impl_lib = _python_rsa
         else:
             raise ValueError(f"unrecognized private key type: {type(private_key)}")