Add a deprecation warning when jwt.decode() is called with the legacy…

… verify= argument (#742)

Since the arbitrary/unused `**kwargs` can't quite be dropped (as #657 would do) without
a major version bump (as reverted in #701), it's still a good idea to warn users if they
are attempting to use contradictory arguments for the security-sensitive `verify=` argument.

jwt/api_jwt.py

@@ -1,4 +1,5 @@
 import json
+import warnings
 from calendar import timegm
 from collections.abc import Iterable, Mapping
 from datetime import datetime, timedelta, timezone
@@ -75,6 +76,17 @@ def decode_complete(
         else:
             options.setdefault("verify_signature", True)
 
+        # If the user has set the legacy `verify` argument, and it doesn't match
+        # what the relevant `options` entry for the argument is, inform the user
+        # that they're likely making a mistake.
+        if "verify" in kwargs and kwargs["verify"] != options["verify_signature"]:
+            warnings.warn(
+                "The `verify` argument to `decode` does nothing in PyJWT 2.0 and newer. "
+                "The equivalent is setting `verify_signature` to False in the `options` dictionary. "
+                "This invocation has a mismatch between the kwarg and the option entry.",
+                category=DeprecationWarning,
+            )
+
         if not options["verify_signature"]:
             options.setdefault("verify_exp", False)
             options.setdefault("verify_nbf", False)

tests/test_api_jwt.py

@@ -658,3 +658,19 @@ def test_decode_no_algorithms_verify_signature_false(self, jwt, payload):
         jwt_message = jwt.encode(payload, secret)
 
         jwt.decode(jwt_message, secret, options={"verify_signature": False})
+
+    def test_decode_legacy_verify_warning(self, jwt, payload):
+        secret = "secret"
+        jwt_message = jwt.encode(payload, secret)
+
+        with pytest.deprecated_call():
+            # The implicit default for options.verify_signature is True,
+            # but the user sets verify to False.
+            jwt.decode(jwt_message, secret, verify=False, algorithms=["HS256"])
+
+        with pytest.deprecated_call():
+            # The user explicitly sets verify=True,
+            # but contradicts it in verify_signature.
+            jwt.decode(
+                jwt_message, secret, verify=True, options={"verify_signature": False}
+            )