Expose get_algorithm_by_name as new method

Looking up an algorithm by name is used internally for signature
generation. This encapsulates that functionality in a dedicated method
and adds it to the public API. No new tests are needed to exercise the
functionality.

Rationale:

1. Inside of PyJWS, this improves the code. The KeyError handler is
   better scoped and the signing code reads more directly.

2. This is part of the path to supporting OIDC at_hash validation as a
   use-case (see: jpadilla#295, jpadilla#296, jpadilla#314).

This is arguably sufficient to consider that use-case supported and
close it. However, it is an improvement and step in the right
direction in either case.

A minor change was needed to satisfy mypy, as a union-typed variable
does not narrow its type based on assignments. The easiest resolution
is to use a new name, in this case, simply `algorithm -> algorithm_`.

CHANGELOG.rst

@@ -19,6 +19,8 @@ Fixed
 Added
 ~~~~~
 - Add to_jwk static method to ECAlgorithm by @leonsmith in https://github.com/jpadilla/pyjwt/pull/732
+- Add ``get_algorithm_by_name`` as a method of ``PyJWS`` objects, and expose
+  the global PyJWS method as part of the public API
 
 `v2.4.0 <https://github.com/jpadilla/pyjwt/compare/2.3.0...2.4.0>`__
 -----------------------------------------------------------------------

jwt/__init__.py

@@ -1,6 +1,7 @@
 from .api_jwk import PyJWK, PyJWKSet
 from .api_jws import (
     PyJWS,
+    get_algorithm_by_name,
     get_unverified_header,
     register_algorithm,
     unregister_algorithm,
@@ -51,6 +52,7 @@
     "get_unverified_header",
     "register_algorithm",
     "unregister_algorithm",
+    "get_algorithm_by_name",
     # Exceptions
     "DecodeError",
     "ExpiredSignatureError",

jwt/api_jws.py

@@ -73,6 +73,23 @@ def get_algorithms(self):
         """
         return list(self._valid_algs)
 
+    def get_algorithm_by_name(self, alg_name: str) -> Algorithm:
+        """
+        For a given string name, return the matching Algorithm object.
+
+        Example usage:
+
+        >>> jws_obj.get_algorithm_by_name("RS256")
+        """
+        try:
+            return self._algorithms[alg_name]
+        except KeyError as e:
+            if not has_crypto and alg_name in requires_cryptography:
+                raise NotImplementedError(
+                    f"Algorithm '{alg_name}' could not be found. Do you have cryptography installed?"
+                ) from e
+            raise NotImplementedError("Algorithm not supported") from e
+
     def encode(
         self,
         payload: bytes,
@@ -84,21 +101,21 @@ def encode(
     ) -> str:
         segments = []
 
-        if algorithm is None:
-            algorithm = "none"
+        # declare a new var to narrow the type for type checkers
+        algorithm_: str = algorithm if algorithm is not None else "none"
 
         # Prefer headers values if present to function parameters.
         if headers:
             headers_alg = headers.get("alg")
             if headers_alg:
-                algorithm = headers["alg"]
+                algorithm_ = headers["alg"]
 
             headers_b64 = headers.get("b64")
             if headers_b64 is False:
                 is_payload_detached = True
 
         # Header
-        header = {"typ": self.header_typ, "alg": algorithm}  # type: Dict[str, Any]
+        header = {"typ": self.header_typ, "alg": algorithm_}  # type: Dict[str, Any]
 
         if headers:
             self._validate_headers(headers)
@@ -128,17 +145,9 @@ def encode(
         # Segments
         signing_input = b".".join(segments)
 
-        try:
-            alg_obj = self._algorithms[algorithm]
-            key = alg_obj.prepare_key(key)
-            signature = alg_obj.sign(signing_input, key)
-
-        except KeyError as e:
-            if not has_crypto and algorithm in requires_cryptography:
-                raise NotImplementedError(
-                    f"Algorithm '{algorithm}' could not be found. Do you have cryptography installed?"
-                ) from e
-            raise NotImplementedError("Algorithm not supported") from e
+        alg_obj = self.get_algorithm_by_name(algorithm_)
+        key = alg_obj.prepare_key(key)
+        signature = alg_obj.sign(signing_input, key)
 
         segments.append(base64url_encode(signature))
 
@@ -286,4 +295,5 @@ def _validate_kid(self, kid):
 decode = _jws_global_obj.decode
 register_algorithm = _jws_global_obj.register_algorithm
 unregister_algorithm = _jws_global_obj.unregister_algorithm
+get_algorithm_by_name = _jws_global_obj.get_algorithm_by_name
 get_unverified_header = _jws_global_obj.get_unverified_header