Do not require typing_extensions at runtime

There are type annotations that indirectly depend on the
typing_extensions module (on Python 3.5-3.7: Protocol was added to the
core typing module in 3.8). The typing_extensions module is not
installed by default, so the code didn't run on a pristine Python
installation.

To avoid depending on a non-default module, make the dependency on
typing_extensions optional. (It's still required to run mypy, but
installing mypy takes care of providing typing_extensions.) If it
isn't available, provide a substitute definition that's just good
enough to get the scripts to run.

Move this ugly code to its own module to avoid the temptation of
spreading such ugliness all over the place. It's likely to be used in
other modules anyway.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/mbedtls_dev/test_case.py

@@ -19,15 +19,9 @@
 import binascii
 import os
 import sys
-from typing import Any, Iterable, List, Optional
+from typing import Iterable, List, Optional
-import typing_extensions #pylint: disable=import-error
-
-class Writable(typing_extensions.Protocol):
-    """Abstract class for typing hints."""
-    # pylint: disable=no-self-use,too-few-public-methods,unused-argument
-    def write(self, text: str) -> Any:
-        ...
 
+from mbedtls_dev import typing_util
 
 def hex_string(data: bytes) -> str:
     return '"' + binascii.hexlify(data).decode('ascii') + '"'
@@ -70,7 +64,7 @@ class TestCase:
         if self.function is None:
             raise MissingFunction
 
-    def write(self, out: Writable) -> None:
+    def write(self, out: typing_util.Writable) -> None:
         """Write the .data file paragraph for this test case.
 
         The output starts and ends with a single newline character. If the

scripts/mbedtls_dev/typing_util.py

@@ -0,0 +1,39 @@
+"""Auxiliary definitions used in type annotations.
+"""
+
+# Copyright The Mbed TLS Contributors
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from typing import Any
+
+# The typing_extensions module is necessary for type annotations that are
+# checked with mypy. It is only used for type annotations or to define
+# things that are themselves only used for type annotations. It is not
+# available on a default Python installation. Therefore, try loading
+# what we need from it for the sake of mypy (which depends on, or comes
+# with, typing_extensions), and if not define substitutes that lack the
+# static type information but are good enough at runtime.
+try:
+    from typing_extensions import Protocol #pylint: disable=import-error
+except ImportError:
+    class Protocol: #type: ignore
+        #pylint: disable=too-few-public-methods
+        pass
+
+class Writable(Protocol):
+    """Abstract class for typing hints."""
+    # pylint: disable=no-self-use,too-few-public-methods,unused-argument
+    def write(self, text: str) -> Any:
+        ...