gh-104050: Run mypy on `clinic.py` in CI (#104421)

* Add basic mypy workflow to CI
* Make the type check pass

---------

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
Co-authored-by: Nikita Sobolev <mail@sobolevn.me>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

1 files changed, 16 insertions, 10 deletions

diff --git a/Tools/clinic/cpp.py b/Tools/clinic/cpp.py
index 77f5f96..bc2cc71 100644
--- a/Tools/clinic/cpp.py
+++ b/Tools/clinic/cpp.py
@@ -1,7 +1,12 @@
 import re
 import sys
+from collections.abc import Callable
 
-def negate(condition):
+
+TokenAndCondition = tuple[str, str]
+TokenStack = list[TokenAndCondition]
+
+def negate(condition: str) -> str:
     """
     Returns a CPP conditional that is the opposite of the conditional passed in.
     """
@@ -22,17 +27,18 @@ class Monitor:
     Anyway this implementation seems to work well enough for the CPython sources.
     """
 
+    is_a_simple_defined: Callable[[str], re.Match[str] | None]
     is_a_simple_defined = re.compile(r'^defined\s*\(\s*[A-Za-z0-9_]+\s*\)$').match
 
-    def __init__(self, filename=None, *, verbose=False):
-        self.stack = []
+    def __init__(self, filename=None, *, verbose: bool = False):
+        self.stack: TokenStack = []
         self.in_comment = False
-        self.continuation = None
+        self.continuation: str | None = None
         self.line_number = 0
         self.filename = filename
         self.verbose = verbose
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return ''.join((
             '<Monitor ',
             str(id(self)),
@@ -40,10 +46,10 @@ class Monitor:
             " condition=", repr(self.condition()),
             ">"))
 
-    def status(self):
+    def status(self) -> str:
         return str(self.line_number).rjust(4) + ": " + self.condition()
 
-    def condition(self):
+    def condition(self) -> str:
         """
         Returns the current preprocessor state, as a single #if condition.
         """
@@ -62,15 +68,15 @@ class Monitor:
         if self.stack:
             self.fail("Ended file while still in a preprocessor conditional block!")
 
-    def write(self, s):
+    def write(self, s: str) -> None:
         for line in s.split("\n"):
             self.writeline(line)
 
-    def writeline(self, line):
+    def writeline(self, line: str) -> None:
         self.line_number += 1
         line = line.strip()
 
-        def pop_stack():
+        def pop_stack() -> TokenAndCondition:
             if not self.stack:
                 self.fail("#" + token + " without matching #if / #ifdef / #ifndef!")
             return self.stack.pop()