Only flag __aenter__ when neither CM method checkpoints

When both `__aenter__` and `__aexit__` are defined and neither contains
a checkpoint, we used to flag (and autofix) both methods, which produced
redundant `lowlevel.checkpoint()` calls -- only one is needed for the
async context manager to checkpoint.

Prefer to report and fix `__aenter__` in this case; `__aexit__` is
exempted since adding a checkpoint to either satisfies the rule.

https://claude.ai/code/session_014jAydKywq31Ew4fVYGJdiG

Author: claude

flake8_async/visitors/visitor91x.py

@@ -590,12 +590,16 @@ def _is_exempt_async_cm_method(self, node: cst.FunctionDef) -> bool:
         if self.async_cm_class[name]:
             return False
         partner = "__aexit__" if name == "__aenter__" else "__aenter__"
-        if partner not in self.async_cm_class:
-            # Partner is not defined on this class; only assume it is inherited
-            # (and contains a checkpoint) if the class inherits from something.
-            return self.async_cm_class_has_bases
-        # Partner defined; exempt iff it contains a checkpoint.
-        return self.async_cm_class[partner]
+        if partner in self.async_cm_class:
+            # Partner is defined on the class; if it checkpoints, we're fine.
+            if self.async_cm_class[partner]:
+                return True
+            # Neither method checkpoints -- to avoid double-flagging (and a
+            # redundant autofix), we report and fix only `__aenter__`.
+            return name == "__aexit__"
+        # Partner is not defined on this class; only assume it is inherited
+        # (and contains a checkpoint) if the class inherits from something.
+        return self.async_cm_class_has_bases
 
     def visit_FunctionDef(self, node: cst.FunctionDef) -> bool:
         # `await` in default values happen in parent scope

tests/autofix_files/async910.py

@@ -674,9 +674,8 @@ async def __aenter__(self):  # error: 4, "exit", Stmt("function definition", lin
         print("setup")
         await trio.lowlevel.checkpoint()
 
-    async def __aexit__(self, *a):  # error: 4, "exit", Stmt("function definition", line)
+    async def __aexit__(self, *a):  # only __aenter__ is flagged to avoid redundancy
         print("teardown")
-        await trio.lowlevel.checkpoint()
 # fmt: on
 
 

tests/autofix_files/async910.py.diff

@@ -223,18 +223,14 @@
 
 
  async def foo_nested_empty_async():
-@@ x,9 x,11 @@
+@@ x,6 x,7 @@
  class CtxNeitherCheckpoint:
      async def __aenter__(self):  # error: 4, "exit", Stmt("function definition", line)
          print("setup")
 +        await trio.lowlevel.checkpoint()
 
-     async def __aexit__(self, *a):  # error: 4, "exit", Stmt("function definition", line)
+     async def __aexit__(self, *a):  # only __aenter__ is flagged to avoid redundancy
          print("teardown")
-+        await trio.lowlevel.checkpoint()
- # fmt: on
-
-
 @@ x,6 x,7 @@
      async def __aenter__(self):  # error: 4, "exit", Stmt("function definition", line)
          if _:

tests/eval_files/async910.py

@@ -643,7 +643,7 @@ class CtxNeitherCheckpoint:
     async def __aenter__(self):  # error: 4, "exit", Stmt("function definition", line)
         print("setup")
 
-    async def __aexit__(self, *a):  # error: 4, "exit", Stmt("function definition", line)
+    async def __aexit__(self, *a):  # only __aenter__ is flagged to avoid redundancy
         print("teardown")
 # fmt: on