From 319b15501d0d81ad67548eb2883dbbf5e1897565 Mon Sep 17 00:00:00 2001
From: StressTestor <212606152+StressTestor@users.noreply.github.com>
Date: Mon, 15 Jun 2026 19:10:23 -0600
Subject: [PATCH] B031: don't count store-context references as generator uses

A reference to the groupby loop variable in store context (an annotation
target like `group: T`, or an assignment target) was counted as a use of
the generator, so a single real use plus an annotation tripped a
false-positive B031. Count only loads.

Addresses #465 (the annotation/store-context case; the if/else case is
handled separately by #557).
---
 README.rst               |  1 +
 bugbear.py               | 11 +++++++++--
 tests/eval_files/b031.py |  6 ++++++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/README.rst b/README.rst
index d8cfd18..c806a12 100644
--- a/README.rst
+++ b/README.rst
@@ -499,6 +499,7 @@ UNRELEASED
 ~~~~~~~~~~
 
 * B018: handle also useless calls such as `isinstance(x, int)` without assigning or using the result
+* B031: don't count a store-context reference (e.g. an annotation target like `group: T`) as a use of the `groupby` generator (#465)
 
 25.11.29
 ~~~~~~~~
diff --git a/bugbear.py b/bugbear.py
index 329c435..c7d67cc 100644
--- a/bugbear.py
+++ b/bugbear.py
@@ -1215,11 +1215,18 @@ def check_for_b031(self, loop_node: ast.For) -> None:  # noqa: C901
                             if (
                                 isinstance(nested_node, ast.Name)
                                 and nested_node.id == group_name
+                                and isinstance(nested_node.ctx, ast.Load)
                             ):
                                 self.add_error("B031", nested_node, nested_node.id)
 
-                    # Handle multiple uses
-                    if isinstance(node, ast.Name) and node.id == group_name:
+                    # Handle multiple uses. Count only loads: a store-context
+                    # reference, such as an annotation target (`group: T`), is
+                    # not a read of the generator (#465).
+                    if (
+                        isinstance(node, ast.Name)
+                        and node.id == group_name
+                        and isinstance(node.ctx, ast.Load)
+                    ):
                         num_usages += 1
                         if num_usages > 1:
                             self.add_error("B031", node, node.id)
diff --git a/tests/eval_files/b031.py b/tests/eval_files/b031.py
index 6580076..fd51a70 100644
--- a/tests/eval_files/b031.py
+++ b/tests/eval_files/b031.py
@@ -63,3 +63,9 @@ def collect_shop_items(shopper, items):
 ):
     collect_shop_items("Jane", group[1])
     collect_shop_items("Joe", group[1])
+
+
+# Annotating the loop variable is not a second usage of the generator (#465)
+for _section, section_items in groupby(items, key=lambda p: p[1]):
+    section_items: list
+    collect_shop_items("Jane", section_items)