B031: don't count store-context references as generator uses (#558)

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).

Author: StressTestor

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
 ~~~~~~~~

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)

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)