B909: visit if conditions and alternate branches

README.rst

@@ -611,6 +611,8 @@ UNRELEASED
 * B018: also report ``map()``, ``filter()``, ``reversed()``, ``zip()`` and
   ``enumerate()`` calls used as statements. They return lazy iterators, so
   ``map(print, items)`` on its own line never calls ``print`` (#452)
+* B909: also check mutations in ``if`` conditions and ``elif``/``else``
+  branches inside loops (#606)
 
 26.9.30
 ~~~~~~~

bugbear.py

@@ -3318,9 +3318,12 @@ def visit_Call(self, node: ast.Call) -> None:
         self.generic_visit(node)
 
     def visit_If(self, node: ast.If) -> None:
+        self.visit(node.test)
         self._conditional_block += 1
         self.visit(node.body)
         self._conditional_block += 1
+        self.visit(node.orelse)
+        self._conditional_block += 1
 
     def visit(self, node: ast.AST | list[ast.stmt]) -> ast.AST | list[ast.stmt] | None:
         """Like super-visit but supports iteration over lists."""

tests/eval_files/b909.py

@@ -177,3 +177,33 @@ def __init__(self, ls):
 
 for key in list(some_dict.keys()):
     some_dict.pop(key) # no error
+
+
+# Visit every branch and the condition of an if statement.
+some_list = [1, 2, 3]
+for elem in some_list:
+    if elem == 1:
+        pass
+    elif elem == 2:
+        some_list.remove(elem) # B909: 8
+    else:
+        some_list.clear() # B909: 8
+
+for _ in some_list:
+    if some_list.pop(): # B909: 7
+        break
+
+for elem in some_list:
+    if elem == 1:
+        some_list.remove(elem) # B909: 8
+    else:
+        some_list.clear()
+        break
+
+for elem in some_list:
+    if elem == 1:
+        pass
+    else:
+        some_list.remove(elem) # B909: 8
+        if elem == 2:
+            break