fix(b038): Restrict rule to mutating functions only (#453)

* fix(b038): Restrict rule to mutating functions only

Previous implementation produced false positives.
This fixes those and adds some more tests.

See #451

* fix(b038): Add dict.popitem() to list of mutating functions

bugbear.py

@@ -1598,6 +1598,19 @@ def compose_call_path(node):
 
 
 class B038Checker(ast.NodeVisitor):
+    # https://docs.python.org/3/library/stdtypes.html#mutable-sequence-types
+    MUTATING_FUNCTIONS = (
+        "append",
+        "sort",
+        "reverse",
+        "remove",
+        "clear",
+        "extend",
+        "insert",
+        "pop",
+        "popitem",
+    )
+
     def __init__(self, name: str):
         self.name = name
         self.mutations = []
@@ -1619,8 +1632,12 @@ def visit_Call(self, node: ast.Call):
         if isinstance(node.func, ast.Attribute):
             name = _to_name_str(node.func.value)
             function_object = name
+            function_name = node.func.attr
 
-            if function_object == self.name:
+            if (
+                function_object == self.name
+                and function_name in self.MUTATING_FUNCTIONS
+            ):
                 self.mutations.append(node)
 
         self.generic_visit(node)

tests/b038.py

@@ -45,3 +45,31 @@ def __init__(self, ls):
     print(elem)
     if elem % 2 == 0:
         del a.some_list[2]  # should error
+
+
+
+some_list = [1, 2, 3]
+for elem in some_list:
+    print(elem)
+    if elem == 2:
+        found_idx = some_list.index(elem)  # should not error
+        some_list.append(elem)  # should error
+        some_list.sort()  # should error
+        some_list.reverse()  # should error
+        some_list.clear()  # should error
+        some_list.extend([1,2])  # should error
+        some_list.insert(1, 1)  # should error
+        some_list.pop(1) # should error
+        some_list.pop() # should error
+        some_list = 3 # should error
+        break
+
+
+
+mydicts = {'a': {'foo': 1, 'bar': 2}}
+
+for mydict in mydicts:
+    if mydicts.get('a', ''):
+        print(mydict['foo'])  # should not error
+        mydicts.popitem() # should error
+

tests/test_bugbear.py

@@ -981,6 +981,15 @@ def test_b038(self):
             B038(27, 8),
             B038(41, 8),
             B038(47, 8),
+            B038(56, 8),
+            B038(57, 8),
+            B038(58, 8),
+            B038(59, 8),
+            B038(60, 8),
+            B038(61, 8),
+            B038(62, 8),
+            B038(63, 8),
+            B038(74, 8),
         ]
         self.assertEqual(errors, self.errors(*expected))