fix(b909): Fix false positive affecting containers of mutables (#469)

* fix(b909): Fix false positive affecting containers of mutables

The false positives occurred when trying to edit a dictionary while
iterating over a list of dictionaries:

```
lst: list[dict] = [{}, {}, {}]
for dic in lst:
    dic["key"] = False # was false positive - fixed now
```

* fix(b909): Allow mutation of dict[key] form

These changes allow the following:
```
some_dict = {"foo": "bar"}
for key in some_dict:
    some_dict[key] = 3 # no error (previously error'd)
```

* fix(b909): Fix python 3.8 incompatibility

Turns out, that the slice type was changed in python 3.9.

> Changed in version 3.9: Simple indices are represented by their value,
> extended slices are represented as tuples.
from https://docs.python.org/3/library/ast.html#module-ast

bugbear.py

@@ -1581,11 +1581,13 @@ def check(num_args, param_name):
     def check_for_b909(self, node: ast.For):
         if isinstance(node.iter, ast.Name):
             name = _to_name_str(node.iter)
+            key = _to_name_str(node.target)
         elif isinstance(node.iter, ast.Attribute):
             name = _to_name_str(node.iter)
+            key = _to_name_str(node.target)
         else:
             return
-        checker = B909Checker(name)
+        checker = B909Checker(name, key)
         checker.visit(node.body)
         for mutation in itertools.chain.from_iterable(
             m for m in checker.mutations.values()
@@ -1603,6 +1605,46 @@ def compose_call_path(node):
         yield node.id
 
 
+def _tansform_slice_to_py39(slice: ast.Slice) -> ast.Slice | ast.Name:
+    """Transform a py38 style slice to a py39 style slice.
+
+    In py39 the slice was changed to have simple names directly assigned:
+    ```py
+    # code:
+    some_dict[key]
+    # py38:
+    slice=Index(
+        value=Name(
+                lineno=152,
+                col_offset=14,
+                end_lineno=152,
+                end_col_offset=17,
+                id='key',
+                ctx=Load()
+            ),
+    )
+    # py39 onwards:
+    slice=Name(
+                lineno=152,
+                col_offset=14,
+                end_lineno=152,
+                end_col_offset=17,
+                id='key',
+                ctx=Load()
+            ),
+    ```
+
+    > Changed in version 3.9: Simple indices are represented by their value,
+    > extended slices are represented as tuples.
+    from https://docs.python.org/3/library/ast.html#module-ast
+    """
+    if sys.version_info >= (3, 9):
+        return slice
+    if isinstance(slice, ast.Index) and isinstance(slice.value, ast.Name):
+        slice = slice.value
+    return slice
+
+
 class B909Checker(ast.NodeVisitor):
     # https://docs.python.org/3/library/stdtypes.html#mutable-sequence-types
     MUTATING_FUNCTIONS = (
@@ -1624,14 +1666,19 @@ class B909Checker(ast.NodeVisitor):
         "discard",
     )
 
-    def __init__(self, name: str):
+    def __init__(self, name: str, key: str):
         self.name = name
+        self.key = key
         self.mutations = defaultdict(list)
         self._conditional_block = 0
 
     def visit_Assign(self, node: ast.Assign):
         for target in node.targets:
-            if isinstance(target, ast.Subscript) and _to_name_str(target.value):
+            if (
+                isinstance(target, ast.Subscript)
+                and _to_name_str(target.value) == self.name
+                and _to_name_str(_tansform_slice_to_py39(target.slice)) != self.key
+            ):
                 self.mutations[self._conditional_block].append(node)
         self.generic_visit(node)
 

tests/b909.py

@@ -127,3 +127,26 @@ def __init__(self, ls):
         bar.remove(1)
         break
     break
+
+lst: list[dict] = [{}, {}, {}]
+for dic in lst:
+    dic["key"] = False # no error
+
+
+
+for grammar in grammars:
+    errors[grammar.version] = InvalidInput() # no error
+
+
+
+for key in self.hpo_params:
+    if key in nni_config:
+        nni_config[key] = self.hpo_params[key] # no error
+
+
+some_dict = {"foo": "bar"}
+for key in some_dict:
+    some_dict[key] = 3 # no error
+
+for key in some_dict.keys():
+    some_dict[key] = 3 # no error