implement B035 check for static keys in dict-comprehension

README.rst

@@ -190,6 +190,8 @@ second usage. Save the result to a list if the result is needed multiple times.
 
 **B034**: Calls to `re.sub`, `re.subn` or `re.split` should pass `flags` or `count`/`maxsplit` as keyword arguments. It is commonly assumed that `flags` is the third positional parameter, forgetting about `count`/`maxsplit`, since many other `re` module functions are of the form `f(pattern, string, flags)`.
 
+**B035**: Found dict comprehension with a static key - either a constant value or variable not from the comprehension expression. This will result in a dict with a single key that was repeatedly overwritten.
+
 Opinionated warnings
 ~~~~~~~~~~~~~~~~~~~~
 

bugbear.py

@@ -483,6 +483,7 @@ def visit_SetComp(self, node):
 
     def visit_DictComp(self, node):
         self.check_for_b023(node)
+        self.check_for_b035(node)
         self.generic_visit(node)
 
     def visit_GeneratorExp(self, node):
@@ -954,6 +955,36 @@ def check_for_b031(self, loop_node):  # noqa: C901
                                 B031(node.lineno, node.col_offset, vars=(node.id,))
                             )
 
+    def _get_names_from_tuple(self, node: ast.Tuple):
+        for dim in node.elts:
+            if isinstance(dim, ast.Name):
+                yield dim.id
+            elif isinstance(dim, ast.Tuple):
+                yield from self._get_names_from_tuple(dim)
+
+    def _get_dict_comp_loop_var_names(self, node: ast.DictComp):
+        for gen in node.generators:
+            if isinstance(gen.target, ast.Name):
+                yield gen.target.id
+            elif isinstance(gen.target, ast.Tuple):
+                yield from self._get_names_from_tuple(gen.target)
+
+    def check_for_b035(self, node: ast.DictComp):
+        """Check that a static key isn't used in a dict comprehension.
+
+        Emit a warning if a likely unchanging key is used - either a constant,
+        or a variable that isn't coming from the generator expression.
+        """
+        if isinstance(node.key, ast.Constant):
+            self.errors.append(
+                B035(node.key.lineno, node.key.col_offset, vars=(node.key.value,))
+            )
+        elif isinstance(node.key, ast.Name):
+            if node.key.id not in self._get_dict_comp_loop_var_names(node):
+                self.errors.append(
+                    B035(node.key.lineno, node.key.col_offset, vars=(node.key.id,))
+                )
+
     def _get_assigned_names(self, loop_node):
         loop_targets = (ast.For, ast.AsyncFor, ast.comprehension)
         for node in children_in_scope(loop_node):
@@ -1884,6 +1915,8 @@ def visit_Lambda(self, node):
         " due to unintuitive argument positions."
     )
 )
+B035 = Error(message="B035 Static key in dict comprehension {!r}.")
+
 
 # Warnings disabled by default.
 B901 = Error(

tests/b035.py

@@ -0,0 +1,35 @@
+# OK - consts in regular dict
+regular_dict = {"a": 1, "b": 2}
+regular_nested_dict = {"a": 1, "nested": {"b": 2, "c": "three"}}
+
+# bad - const key in dict comprehension
+bad_const_key_str = {"a": i for i in range(3)}
+bad_const_key_int = {1: i for i in range(3)}
+
+# OK - const value in dict comp
+const_val = {i: "a" for i in range(3)}
+
+# OK - expression with const in dict comp
+key_expr_with_const = {i * i: i for i in range(3)}
+key_expr_with_const2 = {"a" * i: i for i in range(3)}
+
+# nested
+nested_bad_and_good = {
+    "good": {"a": 1, "b": 2},
+    "bad": {"a": i for i in range(3)},
+}
+
+CONST_KEY_VAR = "KEY"
+
+# bad
+bad_const_key_var = {CONST_KEY_VAR: i for i in range(3)}
+
+# OK - variable from tuple
+var_from_tuple = {k: v for k, v in {}.items()}
+
+# OK - variable from nested tuple
+var_from_nested_tuple = {v2: k for k, (v1, v2) in {"a": (1, 2)}.items()}
+
+# bad - variabe not from generator
+v3 = 1
+bad_var_not_from_nested_tuple = {v3: k for k, (v1, v2) in {"a": (1, 2)}.items()}

tests/test_bugbear.py

@@ -43,6 +43,7 @@
     B032,
     B033,
     B034,
+    B035,
     B901,
     B902,
     B903,
@@ -521,6 +522,19 @@ def test_b034(self):
         )
         self.assertEqual(errors, expected)
 
+    def test_b035(self):
+        filename = Path(__file__).absolute().parent / "b035.py"
+        bbc = BugBearChecker(filename=str(filename))
+        errors = list(bbc.run())
+        expected = self.errors(
+            B035(6, 21, vars=("a",)),
+            B035(7, 21, vars=(1,)),
+            B035(19, 12, vars=("a",)),
+            B035(25, 21, vars=("CONST_KEY_VAR",)),
+            B035(35, 33, vars=("v3",)),
+        )
+        self.assertEqual(errors, expected)
+
     def test_b908(self):
         filename = Path(__file__).absolute().parent / "b908.py"
         bbc = BugBearChecker(filename=str(filename))