B032: Possible unintentional type annotations instead of assignments

README.rst

@@ -184,6 +184,8 @@ It is therefore recommended to use a stacklevel of 2 or greater to provide more
 **B031**: Using the generator returned from `itertools.groupby()` more than once will do nothing on the
 second usage. Save the result to a list if the result is needed multiple times.
 
+**B032**: Possible unintentional type annotation (using ``:``). Did you mean to assign (using ``=``)?
+
 Opinionated warnings
 ~~~~~~~~~~~~~~~~~~~~
 
@@ -333,6 +335,7 @@ Future
 * B016: Warn when raising f-strings.
 * Add B028: Check for an explicit stacklevel keyword argument on the warn method from the warnings module.
 * Add B029: Check when trying to use ``except`` with an empty tuple i.e. ``except: ()``.
+* Add B032: Check for possible unintentional type annotations instead of assignments.
 
 23.1.20
 ~~~~~~~~~

bugbear.py

@@ -482,6 +482,10 @@ def visit_JoinedStr(self, node):
         self.check_for_b907(node)
         self.generic_visit(node)
 
+    def visit_AnnAssign(self, node):
+        self.check_for_b032(node)
+        self.generic_visit(node)
+
     def check_for_b005(self, node):
         if node.func.attr not in B005.methods:
             return  # method name doesn't match
@@ -1235,6 +1239,21 @@ def check_for_b028(self, node):
         ):
             self.errors.append(B028(node.lineno, node.col_offset))
 
+    def check_for_b032(self, node):
+        if (
+            node.value is None
+            and hasattr(node.target, "value")
+            and isinstance(node.target.value, ast.Name)
+            and (
+                isinstance(node.target, ast.Subscript)
+                or (
+                    isinstance(node.target, ast.Attribute)
+                    and node.target.value.id != "self"
+                )
+            )
+        ):
+            self.errors.append(B032(node.lineno, node.col_offset))
+
 
 def compose_call_path(node):
     if isinstance(node, ast.Attribute):
@@ -1625,6 +1644,13 @@ def visit_Lambda(self, node):
     )
 )
 
+B032 = Error(
+    message=(
+        "B032 Possible unintentional type annotation (using `:`). Did you mean to"
+        " assign (using `=`)?"
+    )
+)
+
 # Warnings disabled by default.
 B901 = Error(
     message=(

tests/b032.py

@@ -0,0 +1,29 @@
+"""
+Should emit:
+B032 - on lines 9, 10, 12, 13, 16-19
+"""
+
+# Flag these
+dct = {"a": 1}
+
+dct["b"]: 2
+dct.b: 2
+
+dct["b"]: "test"
+dct.b: "test"
+
+test = "test"
+dct["b"]: test
+dct["b"]: test.lower()
+dct.b: test
+dct.b: test.lower()
+
+# Do not flag below
+typed_dct: dict[str, int] = {"a": 1}
+typed_dct["b"] = 2
+typed_dct.b = 2
+
+
+class TestClass:
+    def test_self(self):
+        self.test: int

tests/test_bugbear.py

@@ -43,6 +43,7 @@
     B029,
     B030,
     B031,
+    B032,
     B901,
     B902,
     B903,
@@ -471,6 +472,22 @@ def test_b031(self):
         )
         self.assertEqual(errors, expected)
 
+    def test_b032(self):
+        filename = Path(__file__).absolute().parent / "b032.py"
+        bbc = BugBearChecker(filename=str(filename))
+        errors = list(bbc.run())
+        expected = self.errors(
+            B032(9, 0),
+            B032(10, 0),
+            B032(12, 0),
+            B032(13, 0),
+            B032(16, 0),
+            B032(17, 0),
+            B032(18, 0),
+            B032(19, 0),
+        )
+        self.assertEqual(errors, expected)
+
     @unittest.skipIf(sys.version_info < (3, 8), "not implemented for <3.8")
     def test_b907(self):
         filename = Path(__file__).absolute().parent / "b907.py"