Add B041: Duplicate key-value pairs in dictionary literals

README.rst

@@ -205,6 +205,8 @@ second usage. Save the result to a list if the result is needed multiple times.
 
 **B040**: Caught exception with call to ``add_note`` not used. Did you forget to ``raise`` it?
 
+**B041**: Repeated key-value pair in dictionary literal.
+
 Opinionated warnings
 ~~~~~~~~~~~~~~~~~~~~
 

bugbear.py

@@ -8,7 +8,7 @@
 import re
 import sys
 import warnings
-from collections import defaultdict, namedtuple
+from collections import Counter, defaultdict, namedtuple
 from contextlib import suppress
 from functools import lru_cache, partial
 from keyword import iskeyword
@@ -362,6 +362,17 @@ class B040CaughtException:
     has_note: bool
 
 
+class B041UnhandledKeyType:
+    """
+    A dictionary key of a type that we do not check for duplicates.
+    """
+
+
+@attr.define(frozen=True)
+class B041VariableKeyType:
+    name: str
+
+
 @attr.s
 class BugBearVisitor(ast.NodeVisitor):
     filename = attr.ib()
@@ -633,6 +644,35 @@ def visit_Set(self, node) -> None:
         self.check_for_b033(node)
         self.generic_visit(node)
 
+    def visit_Dict(self, node) -> None:
+        self.check_for_b041(node)
+        self.generic_visit(node)
+
+    def check_for_b041(self, node) -> None:
+        # Complain if there are duplicate key-value pairs in a dictionary literal.
+        def convert_to_value(item):
+            if isinstance(item, ast.Constant):
+                return item.value
+            elif isinstance(item, ast.Tuple):
+                return tuple(convert_to_value(i) for i in item.elts)
+            elif isinstance(item, ast.Name):
+                return B041VariableKeyType(item.id)
+            else:
+                return B041UnhandledKeyType()
+
+        keys = [convert_to_value(key) for key in node.keys]
+        key_counts = Counter(keys)
+        duplicate_keys = [key for key, count in key_counts.items() if count > 1]
+        for key in duplicate_keys:
+            key_indices = [i for i, i_key in enumerate(keys) if i_key == key]
+            seen = set()
+            for index in key_indices:
+                value = convert_to_value(node.values[index])
+                if value in seen:
+                    key_node = node.keys[index]
+                    self.errors.append(B041(key_node.lineno, key_node.col_offset))
+                seen.add(value)
+
     def check_for_b005(self, node) -> None:
         if isinstance(node, ast.Import):
             for name in node.names:
@@ -2327,6 +2367,8 @@ def visit_Lambda(self, node) -> None:
     message="B040 Exception with added note not used. Did you forget to raise it?"
 )
 
+B041 = Error(message=("B041 Repeated key-value pair in dictionary literal."))
+
 # Warnings disabled by default.
 B901 = Error(
     message=(

tests/b041.py

@@ -0,0 +1,24 @@
+a = 1
+test = {'yes': 1, 'yes': 1}
+test = {'yes': 1, 'yes': 1, 'no': 2, 'no': 2}
+test = {'yes': 1, 'yes': 1, 'yes': 1}
+test = {1: 1, 1.0: 1}
+test = {True: 1, True: 1}
+test = {None: 1, None: 1}
+test = {a: a, a: a}
+
+# no error if either keys or values are different
+test = {'yes': 1, 'yes': 2}
+test = {1: 1, 2: 1}
+test = {(0, 1): 1, (0, 2): 1}
+test = {(0, 1): 1, (0, 1): 2}
+b = 1
+test = {a: a, b: a}
+test = {a: a, a: b}
+class TestClass:
+    pass
+f = TestClass()
+f.a = 1
+test = {f.a: 1, f.a: 1}
+
+

tests/test_bugbear.py

@@ -48,6 +48,7 @@
     B037,
     B039,
     B040,
+    B041,
     B901,
     B902,
     B903,
@@ -666,6 +667,23 @@ def test_b040(self) -> None:
         )
         self.assertEqual(errors, expected)
 
+    def test_b041(self) -> None:
+        filename = Path(__file__).absolute().parent / "b041.py"
+        bbc = BugBearChecker(filename=str(filename))
+        errors = list(bbc.run())
+        expected = self.errors(
+            B041(2, 18),
+            B041(3, 18),
+            B041(3, 37),
+            B041(4, 18),
+            B041(4, 28),
+            B041(5, 14),
+            B041(6, 17),
+            B041(7, 17),
+            B041(8, 14),
+        )
+        self.assertEqual(errors, expected)
+
     def test_b908(self):
         filename = Path(__file__).absolute().parent / "b908.py"
         bbc = BugBearChecker(filename=str(filename))