Add rule for unnecessary dict() calls (#500)

Author: adamchainz

CHANGELOG.rst

@@ -2,6 +2,8 @@
 Changelog
 =========
 
+* Add rule C418 to check for calls passing a dict literal or dict comprehension to ``dict()``.
+
 3.11.1 (2023-03-21)
 -------------------
 

README.rst

@@ -113,7 +113,7 @@ C409-410: Unnecessary ``<list/tuple>`` passed to ``<list/tuple>``\() - ``<advice
 Rules:
 
 * C409 Unnecessary ``<list/tuple>`` passed to tuple() - ``<advice>``.
-* C410 Unnecessary ``list passed to list() - ``<advice>``.
+* C410 Unnecessary list passed to list() - ``<advice>``.
 
 Where ``<advice>`` is either:
 
@@ -207,3 +207,12 @@ For example:
 * Rewrite ``list(map(lambda num: num * 2, nums))`` to ``[num * 2 for num in nums]``
 * Rewrite ``set(map(lambda num: num % 2 == 0, nums))`` to ``{num % 2 == 0 for num in nums}``
 * Rewrite ``dict(map(lambda v: (v, v ** 2), values))`` to ``{v : v ** 2 for v in values}``
+
+C418: Unnecessary ``<dict/dict comprehension>`` passed to dict() - remove the outer call to dict()
+--------------------------------------------------------------------------------------------------
+
+It's unnecessary to use a ``dict`` around a dict literal or dict comprehension, since either syntax already constructs a dict.
+For example:
+
+* Rewrite ``dict({})`` as ``{}``
+* Rewrite ``dict({"a": 1})`` as ``{"a": 1}``

src/flake8_comprehensions/__init__.py

@@ -43,6 +43,10 @@ def __init__(self, tree: ast.AST) -> None:
         "C415": "C415 Unnecessary subscript reversal of iterable within {func}().",
         "C416": "C416 Unnecessary {type} comprehension - rewrite using {type}().",
         "C417": "C417 Unnecessary use of map - use a {comp} instead.",
+        "C418": (
+            "C418 Unnecessary {type} passed to dict() - "
+            + "remove the outer call to dict()."
+        ),
     }
 
     def run(self) -> Generator[tuple[int, int, str, type[Any]], None, None]:
@@ -117,6 +121,23 @@ def run(self) -> Generator[tuple[int, int, str, type[Any]], None, None]:
                         type(self),
                     )
 
+                elif (
+                    num_positional_args == 1
+                    and num_keyword_args == 0
+                    and isinstance(node.args[0], (ast.Dict, ast.DictComp))
+                    and node.func.id == "dict"
+                ):
+                    if isinstance(node.args[0], ast.Dict):
+                        type_ = "dict"
+                    else:
+                        type_ = "dict comprehension"
+                    yield (
+                        node.lineno,
+                        node.col_offset,
+                        self.messages["C418"].format(type=type_),
+                        type(self),
+                    )
+
                 elif (
                     num_positional_args == 1
                     and isinstance(node.args[0], (ast.Tuple, ast.List))

tests/test_flake8_comprehensions.py

@@ -893,3 +893,48 @@ def test_C417_fail(code, failures, flake8_path):
     (flake8_path / "example.py").write_text(dedent(code))
     result = flake8_path.run_flake8()
     assert result.out_lines == failures
+
+
+@pytest.mark.parametrize(
+    "code",
+    [
+        "dict({}, a=1)",
+        "dict({x: 1 for x in range(1)}, a=1)",
+    ],
+)
+def test_C418_pass(code, flake8_path):
+    (flake8_path / "example.py").write_text(dedent(code))
+    result = flake8_path.run_flake8()
+    assert result.out_lines == []
+
+
+@pytest.mark.parametrize(
+    "code,failures",
+    [
+        (
+            "dict({})",
+            [
+                "./example.py:1:1: C418 Unnecessary dict passed to dict() - "
+                + "remove the outer call to dict()."
+            ],
+        ),
+        (
+            "dict({'a': 1})",
+            [
+                "./example.py:1:1: C418 Unnecessary dict passed to dict() - "
+                + "remove the outer call to dict()."
+            ],
+        ),
+        (
+            "dict({'x': 1 for x in range(10)})",
+            [
+                "./example.py:1:1: C418 Unnecessary dict comprehension passed "
+                + "to dict() - remove the outer call to dict()."
+            ],
+        ),
+    ],
+)
+def test_C418_fail(code, failures, flake8_path):
+    (flake8_path / "example.py").write_text(dedent(code))
+    result = flake8_path.run_flake8()
+    assert result.out_lines == failures