Repository: adamchainz/flake8-comprehensions (python)

Issue:
Feature Request: Detect dictionary comprehensions with constant values
### Description

Hi! Thanks for this helpful tool.

I'm working with code that has a lot of 

```python
{elt: None for elt in foo}
```

which is slower and (IMO) harder to read than

```python
dict.fromkeys(foo)
```

Would there be interest in a rule that checks for dictionary comprehensions with constant values?

This has rule has been accepted by [another project](https://github.com/astral-sh/ruff/issues/9592) and it was suggested to me that I create an issue here too.

Pull request description:
Add rule for dict comprehensions with constant values
Adds `C420` that checks for dictionary comprehensions that use constant values.

Changed the fixture for `test_C418_pass` as it was failing (`dict({x: 1 for x in range(1)}, a=1)` triggers the new rule).

Closes #552.

Patch:
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 25b7c11..da54167 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -2,6 +2,10 @@
 Changelog
 =========
 
+* Add rule C420 to check for dict comprehensions with constant values, encouraging replacement with ``dict.fromkeys()``.
+
+  Thanks to Tom Kuson in `PR #553 <https://github.com/adamchainz/flake8-comprehensions/pull/553>`__.
+
 3.14.0 (2023-07-10)
 -------------------
 
diff --git a/README.rst b/README.rst
index 3b6cbbd..ba9d844 100644
--- a/README.rst
+++ b/README.rst
@@ -227,3 +227,13 @@ For example:
 
 * Rewrite ``all([condition(x) for x in iterable])`` as ``all(condition(x) for x in iterable)``
 * Rewrite ``any([condition(x) for x in iterable])`` as ``any(condition(x) for x in iterable)``
+
+C420: Unnecessary dict comprehension - rewrite using dict.fromkeys().
+----------------------------------------------------------------------
+
+It's unnecessary to use a dict comprehension to build a dict with all values set to the same constant.
+Use ``dict.fromkeys()`` instead, which is faster.
+For example:
+
+* Rewrite ``{x: 1 for x in iterable}`` as ``dict.fromkeys(iterable, 1)``
+* Rewrite ``{x: None for x in iterable}`` as ``dict.fromkeys(iterable)``
diff --git a/src/flake8_comprehensions/__init__.py b/src/flake8_comprehensions/__init__.py
index 3a48a38..dbe97a1 100644
--- a/src/flake8_comprehensions/__init__.py
+++ b/src/flake8_comprehensions/__init__.py
@@ -46,6 +46,9 @@ class ComprehensionChecker:
             "C419 Unnecessary list comprehension passed to {func}() prevents "
             + "short-circuiting - rewrite as a generator."
         ),
+        "C420": (
+            "C420 Unnecessary {type} comprehension - rewrite using dict.fromkeys()."
+        ),
     }
 
     def run(self) -> Generator[tuple[int, int, str, type[Any]], None, None]:
@@ -335,32 +338,47 @@ class ComprehensionChecker:
                     len(node.generators) == 1
                     and not node.generators[0].ifs
                     and not node.generators[0].is_async
-                    and (
-                        (
-                            isinstance(node, (ast.ListComp, ast.SetComp))
-                            and isinstance(node.elt, ast.Name)
-                            and isinstance(node.generators[0].target, ast.Name)
-                            and node.elt.id == node.generators[0].target.id
+                ):
+                    if (
+                        isinstance(node, (ast.ListComp, ast.SetComp))
+                        and isinstance(node.elt, ast.Name)
+                        and isinstance(node.generators[0].target, ast.Name)
+                        and node.elt.id == node.generators[0].target.id
+                    ) or (
+                        isinstance(node, ast.DictComp)
+                        and isinstance(node.key, ast.Name)
+                        and isinstance(node.value, ast.Name)
+                        and isinstance(node.generators[0].target, ast.Tuple)
+                        and len(node.generators[0].target.elts) == 2
+                        and isinstance(node.generators[0].target.elts[0], ast.Name)
+                        and node.generators[0].target.elts[0].id == node.key.id
+                        and isinstance(node.generators[0].target.elts[1], ast.Name)
+                        and node.generators[0].target.elts[1].id == node.value.id
+                    ):
+                        yield (
+                            node.lineno,
+                            node.col_offset,
+                            self.messages["C416"].format(
+                                type=comp_type[node.__class__]
+                            ),
+                            type(self),
                         )
-                        or (
-                            isinstance(node, ast.DictComp)
-                            and isinstance(node.key, ast.Name)
-                            and isinstance(node.value, ast.Name)
-                            and isinstance(node.generators[0].target, ast.Tuple)
-                            and len(node.generators[0].target.elts) == 2
-                            and isinstance(node.generators[0].target.elts[0], ast.Name)
-                            and node.generators[0].target.elts[0].id == node.key.id
-                            and isinstance(node.generators[0].target.elts[1], ast.Name)
-                            and node.generators[0].target.elts[1].id == node.value.id
+
+                    elif (
+                        isinstance(node, ast.DictComp)
+                        and isinstance(node.key, ast.Name)
+                        and isinstance(node.value, ast.Constant)
+                        and isinstance(node.generators[0].target, ast.Name)
+                        and node.key.id == node.generators[0].target.id
+                    ):
+                        yield (
+                            node.lineno,
+                            node.col_offset,
+                            self.messages["C420"].format(
+                                type=comp_type[node.__class__]
+                            ),
+                            type(self),
                         )
-                    )
-                ):
-                    yield (
-                        node.lineno,
-                        node.col_offset,
-                        self.messages["C416"].format(type=comp_type[node.__class__]),
-                        type(self),
-                    )
 
 
 def has_star_args(call_node: ast.Call) -> bool:


Test patch:
diff --git a/tests/test_flake8_comprehensions.py b/tests/test_flake8_comprehensions.py
index 454cc31..3843b6e 100644
--- a/tests/test_flake8_comprehensions.py
+++ b/tests/test_flake8_comprehensions.py
@@ -886,7 +886,7 @@ def test_C417_fail(code, failures, flake8_path):
     "code",
     [
         "dict({}, a=1)",
-        "dict({x: 1 for x in range(1)}, a=1)",
+        "dict({x: [] for x in range(1)}, a=1)",
     ],
 )
 def test_C418_pass(code, flake8_path):
@@ -963,3 +963,58 @@ def test_C419_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",
+    [
+        "{elt: elt * 2 for elt in range(5)}",
+        "{elt: [] for elt in foo}",
+        "{elt: {1, 2, 3} for elt in ['a', 'b', 'c']}",
+        "{elt: some_func() for elt in ['a', 'b', 'c']}",
+        "{elt: SomeClass() for elt in ['a', 'b', 'c']}",
+    ],
+)
+def test_C420_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",
+    [
+        (
+            "{elt: None for elt in range(5)}",
+            [
+                "./example.py:1:1: C420 Unnecessary dict comprehension - "
+                + "rewrite using dict.fromkeys()."
+            ],
+        ),
+        (
+            "{elt: 1 for elt in foo}",
+            [
+                "./example.py:1:1: C420 Unnecessary dict comprehension - "
+                + "rewrite using dict.fromkeys()."
+            ],
+        ),
+        (
+            "{elt: 'value' for elt in ['a', 'b', 'c']}",
+            [
+                "./example.py:1:1: C420 Unnecessary dict comprehension - "
+                + "rewrite using dict.fromkeys()."
+            ],
+        ),
+        (
+            "{elt: True for elt in some_func()}",
+            [
+                "./example.py:1:1: C420 Unnecessary dict comprehension - "
+                + "rewrite using dict.fromkeys()."
+            ],
+        ),
+    ],
+)
+def test_C420_fail(code, failures, flake8_path):
+    (flake8_path / "example.py").write_text(dedent(code))
+    result = flake8_path.run_flake8()
+    assert result.out_lines == failures