Made C408 only apply when no arguments are passed (#250)

Fixes #249.

HISTORY.rst

@@ -2,6 +2,9 @@
 History
 =======
 
+* Made ``C408`` only apply when no arguments are passed to
+  ``dict``/``list``/``tuple``.
+
 3.2.2 (2020-01-20)
 ------------------
 

README.rst

@@ -130,9 +130,10 @@ The list of builtins that are checked for are:
 C408: Unnecessary (dict/list/tuple) call - rewrite as a literal.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-It's slower to call e.g. ``dict()`` than using the empty literal, because the
-name ``dict`` must be looked up in the global scope in case it has been
-rebound. Same for the other two basic types here. For example:
+It's slower to call these types than to construct them with literals, e.g.
+``dict()`` is slower than ``{}``, because the name ``dict`` must be looked up
+in the global scope in case it has been rebound, and it includes the overhead
+of a function call. Same for the other two builtin types here. For example:
 
 * Rewrite ``dict()`` as ``{}``
 * Rewrite ``list()`` as ``[]``

src/flake8_comprehensions.py

@@ -187,9 +187,8 @@ def run(self):
 
                 elif (
                     num_positional_args == 0
-                    and not has_star_args(node)
-                    and not has_keyword_args(node)
                     and node.func.id in ("tuple", "list", "dict")
+                    and len(node.keywords) == 0
                 ):
                     yield (
                         node.lineno,
@@ -321,14 +320,6 @@ def run(self):
                     )
 
 
-def has_star_args(call_node):
-    return any(isinstance(a, ast.Starred) for a in call_node.args)
-
-
-def has_keyword_args(call_node):
-    return any(k.arg is None for k in call_node.keywords)
-
-
 comp_type = {
     ast.DictComp: "dict",
     ast.ListComp: "list",

tests/test_flake8_comprehensions.py

@@ -618,6 +618,12 @@ def test_C408_pass_6(flake8dir):
     assert result.out_lines == []
 
 
+def test_C408_pass_7(flake8dir):
+    flake8dir.make_example_py("dict(a=1)")
+    result = flake8dir.run_flake8()
+    assert result.out_lines == []
+
+
 def test_C408_fail_1(flake8dir):
     flake8dir.make_example_py("tuple()")
     result = flake8dir.run_flake8()
@@ -642,14 +648,6 @@ def test_C408_fail_3(flake8dir):
     ]
 
 
-def test_C408_fail_4(flake8dir):
-    flake8dir.make_example_py("dict(a=1)")
-    result = flake8dir.run_flake8()
-    assert result.out_lines == [
-        "./example.py:1:1: C408 Unnecessary dict call - rewrite as a literal."
-    ]
-
-
 # C409