B026 - Argument unpacking after keyword argument (#287)

* add B026 - Argument unpacking after keyword argument
* add support for multiple unpackings
* fix wording

Author: pmeier

README.rst

@@ -160,6 +160,13 @@ the loop, because `late-binding closures are a classic gotcha
 This check identifies exception types that are specified in multiple ``except``
 clauses. The first specification is the only one ever considered, so all others can be removed.
 
+**B026**: Star-arg unpacking after a keyword argument is strongly discouraged, because
+it only works when the keyword parameter is declared after all parameters supplied by
+the unpacked sequence, and this change of ordering can surprise and mislead readers.
+There was `cpython discussion of disallowing this syntax
+<https://github.com/python/cpython/issues/82741>`_, but legacy usage and parser
+limitations make it difficult.
+
 Opinionated warnings
 ~~~~~~~~~~~~~~~~~~~~
 

bugbear.py

@@ -354,6 +354,8 @@ def visit_Call(self, node):
                 ):
                     self.errors.append(B010(node.lineno, node.col_offset))
 
+            self.check_for_b026(node)
+
         self.generic_visit(node)
 
     def visit_Assign(self, node):
@@ -641,6 +643,22 @@ def is_abstract_decorator(expr):
 
         self.errors.append(B024(node.lineno, node.col_offset, vars=(node.name,)))
 
+    def check_for_b026(self, call: ast.Call):
+        if not call.keywords:
+            return
+
+        starreds = [arg for arg in call.args if isinstance(arg, ast.Starred)]
+        if not starreds:
+            return
+
+        first_keyword = call.keywords[0].value
+        for starred in starreds:
+            if (starred.lineno, starred.col_offset) > (
+                first_keyword.lineno,
+                first_keyword.col_offset,
+            ):
+                self.errors.append(B026(starred.lineno, starred.col_offset))
+
     def _get_assigned_names(self, loop_node):
         loop_targets = (ast.For, ast.AsyncFor, ast.comprehension)
         for node in children_in_scope(loop_node):
@@ -1203,6 +1221,14 @@ def visit_Lambda(self, node):
         " will be considered and all other except catches can be safely removed."
     )
 )
+B026 = Error(
+    message=(
+        "B026 Star-arg unpacking after a keyword argument is strongly discouraged, "
+        "because it only works when the keyword parameter is declared after all "
+        "parameters supplied by the unpacked sequence, and this change of ordering can "
+        "surprise and mislead readers."
+    )
+)
 
 # Warnings disabled by default.
 B901 = Error(

tests/b026.py

@@ -0,0 +1,21 @@
+"""
+Should emit:
+B026 - on lines 16, 17, 18, 19, 20, 21
+"""
+
+
+def foo(bar, baz, bam):
+    pass
+
+
+bar_baz = ["bar", "baz"]
+
+foo("bar", "baz", bam="bam")
+foo("bar", baz="baz", bam="bam")
+foo(bar="bar", baz="baz", bam="bam")
+foo(bam="bam", *["bar", "baz"])
+foo(bam="bam", *bar_baz)
+foo(baz="baz", bam="bam", *["bar"])
+foo(bar="bar", baz="baz", bam="bam", *[])
+foo(bam="bam", *["bar"], *["baz"])
+foo(*["bar"], bam="bam", *["baz"])

tests/test_bugbear.py

@@ -36,6 +36,7 @@
     B023,
     B024,
     B025,
+    B026,
     B901,
     B902,
     B903,
@@ -381,6 +382,23 @@ def test_b025(self):
             ),
         )
 
+    def test_b026(self):
+        filename = Path(__file__).absolute().parent / "b026.py"
+        bbc = BugBearChecker(filename=str(filename))
+        errors = list(bbc.run())
+        self.assertEqual(
+            errors,
+            self.errors(
+                B026(16, 15),
+                B026(17, 15),
+                B026(18, 26),
+                B026(19, 37),
+                B026(20, 15),
+                B026(20, 25),
+                B026(21, 25),
+            ),
+        )
+
     def test_b901(self):
         filename = Path(__file__).absolute().parent / "b901.py"
         bbc = BugBearChecker(filename=str(filename))