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B026 - Argument unpacking after keyword argument #287

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merged 4 commits into from Sep 22, 2022
Merged

B026 - Argument unpacking after keyword argument #287

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B026 - Argument unpacking after keyword argument
pmeier Sep 20, 2022
a3e5594
format
pmeier Sep 20, 2022
a468ce8
add support for multiple unpackings
pmeier Sep 21, 2022
9d36545
fix wording
pmeier Sep 22, 2022
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2 changes: 2 additions & 0 deletions README.rst
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Expand Up @@ -160,6 +160,8 @@ 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**: Argument unpacking after keyword argument.
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Suggested change
**B026**: Argument unpacking after keyword argument.
**B026**: Argument unpacking after keyword argument is strongly discouraged. This behavior is uncommon and can lead to unnecessary confusion when reading the code thus making it prone to causing bugs. There was cpython discussion of disallowing but legacy usage and parser limitations make it difficult.

+1 on more description here. A suggestion that I'm more than happy to have reworded + made better.

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Suggested change
**B026**: Argument unpacking after keyword argument.
**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.

I think this is compact enough to also use as the runtime message, and explains specifically what we think is wrong (the mismatched order can be misleading) rather than non-actionable historical details.

I do like the historical details and I'd be happy to keep "There was cpython discussion of disallowing this syntax, but legacy usage and parser limitations make it difficult." with that link in the readme too, but not print it at runtime.

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I'm not super happy about the wording and needed to read it multiple times to understand that it indeed correctly explains the issue. Still, I have no better way so what gives. This is really hard to explain without an example TBH.

+1 on keeping the historical details to the README.


Opinionated warnings
~~~~~~~~~~~~~~~~~~~~

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16 changes: 16 additions & 0 deletions bugbear.py
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Expand Up @@ -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):
Expand Down Expand Up @@ -641,6 +643,19 @@ 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):
try:
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starred = next(arg for arg in call.args if isinstance(arg, ast.Starred))
except StopIteration:
return

if any(
(keyword.value.lineno, keyword.value.col_offset)
< (starred.lineno, starred.col_offset)
for keyword in call.keywords
):
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self.errors.append(B026(call.lineno, call.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):
Expand Down Expand Up @@ -1203,6 +1218,7 @@ def visit_Lambda(self, node):
" will be considered and all other except catches can be safely removed."
)
)
B026 = Error(message="B026 Argument unpacking after keyword argument.")

# Warnings disabled by default.
B901 = Error(
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16 changes: 16 additions & 0 deletions tests/b026.py
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@@ -0,0 +1,16 @@
"""
Should emit:
B026 - on lines 14, 15, 16
"""


def foo(bar, baz, bam):
pass


foo("bar", "baz", bam="bam")
foo("bar", baz="baz", bam="bam")
foo(bar="bar", baz="baz", bam="bam")
foo(bam="bam", *["bar", "baz"])
foo(baz="baz", bam="bam", *["bar"])
foo(bar="bar", baz="baz", bam="bam", *[])
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14 changes: 14 additions & 0 deletions tests/test_bugbear.py
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Expand Up @@ -36,6 +36,7 @@
B023,
B024,
B025,
B026,
B901,
B902,
B903,
Expand Down Expand Up @@ -381,6 +382,19 @@ 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(14, 0),
B026(15, 0),
B026(16, 0),
),
)

def test_b901(self):
filename = Path(__file__).absolute().parent / "b901.py"
bbc = BugBearChecker(filename=str(filename))
Expand Down