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Move dataclass kw_only fields to the end of the signature #19018

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Move dataclass kw_only fields to the end of the signature #19018

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ceef876
Kwonly attributes are moved to the end in all cases, this behaviour d…
sterliakov May 3, 2025
63edf49
Add regression test for another issue
sterliakov May 3, 2025
39138a7
Revert previous attempt...
sterliakov May 3, 2025
b9503ad
Yes, this test was also wrong
sterliakov May 3, 2025
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5 changes: 1 addition & 4 deletions mypy/plugins/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -546,7 +546,6 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
# in the parent. We can implement this via a dict without disrupting the attr order
# because dicts preserve insertion order in Python 3.7+.
found_attrs: dict[str, DataclassAttribute] = {}
found_dataclass_supertype = False
for info in reversed(cls.info.mro[1:-1]):
if "dataclass_tag" in info.metadata and "dataclass" not in info.metadata:
# We haven't processed the base class yet. Need another pass.
Expand All @@ -556,7 +555,6 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:

# Each class depends on the set of attributes in its dataclass ancestors.
self._api.add_plugin_dependency(make_wildcard_trigger(info.fullname))
found_dataclass_supertype = True

for data in info.metadata["dataclass"]["attributes"]:
name: str = data["name"]
Expand Down Expand Up @@ -720,8 +718,7 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
)

all_attrs = list(found_attrs.values())
if found_dataclass_supertype:
all_attrs.sort(key=lambda a: a.kw_only)
all_attrs.sort(key=lambda a: a.kw_only)

# Third, ensure that arguments without a default don't follow
# arguments that have a default and that the KW_ONLY sentinel
Expand Down
2 changes: 1 addition & 1 deletion test-data/unit/check-dataclass-transform.test
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Expand Up @@ -265,7 +265,7 @@ class Foo:

Foo(a=5, b_=1) # E: Unexpected keyword argument "a" for "Foo"
Foo(a_=1, b_=1, noinit=1) # E: Unexpected keyword argument "noinit" for "Foo"
Foo(1, 2, 3) # E: Too many positional arguments for "Foo"
Foo(1, 2, 3) # (a, b, unused1)
foo = Foo(1, 2, kwonly=3)
reveal_type(foo.noinit) # N: Revealed type is "builtins.int"
reveal_type(foo.unused1) # N: Revealed type is "builtins.int"
Expand Down
41 changes: 35 additions & 6 deletions test-data/unit/check-dataclasses.test
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Expand Up @@ -460,14 +460,16 @@ from dataclasses import dataclass, field, KW_ONLY
class Application:
_: KW_ONLY
name: str = 'Unnamed'
rating: int = field(kw_only=False) # E: Attributes without a default cannot follow attributes with one
rating: int = field(kw_only=False)

Application(name='name', rating=5)
Application() # E: Missing positional argument "name" in call to "Application"
Application('name') # E: Too many positional arguments for "Application" # E: Too few arguments for "Application"
Application('name', 123) # E: Too many positional arguments for "Application"
Application('name', rating=123) # E: Too many positional arguments for "Application"

Application() # E: Missing positional argument "rating" in call to "Application"
Application(123)
Application('name') # E: Argument 1 to "Application" has incompatible type "str"; expected "int"
Application('name', 123) # E: Too many positional arguments for "Application" \
# E: Argument 1 to "Application" has incompatible type "str"; expected "int" \
# E: Argument 2 to "Application" has incompatible type "int"; expected "str"
Application(123, rating=123) # E: "Application" gets multiple values for keyword argument "rating"
[builtins fixtures/dataclasses.pyi]

[case testDataclassesOrderingKwOnlyWithSentinelAndSubclass]
Expand Down Expand Up @@ -2610,3 +2612,30 @@ class B2(B1): # E: A NamedTuple cannot be a dataclass
pass

[builtins fixtures/tuple.pyi]

[case testDataclassKwOnlyArgsLast]
from dataclasses import dataclass, field

@dataclass
class User:
id: int = field(kw_only=True)
name: str

User("Foo", id=0)
[builtins fixtures/tuple.pyi]

[case testDataclassKwOnlyArgsDefaultAllowedNonLast]
from dataclasses import dataclass, field

@dataclass
class User:
id: int = field(kw_only=True, default=0)
name: str

User() # E: Missing positional argument "name" in call to "User"
User("")
User(0) # E: Argument 1 to "User" has incompatible type "int"; expected "str"
User("", 0) # E: Too many positional arguments for "User"
User("", id=0)
User("", name="") # E: "User" gets multiple values for keyword argument "name"
[builtins fixtures/tuple.pyi]