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Fix copy.replace widening a bound TypeVar to its bound #15997

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Fix copy.replace widening a bound TypeVar to its bound #15997

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Fix copy.replace widening a bound TypeVar to its bound
apoorva-01 Jul 11, 2026
9afdca5
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Jul 11, 2026
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26 changes: 26 additions & 0 deletions stdlib/@tests/test_cases/check_copy.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@

import copy
import sys
from dataclasses import dataclass
from typing import Generic, TypeVar
from typing_extensions import Self, assert_type

Expand Down Expand Up @@ -37,3 +38,28 @@ def __replace__(self, value: str) -> Box[str]:
box1: Box[int] = Box(42)
box2 = copy.replace(box1, val="spam")
assert_type(box2, Box[str])


# Regression test for #15973: replace() must preserve a bound TypeVar whose
# __replace__ returns Self (the dataclass/namedtuple case), rather than widening
# to the bound.
@dataclass(frozen=True)
class BaseConfig:
pg_ssl_key: str | None = None


@dataclass(frozen=True)
class SubConfig(BaseConfig):
pg_host: str = "localhost"


_ConfigT = TypeVar("_ConfigT", bound=BaseConfig)

if sys.version_info >= (3, 13):

def replace_config(config: _ConfigT) -> _ConfigT:
result = copy.replace(config, pg_ssl_key="replaced")
assert_type(result, _ConfigT)
return result

assert_type(copy.replace(SubConfig(), pg_host="example.com"), SubConfig)
17 changes: 15 additions & 2 deletions stdlib/copy.pyi
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Original file line number Diff line number Diff line change
@@ -1,16 +1,23 @@
import sys
from typing import Any, Protocol, TypeVar, type_check_only
from typing import Any, Protocol, TypeVar, overload, type_check_only
from typing_extensions import Self

__all__ = ["Error", "copy", "deepcopy"]

_T = TypeVar("_T")
_RT_co = TypeVar("_RT_co", covariant=True)

@type_check_only
class _SupportsReplace(Protocol[_RT_co]):
class _SupportsReplaceSelf(Protocol):
# In reality doesn't support args, but there's no great way to express this.
def __replace__(self, /, *_: Any, **changes: Any) -> Self: ...

@type_check_only
class _SupportsReplace(Protocol[_RT_co]):
def __replace__(self, /, *_: Any, **changes: Any) -> _RT_co: ...

_SR = TypeVar("_SR", bound=_SupportsReplaceSelf)

# None in CPython but non-None in Jython
PyStringMap: Any

Expand All @@ -20,7 +27,13 @@ def copy(x: _T) -> _T: ...

if sys.version_info >= (3, 13):
__all__ += ["replace"]

# The types accepted by `**changes` match those of `obj.__replace__`.
# When `__replace__` returns `Self`, keep the argument's own type (so a bound
# TypeVar is preserved); otherwise return whatever `__replace__` declares.
@overload
def replace(obj: _SR, /, **changes: Any) -> _SR: ...
@overload
def replace(obj: _SupportsReplace[_RT_co], /, **changes: Any) -> _RT_co: ...

class Error(Exception): ...
Expand Down
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