https://github.com/python/cpython/commit/f6ed7c0acbd9234226cab5cca12f9d312809103e
commit: f6ed7c0acbd9234226cab5cca12f9d312809103e
branch: main
author: Roman Donchenko <[email protected]>
committer: JelleZijlstra <[email protected]>
date: 2026-04-20T13:19:43-07:00
summary:
gh-108411: Make typing.IO/BinaryIO arguments positional-only (#142906)
`IO` is purported to be the type of the file objects returned by `open`.
However, all methods on those objects take positional-only arguments, while
`IO`'s methods are declared with regular arguments. As such, the file objects
cannot actually be considered to implement `IO`. The same thing applies to
`BinaryIO`.
Fix this by adjusting the definition of these ABCs to match the file objects.
This is technically a breaking change, but it is unlikely to actually break
anything:
* These methods should never be called at runtime, since they are abstract.
Therefore, this should not cause any runtime errors.
* In typeshed these arguments are already positional-only, so this should
not cause any errors during typechecking either.
files:
A Misc/NEWS.d/next/Library/2025-12-17-02-55-03.gh-issue-108411.up7MAc.rst
M Lib/typing.py
diff --git a/Lib/typing.py b/Lib/typing.py
index 868fec9e088cb5..3e7661dd2f877c 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -3612,7 +3612,7 @@ def isatty(self) -> bool:
pass
@abstractmethod
- def read(self, n: int = -1) -> AnyStr:
+ def read(self, n: int = -1, /) -> AnyStr:
pass
@abstractmethod
@@ -3620,15 +3620,15 @@ def readable(self) -> bool:
pass
@abstractmethod
- def readline(self, limit: int = -1) -> AnyStr:
+ def readline(self, limit: int = -1, /) -> AnyStr:
pass
@abstractmethod
- def readlines(self, hint: int = -1) -> list[AnyStr]:
+ def readlines(self, hint: int = -1, /) -> list[AnyStr]:
pass
@abstractmethod
- def seek(self, offset: int, whence: int = 0) -> int:
+ def seek(self, offset: int, whence: int = 0, /) -> int:
pass
@abstractmethod
@@ -3640,7 +3640,7 @@ def tell(self) -> int:
pass
@abstractmethod
- def truncate(self, size: int | None = None) -> int:
+ def truncate(self, size: int | None = None, /) -> int:
pass
@abstractmethod
@@ -3648,11 +3648,11 @@ def writable(self) -> bool:
pass
@abstractmethod
- def write(self, s: AnyStr) -> int:
+ def write(self, s: AnyStr, /) -> int:
pass
@abstractmethod
- def writelines(self, lines: list[AnyStr]) -> None:
+ def writelines(self, lines: list[AnyStr], /) -> None:
pass
@abstractmethod
@@ -3660,7 +3660,7 @@ def __enter__(self) -> IO[AnyStr]:
pass
@abstractmethod
- def __exit__(self, type, value, traceback) -> None:
+ def __exit__(self, type, value, traceback, /) -> None:
pass
@@ -3670,7 +3670,7 @@ class BinaryIO(IO[bytes]):
__slots__ = ()
@abstractmethod
- def write(self, s: bytes | bytearray) -> int:
+ def write(self, s: bytes | bytearray, /) -> int:
pass
@abstractmethod
diff --git
a/Misc/NEWS.d/next/Library/2025-12-17-02-55-03.gh-issue-108411.up7MAc.rst
b/Misc/NEWS.d/next/Library/2025-12-17-02-55-03.gh-issue-108411.up7MAc.rst
new file mode 100644
index 00000000000000..95aa41e922684f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-12-17-02-55-03.gh-issue-108411.up7MAc.rst
@@ -0,0 +1,2 @@
+``typing.IO`` and ``typing.BinaryIO`` method arguments are now
+positional-only.
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