From: Roman Donchenko <rdonchen@outlook.com>
Date: Mon, 20 Apr 2026 20:19:43 +0000 (+0300)
Subject: gh-108411: Make typing.IO/BinaryIO arguments positional-only (#142906)
X-Git-Url: http://git.ipfire.org/index.cgi?a=commitdiff_plain;h=f6ed7c0acbd9234226cab5cca12f9d312809103e;p=thirdparty%2FPython%2Fcpython.git

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.
---

diff --git a/Lib/typing.py b/Lib/typing.py
index 868fec9e088c..3e7661dd2f87 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -3612,7 +3612,7 @@ class IO(Generic[AnyStr]):
         pass
 
     @abstractmethod
-    def read(self, n: int = -1) -> AnyStr:
+    def read(self, n: int = -1, /) -> AnyStr:
         pass
 
     @abstractmethod
@@ -3620,15 +3620,15 @@ class IO(Generic[AnyStr]):
         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 @@ class IO(Generic[AnyStr]):
         pass
 
     @abstractmethod
-    def truncate(self, size: int | None = None) -> int:
+    def truncate(self, size: int | None = None, /) -> int:
         pass
 
     @abstractmethod
@@ -3648,11 +3648,11 @@ class IO(Generic[AnyStr]):
         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 @@ class IO(Generic[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 000000000000..95aa41e92268
--- /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.