Fixed the return type annotations of AsyncFile.readinto(1)

Fixes #825.

docs/versionhistory.rst

@@ -11,6 +11,9 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
   an object with a ``.fileno()`` method or an integer handle, and deprecated
   their now obsolete versions (``wait_socket_readable()`` and
   ``wait_socket_writable()`` (PR by @davidbrochart)
+- Fixed the return type annotations of ``readinto()`` and ``readinto1()`` methods in the
+  ``anyio.AsyncFile`` class
+  (`#825 <https://github.com/agronholm/anyio/issues/825>`_)
 
 **4.6.2**
 

src/anyio/_core/_fileio.py

@@ -92,10 +92,10 @@ async def readline(self) -> AnyStr:
     async def readlines(self) -> list[AnyStr]:
         return await to_thread.run_sync(self._fp.readlines)
 
-    async def readinto(self: AsyncFile[bytes], b: WriteableBuffer) -> bytes:
+    async def readinto(self: AsyncFile[bytes], b: WriteableBuffer) -> int:
         return await to_thread.run_sync(self._fp.readinto, b)
 
-    async def readinto1(self: AsyncFile[bytes], b: WriteableBuffer) -> bytes:
+    async def readinto1(self: AsyncFile[bytes], b: WriteableBuffer) -> int:
         return await to_thread.run_sync(self._fp.readinto1, b)
 
     @overload

tests/test_fileio.py

@@ -39,6 +39,18 @@ async def test_read(self, testdatafile: pathlib.Path, testdata: bytes) -> None:
         assert f.closed
         assert data == testdata
 
+    async def test_readinto(self, testdatafile: pathlib.Path, testdata: bytes) -> None:
+        buffer = bytearray(100)
+        async with await open_file(testdatafile, "rb") as f:
+            assert await f.readinto(buffer) == 100
+            assert bytes(buffer) == testdata[:100]
+
+    async def test_readinto1(self, testdatafile: pathlib.Path, testdata: bytes) -> None:
+        buffer = bytearray(100)
+        async with await open_file(testdatafile, "rb") as f:
+            assert await f.readinto1(buffer) == 100
+            assert bytes(buffer) == testdata[:100]
+
     async def test_write(self, testdatafile: pathlib.Path, testdata: bytes) -> None:
         async with await open_file(testdatafile, "ab") as f:
             await f.write(b"f" * 1000)