gh-85524: Raise "UnsupportedOperation" on FileIO.readall

Lib/test/test_io/test_general.py

@@ -125,6 +125,7 @@ def test_invalid_operations(self):
             self.assertRaises(exc, fp.readline)
         with self.open(os_helper.TESTFN, "wb", buffering=0) as fp:
             self.assertRaises(exc, fp.read)
+            self.assertRaises(exc, fp.readall)
             self.assertRaises(exc, fp.readline)
         with self.open(os_helper.TESTFN, "rb", buffering=0) as fp:
             self.assertRaises(exc, fp.write, b"blah")

Misc/NEWS.d/next/Library/2025-11-07-12-25-46.gh-issue-85524.9SWFIC.rst

@@ -0,0 +1,3 @@
+Update ``io.FileIO.readall``, an implementation of :meth:`io.RawIOBase.readall`,
+to follow :class:`io.IOBase` guidelines and raise :exc:`io.UnsupportedOperation`
+when a file is in "w" mode rather than :exc:`OSError`

Modules/_io/fileio.c

@@ -728,6 +728,9 @@ new_buffersize(fileio *self, size_t currentsize)
 @permit_long_docstring_body
 _io.FileIO.readall
 
+    cls: defining_class
+    /
+
 Read all data from the file, returned as bytes.
 
 Reads until either there is an error or read() returns size 0 (indicates EOF).
@@ -738,8 +741,8 @@ data is available (EAGAIN is returned before bytes are read) returns None.
 [clinic start generated code]*/
 
 static PyObject *
-_io_FileIO_readall_impl(fileio *self)
-/*[clinic end generated code: output=faa0292b213b4022 input=10d8b2ec403302dc]*/
+_io_FileIO_readall_impl(fileio *self, PyTypeObject *cls)
+/*[clinic end generated code: output=d546737ec895c462 input=cecda40bf9961299]*/
 {
     Py_off_t pos, end;
     PyBytesWriter *writer;
@@ -750,6 +753,10 @@ _io_FileIO_readall_impl(fileio *self)
     if (self->fd < 0) {
         return err_closed();
     }
+    if (!self->readable) {
+        _PyIO_State *state = get_io_state_by_cls(cls);
+        return err_mode(state, "reading");
+    }
 
     if (self->stat_atopen != NULL && self->stat_atopen->st_size < _PY_READ_MAX) {
         end = (Py_off_t)self->stat_atopen->st_size;
@@ -873,7 +880,7 @@ _io_FileIO_read_impl(fileio *self, PyTypeObject *cls, Py_ssize_t size)
     }
 
     if (size < 0)
-        return _io_FileIO_readall_impl(self);
+        return _io_FileIO_readall_impl(self, cls);
 
     if (size > _PY_READ_MAX) {
         size = _PY_READ_MAX;