Documented deprecation

docs/deprecations.rst

@@ -53,6 +53,18 @@ Before Pillow 8.3.0, ``ImagePalette`` required palette data of particular length
 default, and the size parameter could be used to override that. Pillow 8.3.0 removed
 the default required length, also removing the need for the size parameter.
 
+ImageShow.Viewer.show_file file argument
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. deprecated:: 9.1.0
+
+The ``file`` argument in :py:meth:`~PIL.ImageShow.Viewer.show_file()` has been
+deprecated, replaced by ``path``.
+
+In effect, ``viewer.show_file("test.jpg")`` will continue to work unchanged.
+``viewer.show_file(file="test.jpg")`` will raise a deprecation warning, and suggest
+``viewer.show_file(path="test.jpg")`` instead.
+
 Removed features
 ----------------
 

src/PIL/ImageShow.py

@@ -107,7 +107,12 @@ def show_image(self, image, **options):
         return self.show_file(self.save_image(image), **options)
 
     def show_file(self, path=None, **options):
-        """Display given file."""
+        """
+        Display given file.
+
+        Before Pillow 9.1.0, the first argument was ``file``. This is now deprecated,
+        and ``path`` should be used instead.
+        """
         if path is None:
             if "file" in options:
                 warnings.warn(