diff --git a/Tests/test_file_jpeg.py b/Tests/test_file_jpeg.py
index 654242148..4858d92e6 100644
--- a/Tests/test_file_jpeg.py
+++ b/Tests/test_file_jpeg.py
@@ -986,13 +986,7 @@ class TestFileJpeg:
             def decode(self, buffer: bytes) -> tuple[int, int]:
                 return 0, 0
 
-        decoder = InfiniteMockPyDecoder(None)
-
-        def closure(mode: str, *args) -> InfiniteMockPyDecoder:
-            decoder.__init__(mode, *args)
-            return decoder
-
-        Image.register_decoder("INFINITE", closure)
+        Image.register_decoder("INFINITE", InfiniteMockPyDecoder)
 
         with Image.open(TEST_FILE) as im:
             im.tile = [
diff --git a/Tests/test_image.py b/Tests/test_image.py
index 4c04e0da4..aae51eaa4 100644
--- a/Tests/test_image.py
+++ b/Tests/test_image.py
@@ -16,6 +16,7 @@ from PIL import (
     ExifTags,
     Image,
     ImageDraw,
+    ImageFile,
     ImagePalette,
     UnidentifiedImageError,
     features,
@@ -1038,25 +1039,20 @@ class TestImage:
             assert im.fp is None
 
 
-class MockEncoder:
-    args: tuple[str, ...]
-
-
-def mock_encode(*args: str) -> MockEncoder:
-    encoder = MockEncoder()
-    encoder.args = args
-    return encoder
+class MockEncoder(ImageFile.PyEncoder):
+    pass
 
 
 class TestRegistry:
     def test_encode_registry(self) -> None:
-        Image.register_encoder("MOCK", mock_encode)
+        Image.register_encoder("MOCK", MockEncoder)
         assert "MOCK" in Image.ENCODERS
 
         enc = Image._getencoder("RGB", "MOCK", ("args",), extra=("extra",))
 
         assert isinstance(enc, MockEncoder)
-        assert enc.args == ("RGB", "args", "extra")
+        assert enc.mode == "RGB"
+        assert enc.args == ("args", "extra")
 
     def test_encode_registry_fail(self) -> None:
         with pytest.raises(OSError):
diff --git a/Tests/test_imagefile.py b/Tests/test_imagefile.py
index 491409781..cf251c9ce 100644
--- a/Tests/test_imagefile.py
+++ b/Tests/test_imagefile.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from io import BytesIO
+from typing import Any
 
 import pytest
 
@@ -201,12 +202,22 @@ class TestImageFile:
 
 
 class MockPyDecoder(ImageFile.PyDecoder):
+    def __init__(self, mode: str, *args: Any) -> None:
+        MockPyDecoder.last = self
+
+        super().__init__(mode, *args)
+
     def decode(self, buffer):
         # eof
         return -1, 0
 
 
 class MockPyEncoder(ImageFile.PyEncoder):
+    def __init__(self, mode: str, *args: Any) -> None:
+        MockPyEncoder.last = self
+
+        super().__init__(mode, *args)
+
     def encode(self, buffer):
         return 1, 1, b""
 
@@ -228,19 +239,8 @@ class MockImageFile(ImageFile.ImageFile):
 class CodecsTest:
     @classmethod
     def setup_class(cls) -> None:
-        cls.decoder = MockPyDecoder(None)
-        cls.encoder = MockPyEncoder(None)
-
-        def decoder_closure(mode, *args):
-            cls.decoder.__init__(mode, *args)
-            return cls.decoder
-
-        def encoder_closure(mode, *args):
-            cls.encoder.__init__(mode, *args)
-            return cls.encoder
-
-        Image.register_decoder("MOCK", decoder_closure)
-        Image.register_encoder("MOCK", encoder_closure)
+        Image.register_decoder("MOCK", MockPyDecoder)
+        Image.register_encoder("MOCK", MockPyEncoder)
 
 
 class TestPyDecoder(CodecsTest):
@@ -251,13 +251,13 @@ class TestPyDecoder(CodecsTest):
 
         im.load()
 
-        assert self.decoder.state.xoff == xoff
-        assert self.decoder.state.yoff == yoff
-        assert self.decoder.state.xsize == xsize
-        assert self.decoder.state.ysize == ysize
+        assert MockPyDecoder.last.state.xoff == xoff
+        assert MockPyDecoder.last.state.yoff == yoff
+        assert MockPyDecoder.last.state.xsize == xsize
+        assert MockPyDecoder.last.state.ysize == ysize
 
         with pytest.raises(ValueError):
-            self.decoder.set_as_raw(b"\x00")
+            MockPyDecoder.last.set_as_raw(b"\x00")
 
     def test_extents_none(self) -> None:
         buf = BytesIO(b"\x00" * 255)
@@ -267,10 +267,10 @@ class TestPyDecoder(CodecsTest):
 
         im.load()
 
-        assert self.decoder.state.xoff == 0
-        assert self.decoder.state.yoff == 0
-        assert self.decoder.state.xsize == 200
-        assert self.decoder.state.ysize == 200
+        assert MockPyDecoder.last.state.xoff == 0
+        assert MockPyDecoder.last.state.yoff == 0
+        assert MockPyDecoder.last.state.xsize == 200
+        assert MockPyDecoder.last.state.ysize == 200
 
     def test_negsize(self) -> None:
         buf = BytesIO(b"\x00" * 255)
@@ -315,10 +315,10 @@ class TestPyEncoder(CodecsTest):
             im, fp, [("MOCK", (xoff, yoff, xoff + xsize, yoff + ysize), 0, "RGB")]
         )
 
-        assert self.encoder.state.xoff == xoff
-        assert self.encoder.state.yoff == yoff
-        assert self.encoder.state.xsize == xsize
-        assert self.encoder.state.ysize == ysize
+        assert MockPyEncoder.last.state.xoff == xoff
+        assert MockPyEncoder.last.state.yoff == yoff
+        assert MockPyEncoder.last.state.xsize == xsize
+        assert MockPyEncoder.last.state.ysize == ysize
 
     def test_extents_none(self) -> None:
         buf = BytesIO(b"\x00" * 255)
@@ -329,10 +329,10 @@ class TestPyEncoder(CodecsTest):
         fp = BytesIO()
         ImageFile._save(im, fp, [("MOCK", None, 0, "RGB")])
 
-        assert self.encoder.state.xoff == 0
-        assert self.encoder.state.yoff == 0
-        assert self.encoder.state.xsize == 200
-        assert self.encoder.state.ysize == 200
+        assert MockPyEncoder.last.state.xoff == 0
+        assert MockPyEncoder.last.state.yoff == 0
+        assert MockPyEncoder.last.state.xsize == 200
+        assert MockPyEncoder.last.state.ysize == 200
 
     def test_negsize(self) -> None:
         buf = BytesIO(b"\x00" * 255)
@@ -340,12 +340,12 @@ class TestPyEncoder(CodecsTest):
         im = MockImageFile(buf)
 
         fp = BytesIO()
-        self.encoder.cleanup_called = False
+        MockPyEncoder.last = None
         with pytest.raises(ValueError):
             ImageFile._save(
                 im, fp, [("MOCK", (xoff, yoff, -10, yoff + ysize), 0, "RGB")]
             )
-        assert self.encoder.cleanup_called
+        assert MockPyEncoder.last.cleanup_called
 
         with pytest.raises(ValueError):
             ImageFile._save(
diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index a770488b7..eba30537f 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -229,8 +229,8 @@ MIME: dict[str, str] = {}
 SAVE: dict[str, Callable[[Image, IO[bytes], str | bytes], None]] = {}
 SAVE_ALL: dict[str, Callable[[Image, IO[bytes], str | bytes], None]] = {}
 EXTENSION: dict[str, str] = {}
-DECODERS: dict[str, object] = {}
-ENCODERS: dict[str, object] = {}
+DECODERS: dict[str, type[ImageFile.PyDecoder]] = {}
+ENCODERS: dict[str, type[ImageFile.PyEncoder]] = {}
 
 # --------------------------------------------------------------------
 # Modes
@@ -3524,28 +3524,26 @@ def registered_extensions():
     return EXTENSION
 
 
-def register_decoder(name: str, decoder) -> None:
+def register_decoder(name: str, decoder: type[ImageFile.PyDecoder]) -> None:
     """
     Registers an image decoder.  This function should not be
     used in application code.
 
     :param name: The name of the decoder
-    :param decoder: A callable(mode, args) that returns an
-                    ImageFile.PyDecoder object
+    :param decoder: An ImageFile.PyDecoder object
 
     .. versionadded:: 4.1.0
     """
     DECODERS[name] = decoder
 
 
-def register_encoder(name, encoder):
+def register_encoder(name: str, encoder: type[ImageFile.PyEncoder]) -> None:
     """
     Registers an image encoder.  This function should not be
     used in application code.
 
     :param name: The name of the encoder
-    :param encoder: A callable(mode, args) that returns an
-                    ImageFile.PyEncoder object
+    :param encoder: An ImageFile.PyEncoder object
 
     .. versionadded:: 4.1.0
     """