Lazy import only required plugin: open 2.3-15.6x & save 2.2-9x faster

Tests/test_file_spider.py

@@ -14,6 +14,10 @@
 TEST_FILE = "Tests/images/hopper.spider"
 
 
+def teardown_module() -> None:
+    del Image.EXTENSION[".spider"]
+
+
 def test_sanity() -> None:
     with Image.open(TEST_FILE) as im:
         im.load()

Tests/test_image.py

@@ -466,6 +466,9 @@ def test_registered_extensions_uninitialized(self) -> None:
         # Assert
         assert Image._initialized == 2
 
+        for extension in Image.EXTENSION:
+            assert extension in Image._EXTENSION_PLUGIN
+
     def test_registered_extensions(self) -> None:
         # Arrange
         # Open an image to trigger plugin registration

src/PIL/Image.py

@@ -323,6 +323,102 @@ def getmodebands(mode: str) -> int:
 
 _initialized = 0
 
+# Mapping from file extension to plugin module name for lazy loading
+_EXTENSION_PLUGIN: dict[str, str] = {
+    # Common formats (preinit)
+    ".bmp": "BmpImagePlugin",
+    ".dib": "BmpImagePlugin",
+    ".gif": "GifImagePlugin",
+    ".jfif": "JpegImagePlugin",
+    ".jpe": "JpegImagePlugin",
+    ".jpg": "JpegImagePlugin",
+    ".jpeg": "JpegImagePlugin",
+    ".pbm": "PpmImagePlugin",
+    ".pgm": "PpmImagePlugin",
+    ".pnm": "PpmImagePlugin",
+    ".ppm": "PpmImagePlugin",
+    ".pfm": "PpmImagePlugin",
+    ".png": "PngImagePlugin",
+    ".apng": "PngImagePlugin",
+    # Less common formats (init)
+    ".avif": "AvifImagePlugin",
+    ".avifs": "AvifImagePlugin",
+    ".blp": "BlpImagePlugin",
+    ".bufr": "BufrStubImagePlugin",
+    ".cur": "CurImagePlugin",
+    ".dcx": "DcxImagePlugin",
+    ".dds": "DdsImagePlugin",
+    ".ps": "EpsImagePlugin",
+    ".eps": "EpsImagePlugin",
+    ".fit": "FitsImagePlugin",
+    ".fits": "FitsImagePlugin",
+    ".fli": "FliImagePlugin",
+    ".flc": "FliImagePlugin",
+    ".fpx": "FpxImagePlugin",
+    ".ftc": "FtexImagePlugin",
+    ".ftu": "FtexImagePlugin",
+    ".gbr": "GbrImagePlugin",
+    ".grib": "GribStubImagePlugin",
+    ".h5": "Hdf5StubImagePlugin",
+    ".hdf": "Hdf5StubImagePlugin",
+    ".icns": "IcnsImagePlugin",
+    ".ico": "IcoImagePlugin",
+    ".im": "ImImagePlugin",
+    ".iim": "IptcImagePlugin",
+    ".jp2": "Jpeg2KImagePlugin",
+    ".j2k": "Jpeg2KImagePlugin",
+    ".jpc": "Jpeg2KImagePlugin",
+    ".jpf": "Jpeg2KImagePlugin",
+    ".jpx": "Jpeg2KImagePlugin",
+    ".j2c": "Jpeg2KImagePlugin",
+    ".mic": "MicImagePlugin",
+    ".mpg": "MpegImagePlugin",
+    ".mpeg": "MpegImagePlugin",
+    ".mpo": "MpoImagePlugin",
+    ".msp": "MspImagePlugin",
+    ".palm": "PalmImagePlugin",
+    ".pcd": "PcdImagePlugin",
+    ".pcx": "PcxImagePlugin",
+    ".pdf": "PdfImagePlugin",
+    ".pxr": "PixarImagePlugin",
+    ".psd": "PsdImagePlugin",
+    ".qoi": "QoiImagePlugin",
+    ".bw": "SgiImagePlugin",
+    ".rgb": "SgiImagePlugin",
+    ".rgba": "SgiImagePlugin",
+    ".sgi": "SgiImagePlugin",
+    ".ras": "SunImagePlugin",
+    ".tga": "TgaImagePlugin",
+    ".icb": "TgaImagePlugin",
+    ".vda": "TgaImagePlugin",
+    ".vst": "TgaImagePlugin",
+    ".tif": "TiffImagePlugin",
+    ".tiff": "TiffImagePlugin",
+    ".webp": "WebPImagePlugin",
+    ".wmf": "WmfImagePlugin",
+    ".emf": "WmfImagePlugin",
+    ".xbm": "XbmImagePlugin",
+    ".xpm": "XpmImagePlugin",
+}
+
+
+def _load_plugin_for_extension(ext: str | bytes) -> bool:
+    """Load only the plugin needed for a specific file extension."""
+    if isinstance(ext, bytes):
+        ext = ext.decode()
+    if ext in EXTENSION:
+        return True
+
+    plugin = _EXTENSION_PLUGIN.get(ext.lower())
+    if plugin is None:
+        return False
+
+    try:
+        __import__(f"PIL.{plugin}", globals(), locals(), [])
+        return True
+    except ImportError:
+        return False
+
 
 def preinit() -> None:
     """
@@ -2535,11 +2631,13 @@ def save(
             # only set the name for metadata purposes
             filename = os.fspath(fp.name)
 
-        preinit()
-
         filename_ext = os.path.splitext(filename)[1].lower()
         ext = filename_ext.decode() if isinstance(filename_ext, bytes) else filename_ext
 
+        # Try loading only the plugin for this extension first
+        if not _load_plugin_for_extension(ext):
+            preinit()
+
         if not format:
             if ext not in EXTENSION:
                 init()
@@ -3524,7 +3622,11 @@ def open(
 
     prefix = fp.read(16)
 
-    preinit()
+    # Try to load just the plugin needed for this file extension
+    # before falling back to preinit() which loads common plugins
+    ext = os.path.splitext(filename)[1] if filename else ""
+    if not (ext and _load_plugin_for_extension(ext)):
+        preinit()
 
     warning_messages: list[str] = []
 
@@ -3560,14 +3662,19 @@ def _open_core(
     im = _open_core(fp, filename, prefix, formats)
 
     if im is None and formats is ID:
-        checked_formats = ID.copy()
-        if init():
-            im = _open_core(
-                fp,
-                filename,
-                prefix,
-                tuple(format for format in formats if format not in checked_formats),
-            )
+        # Try preinit (few common plugins) then init (all plugins)
+        for loader in (preinit, init):
+            checked_formats = ID.copy()
+            loader()
+            if formats != checked_formats:
+                im = _open_core(
+                    fp,
+                    filename,
+                    prefix,
+                    tuple(f for f in formats if f not in checked_formats),
+                )
+                if im is not None:
+                    break
 
     if im:
         im._exclusive_fp = exclusive_fp

src/PIL/SpiderImagePlugin.py

@@ -290,9 +290,9 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
 
 def _save_spider(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
     # get the filename extension and register it with Image
-    filename_ext = os.path.splitext(filename)[1]
-    ext = filename_ext.decode() if isinstance(filename_ext, bytes) else filename_ext
-    Image.register_extension(SpiderImageFile.format, ext)
+    if filename_ext := os.path.splitext(filename)[1]:
+        ext = filename_ext.decode() if isinstance(filename_ext, bytes) else filename_ext
+        Image.register_extension(SpiderImageFile.format, ext)
     _save(im, fp, filename)