Added warnings if image file identification fails due to lack of support

Tests/test_file_webp.py

@@ -1,6 +1,6 @@
 from helper import unittest, PillowTestCase, hopper
 
-from PIL import Image
+from PIL import Image, WebPImagePlugin
 
 try:
     from PIL import _webp
@@ -13,15 +13,27 @@ class TestFileWebp(PillowTestCase):
 
     def setUp(self):
         if not HAVE_WEBP:
-            self.skipTest('WebP support not installed')
             return
 
         self.rgb_mode = "RGB"
 
+    def test_unsupported(self):
+        if HAVE_WEBP:
+            WebPImagePlugin.SUPPORTED = False
+
+        file_path = "Tests/images/hopper.webp"
+        self.assert_warning(UserWarning,
+            lambda: self.assertRaises(IOError, Image.open, file_path))
+
+        if HAVE_WEBP:
+            WebPImagePlugin.SUPPORTED = True
+
+    @unittest.skipIf(not HAVE_WEBP, "WebP support not installed")
     def test_version(self):
         _webp.WebPDecoderVersion()
         _webp.WebPDecoderBuggyAlpha()
 
+    @unittest.skipIf(not HAVE_WEBP, "WebP support not installed")
     def test_read_rgb(self):
         """
         Can we read a RGB mode WebP file without error?
@@ -41,6 +53,7 @@ class TestFileWebp(PillowTestCase):
         self.assert_image_similar_tofile(
             image, 'Tests/images/hopper_webp_bits.ppm', 1.0)
 
+    @unittest.skipIf(not HAVE_WEBP, "WebP support not installed")
     def test_write_rgb(self):
         """
         Can we write a RGB mode file to webp without error.
@@ -70,6 +83,7 @@ class TestFileWebp(PillowTestCase):
         target = hopper(self.rgb_mode)
         self.assert_image_similar(image, target, 12.0)
 
+    @unittest.skipIf(not HAVE_WEBP, "WebP support not installed")
     def test_write_unsupported_mode_L(self):
         """
         Saving a black-and-white file to WebP format should work, and be
@@ -90,6 +104,7 @@ class TestFileWebp(PillowTestCase):
 
         self.assert_image_similar(image, target, 10.0)
 
+    @unittest.skipIf(not HAVE_WEBP, "WebP support not installed")
     def test_write_unsupported_mode_P(self):
         """
         Saving a palette-based file to WebP format should work, and be
@@ -110,6 +125,7 @@ class TestFileWebp(PillowTestCase):
 
         self.assert_image_similar(image, target, 50.0)
 
+    @unittest.skipIf(not HAVE_WEBP, "WebP support not installed")
     def test_WebPEncode_with_invalid_args(self):
         """
         Calling encoder functions with no arguments should result in an error.
@@ -119,6 +135,7 @@ class TestFileWebp(PillowTestCase):
             self.assertRaises(TypeError, _webp.WebPAnimEncoder)
         self.assertRaises(TypeError, _webp.WebPEncode)
 
+    @unittest.skipIf(not HAVE_WEBP, "WebP support not installed")
     def test_WebPDecode_with_invalid_args(self):
         """
         Calling decoder functions with no arguments should result in an error.
@@ -128,6 +145,7 @@ class TestFileWebp(PillowTestCase):
             self.assertRaises(TypeError, _webp.WebPAnimDecoder)
         self.assertRaises(TypeError, _webp.WebPDecode)
 
+    @unittest.skipIf(not HAVE_WEBP, "WebP support not installed")
     def test_no_resource_warning(self):
         file_path = "Tests/images/hopper.webp"
         image = Image.open(file_path)

src/PIL/Image.py

@@ -2615,11 +2615,15 @@ def open(fp, mode="r"):
 
     preinit()
 
+    acceptWarnings = []
     def _open_core(fp, filename, prefix):
         for i in ID:
             try:
                 factory, accept = OPEN[i]
-                if not accept or accept(prefix):
+                result = not accept or accept(prefix)
+                if type(result) in [str, bytes]:
+                    acceptWarnings.append(result)
+                elif result:
                     fp.seek(0)
                     im = factory(fp, filename)
                     _decompression_bomb_check(im.size)
@@ -2643,6 +2647,8 @@ def open(fp, mode="r"):
 
     if exclusive_fp:
         fp.close()
+    for message in acceptWarnings:
+        warnings.warn(message)
     raise IOError("cannot identify image file %r"
                   % (filename if filename else fp))
 

src/PIL/WebPImagePlugin.py

@@ -1,4 +1,9 @@
-from . import Image, ImageFile, _webp
+from . import Image, ImageFile
+try:
+    from . import _webp
+    SUPPORTED = True
+except ImportError as e:
+    SUPPORTED = False
 from io import BytesIO
 
 
@@ -25,7 +30,10 @@ def _accept(prefix):
     is_webp_file = prefix[8:12] == b"WEBP"
     is_valid_vp8_mode = prefix[12:16] in _VP8_MODES_BY_IDENTIFIER
 
-    return is_riff_file_format and is_webp_file and is_valid_vp8_mode
+    if is_riff_file_format and is_webp_file and is_valid_vp8_mode:
+        if not SUPPORTED:
+            return "image file could not be identified because WEBP support not installed"
+        return True
 
 
 class WebPImageFile(ImageFile.ImageFile):
@@ -321,8 +329,9 @@ def _save(im, fp, filename):
 
 
 Image.register_open(WebPImageFile.format, WebPImageFile, _accept)
-Image.register_save(WebPImageFile.format, _save)
-if _webp.HAVE_WEBPANIM:
-    Image.register_save_all(WebPImageFile.format, _save_all)
-Image.register_extension(WebPImageFile.format, ".webp")
-Image.register_mime(WebPImageFile.format, "image/webp")
+if SUPPORTED:
+    Image.register_save(WebPImageFile.format, _save)
+    if _webp.HAVE_WEBPANIM:
+        Image.register_save_all(WebPImageFile.format, _save_all)
+    Image.register_extension(WebPImageFile.format, ".webp")
+    Image.register_mime(WebPImageFile.format, "image/webp")