Raise error if transparency is incorrect type or length when saving

Tests/test_file_png.py

@@ -502,8 +502,9 @@ class TestFilePng:
             im = roundtrip(im)
         assert im.info["transparency"] == (248, 248, 248)
 
-        im = roundtrip(im, transparency=(0, 1, 2))
-        assert im.info["transparency"] == (0, 1, 2)
+        for transparency in ((0, 1, 2), [0, 1, 2]):
+            im = roundtrip(im, transparency=transparency)
+            assert im.info["transparency"] == (0, 1, 2)
 
     def test_trns_p(self, tmp_path: Path) -> None:
         # Check writing a transparency of 0, issue #528
@@ -518,6 +519,36 @@ class TestFilePng:
 
             assert_image_equal(im2.convert("RGBA"), im.convert("RGBA"))
 
+    def test_trns_invalid(self, tmp_path: Path) -> None:
+        out = tmp_path / "temp.png"
+
+        for mode in ("1", "L", "I;16"):
+            im = Image.new(mode, (1, 1))
+            with pytest.raises(
+                ValueError, match=f"transparency for {mode} must be an integer"
+            ):
+                im.save(out, transparency="invalid")
+
+        im = Image.new("I", (1, 1))
+        with pytest.warns(DeprecationWarning, match="Saving I mode images as PNG"):
+            with pytest.raises(ValueError):
+                im.save(out, transparency="invalid")
+
+        im = Image.new("P", (1, 1))
+        with pytest.raises(
+            ValueError, match="transparency for P must be an integer or bytes"
+        ):
+            im.save(out, transparency="invalid")
+
+        im = Image.new("RGB", (1, 1))
+        with pytest.raises(
+            ValueError, match="transparency for RGB must be list or tuple"
+        ):
+            im.save(out, transparency="invalid")
+
+        with pytest.raises(ValueError, match="transparency for RGB must have length 3"):
+            im.save(out, transparency=(1, 2))
+
     def test_trns_null(self) -> None:
         # Check reading images with null tRNS value, issue #1239
         test_file = "Tests/images/tRNS_null_1x1.png"

src/PIL/PngImagePlugin.py

@@ -1451,16 +1451,30 @@ def _save(
             alpha_bytes = colors
             if isinstance(transparency, bytes):
                 chunk(fp, b"tRNS", transparency[:alpha_bytes])
-            else:
+            elif isinstance(transparency, int):
                 transparency = max(0, min(255, transparency))
                 alpha = b"\xff" * transparency + b"\0"
                 chunk(fp, b"tRNS", alpha[:alpha_bytes])
+            else:
+                msg = "transparency for P must be an integer or bytes"
+                raise ValueError(msg)
         elif im.mode in ("1", "L", "I", "I;16"):
-            transparency = max(0, min(65535, transparency))
-            chunk(fp, b"tRNS", o16(transparency))
+            if isinstance(transparency, int):
+                transparency = max(0, min(65535, transparency))
+                chunk(fp, b"tRNS", o16(transparency))
+            else:
+                msg = f"transparency for {im.mode} must be an integer"
+                raise ValueError(msg)
         elif im.mode == "RGB":
-            red, green, blue = transparency
-            chunk(fp, b"tRNS", o16(red) + o16(green) + o16(blue))
+            if not isinstance(transparency, (list, tuple)):
+                msg = "transparency for RGB must be list or tuple"
+                raise ValueError(msg)
+            elif len(transparency) != 3:
+                msg = "transparency for RGB must have length 3"
+                raise ValueError(msg)
+            else:
+                red, green, blue = transparency
+                chunk(fp, b"tRNS", o16(red) + o16(green) + o16(blue))
         elif im.encoderinfo.get("transparency") is not None:
             # don't bother with transparency if it's an RGBA
             # and it's in the info dict. It's probably just stale.