Allow "loop=None" when saving

Tests/test_file_gif.py

@@ -875,6 +875,14 @@ def test_identical_frames_to_single_frame(duration, tmp_path):
         assert reread.info["duration"] == 8500
 
 
+def test_loop_none(tmp_path):
+    out = str(tmp_path / "temp.gif")
+    im = Image.new("L", (100, 100), "#000")
+    im.save(out, loop=None)
+    with Image.open(out) as reread:
+        assert "loop" not in reread.info
+
+
 def test_number_of_loops(tmp_path):
     number_of_loops = 2
 

docs/handbook/image-file-formats.rst

@@ -253,7 +253,7 @@ their :py:attr:`~PIL.Image.Image.info` values.
 
 **loop**
     Integer number of times the GIF should loop. 0 means that it will loop
-    forever. By default, the image will not loop.
+    forever. If omitted or ``None``, the image will not loop.
 
 **comment**
     A comment about the image.

src/PIL/GifImagePlugin.py

@@ -912,7 +912,7 @@ def _get_global_header(im, info):
         info
         and (
             "transparency" in info
-            or "loop" in info
+            or info.get("loop") is not None
             or info.get("duration")
             or info.get("comment")
         )
@@ -937,7 +937,7 @@ def _get_global_header(im, info):
         # Global Color Table
         _get_header_palette(palette_bytes),
     ]
-    if "loop" in info:
+    if info.get("loop") is not None:
         header.append(
             b"!"
             + o8(255)  # extension intro