From 6ad98ba3c0c371c9bddf53b0939e7632ede8c2b7 Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Thu, 18 Jun 2020 21:40:38 +1000
Subject: [PATCH] Do not ignore viewer if order is zero when registering

---
 Tests/test_imageshow.py | 12 +++++++-----
 src/PIL/ImageShow.py    |  2 +-
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/Tests/test_imageshow.py b/Tests/test_imageshow.py
index 64f15326b..fddc73bd1 100644
--- a/Tests/test_imageshow.py
+++ b/Tests/test_imageshow.py
@@ -17,19 +17,21 @@ def test_register():
     ImageShow._viewers.pop()
 
 
-def test_viewer_show():
+@pytest.mark.parametrize(
+    "order", [-1, 0],
+)
+def test_viewer_show(order):
     class TestViewer(ImageShow.Viewer):
-        methodCalled = False
-
         def show_image(self, image, **options):
             self.methodCalled = True
             return True
 
     viewer = TestViewer()
-    ImageShow.register(viewer, -1)
+    ImageShow.register(viewer, order)
 
     for mode in ("1", "I;16", "LA", "RGB", "RGBA"):
-        with hopper() as im:
+        viewer.methodCalled = False
+        with hopper(mode) as im:
             assert ImageShow.show(im)
         assert viewer.methodCalled
 
diff --git a/src/PIL/ImageShow.py b/src/PIL/ImageShow.py
index fc5089423..cd85e81b4 100644
--- a/src/PIL/ImageShow.py
+++ b/src/PIL/ImageShow.py
@@ -31,7 +31,7 @@ def register(viewer, order=1):
         pass  # raised if viewer wasn't a class
     if order > 0:
         _viewers.append(viewer)
-    elif order < 0:
+    else:
         _viewers.insert(0, viewer)