diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -38,6 +38,7 @@
 splitnewlines = bdiff.splitnewlines
 
 
+# TODO: this looks like it could be an attrs, which might help pytype
 class diffopts(object):
     '''context is the number of context lines
     text treats all files as text
@@ -52,6 +53,8 @@
     upgrade generates git diffs to avoid data loss
     '''
 
+    _HAS_DYNAMIC_ATTRIBUTES = True
+
     defaults = {
         b'context': 3,
         b'text': False,