From the docs:
# Check for whether or not a preference exists
if "com.MyName.foo.bar" in Glyphs.defaults:
# do stuff
But this is what happens (Glyphs 3.0.3 (3060)):
Why do you need that? That is not easily possible to verify. If there is a registered default, you can’t really discern if there is a user value or the default. I could add a __in__ method to the proxy that would just do return self[key] is not None.
I don’t need it; checking for None is sufficient in my use-case. It’s just the first thing I tried since it is the first thing the docs suggests doing.