There's also a gotcha with default parameters:
def fun(a = []):
a += [1]
return a
fun()
print(fun()) # [1, 1]
issues I had with the tool you linked
I tried to make a link to this code in this visual debugger, but a link is made broken and can't be used without urlencoding spaces (I guess). Also the tool doesn't produce visual output in DuckDuckGo browser on Android