Trailing commas are really easy to miss. When reading this line of code, I did not notice it immediately; I originally assumed that varname was being assigned a list.
This sort of code would be very confusing when I'm just quickly reading through a procedure trying to find the potential bug.
I had to check that on the REPL. I'm surprised that even works and I can't think of a good reason why should list syntax be allowed as a lvalue, in addition to tuple syntax.
Superficially it looks like an operator, but I suspect that's merely because of whitespace freedom; i.e., a, = [0] is equivalent to a,=[0] and a ,= [0].
Trailing commas are a bit subtle and readers of the code may think it was a mistake. (We spend more time reading code later than writing it in the first place.)
Even Python removed one of its most prominent cases of this, the print command. In Python 2.x you could have a trailing comma after a print to omit the new-line but Python 3's print() function requires print('something', end=' ') to be more explicit about it.
