Feature request: Local variable never read

[alippai]

It'd be useful to track if a local variable was read before assigned again:

def main():
  x = 5
  x = 6
  print(x)

A common use-case is simply typos or using if instead of elif:

if case1:
  x = 1
if case2:
  x = 2
else:
  x = 3

Where x = 1 is never used.
The correct snippet after fixing the mistake would be:

if case1:
  x = 1
elif case2:
  x = 2
else:
  x = 3