mypy should error when code might raise UnboundLocalError

[dgoldstein0]

• Are you reporting a bug, or opening a feature request?

Feature request

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

I found a case in my code recently where I had a typed function that had an UnboundLocalError - one of the branches used a variable that was undefined, but the same name was used in another place so mypy just assumed that it would be written before it was read. A simplified version would be like this:

def g(a,b):
  # type: (bool, bool) -> None
  if a:
    for m in [1,2]:
      pass
  if b:
    return m # bug here

Another related scenario (credit: @gvanrossum for this code):

def g(b):
    # type: (bool) -> None
    if b:
        x = 1
    y = x  # Here we don't actually know whether x exists

An even simpler case from @ilevkivskyi:

y = x
x = 1

• What is the actual behavior/output?

Mypy doesn't give errors about this code

• What is the behavior/output you expect?

I would like mypy to detect these as bugs. In particular it would probably make sense if there were a flag like --strict-optional for this - given that this behavior probably would break existing projects if it were on by default. I would like any variable that mypy can't prove is assigned before use to be an error; if mypy can't prove a variable is written before use, even if it is, I'd consider that poorly-written code. I suppose I just expect some basic control-flow analysis - once a variable is written, it's available to all lines in the same basic block, and any basic block that can only be reached by a basic block that assigns that local.

[dgoldstein0]

another minor point: with statements can technically swallow these errors, e.g. with self.assertRaises(...): and similar. That said I don't really see a use case for where typechecked code should allow UnboundLocalErrors at runtime - it's nice to be able to swallow errors from called functions (e.g. try... except Exception: pass or similar patterns) but if mypy can see a possible error of the unbound local kind, it should probably say something.

[ilevkivskyi]

It looks like this is a duplicate of #2400. I will close this one and raise the priority of the other issue instead.