Not detecting UnboundLocalError

[tlynn]

mypy doesn't seem to warn about control flows which might lead to UnboundLocalError, e.g.:

def func(arg):
    if arg:
        variable = 1
    print(variable)

func(0)

Is this deliberate?

[JukkaL]

This is deliberate -- mypy generally doesn't check for whether a variable is defined or not, as it's not directly related to type checking

It's somewhat difficult to implement without false positives. It could be a useful thing to have, if we can (mostly) avoid false positives.

[tlynn]

Ok. Thanks for the quick response. I haven't found any other linting tool that can do it either :-(

[gvanrossum]

Actually I wonder if the "binder" in mypy could be used to do this -- it's been quite successful in determining whether the end of a block can actually be reached or not (in order to implement --warn-no-return), so maybe it could also keep track of whether a variable is set or not in each branch.

[JukkaL]

The original example should be possible to support without many false positives -- we define a variable conditionally and later read it unconditionally in a parent block, which looks like a pretty clear error, at least if there are no return/continue/break statements in the middle.

Things like this are harder:

if foo():
    var = 1
...
if foo():
    print(var)  # may be safe or unsafe, depending on foo()

Not sure if those are common, but I've seen examples like that.

[JukkaL]

Mypy could also perhaps check if attributes of a class instance are defined, at least in some cases (see #2877 for an example).

[tlynn]

For anyone stumbling across this: I've found that pytype https://github.com/google/pytype seems to be able to detect this issue.