Type error due to repeated use of underscore as dummy assignment target

[spkersten]

Consider the following code:

from typing import Tuple

class A: pass
class B: pass
class C: pass

def f() -> Tuple[A, B, C]:
    pass

a = A()
a, _, _ = f()

mypy reports: line 11: Incompatible types in assignment (expression has type "C", variable has type "B")

Seems like mypy treats _ as just a variable. That prevents it from being used as a dummy in assignments like the one above.

A solution might be to always give _ the Any type.

[refi64]

Technically, Python itself treats it as a variable:

ryan@DevPC-LX:~$ python3
Python 3.3.5 |Continuum Analytics, Inc.| (default, Mar 10 2014, 11:19:31) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-54)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def a(_, _): pass
... 
  File "<stdin>", line 1
SyntaxError: duplicate argument '_' in function definition
>>> 1
1
>>> _
1
>>> 

See here.

[JukkaL]

Hmm, this is a bit tricky. A conservative (but ad-hoc) option would be to give variables named _ the type Any if and only if they are only assigned to but never read. Actually, we could give any local variable that is only assigned to in a multiple assignment but that is never read the type Any, I think.

So this would be accepted, since dummy is never read:

def foo() -> None:
    foobar, dummy, dummy = 1, 2, 'x'
    g(foobar)

Not sure if this generalization would be that useful, though.

We could also give a warning if a local variable is only assigned to, unless this happens in a multiple assignment (or a similar context) where it's okay to ignore the values of all variables except at least one.

[o11c]

I argue that only names that start with an underscore should be considered.

I have some code like a, _b, _c = foo() where I remove the underscore if I'm actually going to use the variable but keep a name attached for documentation reasons.

[o11c]

Further complication: what about people who use _ for gettext ? IMO that is unique enough that it's worth special-casing.

[JukkaL]

But the rule for ignoring only things are only assigned to would work with gettext / _, as then you'd actually read the value of _. We probably want to give a warning if there's a normal-looking local variable such as dummy that is never read.