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mypy is unhappy with repeated use of _ as a throw-away variable #1332
Description
_ is commonly used as a throw-away variable in list unpacking, but this will make mypy unhappy if two lists with different types are unpacked in the same function. For example
x, _ = ["foo", "bar"]
n, _ = [1, 2]results in:
test.py:2: error: Incompatible types in assignment (expression has type "int", variable has type "str")
To support this pattern, we may want to consider special-casing _ and not generating this sort of type error if the variable is never read.
Activity
gvanrossum commentedon Apr 5, 2016
A complication is that
_is also commonly used as an alias forgettext. When we see an import likefrom ... import gettext as _we should not special-case it (and probably complain extra loudly if_is redefined locally).JukkaL commentedon Apr 5, 2016
In the
gettextuse case_it will be accessed within the module. As @ddfisher suggested, perhaps we should only special case_if it's never read.dmoisset commentedon Jun 9, 2016
In my opinion, any variable (no matter its name) which is used only write only can be safely typed to Any (or to object), given that this can never be an unsafe operation, so it's sound and cover this pattern well (and some alternative things like using __ as placeholder)
JukkaL commentedon Jun 9, 2016
At module and class top levels we can't determine whether a variable will be read outside the module/class, so at least here a naming convention helps.
dmoisset commentedon Jun 9, 2016
Yes, my comment was made thinking about locals
dmoisset commentedon Jun 10, 2016
A note that may help clean up the issue tracker: this seems to be the same issue as #465 and #1649, they can probably be unified
ddfisher commentedon Jun 10, 2016
Instead of special casing write-only variables to have the Any type, we could also just be willing to infer Union types for things. E.g.:
Come to think of it, this would likely also solve the "variable must only have a single type" problem (provided that is something we want to solve) because of the way we already understand assignments to Union types.
gvanrossum commentedon Jun 10, 2016
ddfisher commentedon Jun 11, 2016
Not necessarily! We add more specific types to the binder on assignment, so e.g.:
JukkaL commentedon Mar 3, 2017
Closing this as a duplicate of #465.