I'm not sure if this is the right place to bring this up, python-ideas seemed
like language issues and python-dev seemed like CPython issues.
There are several unhashable builtin types present in CPython, as of today the
ones I've noticed are: lists, dicts, sets, and bytearrays.
Two of these are containers that require (at least partially) their members to
be hashable: dicts and sets
By this logic you cannot have a set of lists or a dict of sets to ints.
CPython however does not stop or warn you if you attempt to do such a thing
until it hits `BUILD_MAP` or `BUILD_SET` during runtime.
This is reasonable behavior when it's not possible to infer the member type of
the container i.e. ``{f(x) for x in iterable}`` or ``{f(x): y for zip(xs, ys)}``
However, given the situation where literals are nested i.e. ``{[*gen] for gen
in gens}`` or ``{{green: eggs}, {and_: ham}}`` this presents an unavoidable
exception at runtime.
I suggest emitting a SyntaxWarning when encountering these cases of literals
that produce unhashable types that are used in literals that produce types
where the members must be hashable.
I don't think it should be a SyntaxError because it's not a language issue, its
an implementation issue.
I don't think it should be a linters responsibility because for the most part
linters should consider language issues/idioms not side-effects from the
running implementation.
I do understand that such cases this issue addresses may be uncommon and once
you do get that TypeError raised its a relatively quick and easy fix, but
consider this being present in code paths that don't get taken as frequently,
large codebases where it becomes difficult to keep track of small one liner
literals like this or even for the newer programmers toying with Python through
CPython and naively using unhashables in places they shouldn't be.
Either way I'm interested in hearing what the core team thinks of this
suggestion, thanks in advance! :D
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