[Python-Dev] dict __contains__ raises TypeError on unhashable input

[Ben Hoyt]

"unhashable key" doesn't make sense to me. If it's a "key", it has to be
hashable, and I'd think it'd often be a sign of a bug if you were trying to
do "unhashable_value in mapping", so to me it seems good that it raises a
TypeError. It totally depends on what's on the RHS of the in -- for
instance, you get a TypeError if you do:

42 in 'xyz'

Whereas by your reasoning, it should return False, because the integer 42
can never be "in" that string. But really you want the TypeError, because
that's a bug waiting to happen.

This is something of an aside, but I like the fact that Python is
(relatively) strongly typed, and I think this is an example of that. Just
the other day I was surprised to learn that in both Java and C# you can do:

"foo" + 3 + 4

And get "foo34" without any compile-time or run-time error (in a statically
typed language like Java or C# I would have expected a compile-time error).
A friend had written code equivalent to that, except with the 3 and 4 being
integer variables. He was expecting to see "foo7" because that was his
intent, but then realized what was going on -- numbers are automatically
converted to strings in this context, and the + operators are applied
left-to-right.

Whereas if you try that in Python, bomp -- TypeError: cannot concatenate
'str' and 'int' objects!

-Ben


On Sat, Jul 20, 2013 at 1:30 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

> http://bugs.python.org/**issue18510 <http://bugs.python.org/issue18510>
>
> Commenting further:
>
>     some_key in dict
>
> is conceptually the same as
>
>     some_key in dict.keys()
>
> which /would/ return False for an unhashable key -- at least it did in
> 2.x; for 3.x you have to say
>
>     some_key in list(dict.keys())
>
> which seems like a step backwards.
>
> Is it worth changing __contains__ and keys() to be in line with equality?
>
>
> --
> ~Ethan~
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