[Python-Dev] <int> in <str>

[Jonathan Goble]

On Tue, Apr 3, 2018 at 7:34 PM Ethan Furman <ethan at stoneleaf.us> wrote:

> This behavior was recently brought to my attention [1]:
>
> --> 1 in 'hello'
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: 'in <string>' requires string as left operand, not int
>
> However, in any other collection (set, dict, list, tuple, etc), the answer
> would be False.
>
> Does anyone remember the reason why an exception is raised in the string
> instance instead of returning False?
>

If I had to hazard a guess, I'd say it's because strings by definition, in
the sense that they are a container, can only contain strings of length 1,
whereas the other containers you listed can contain a wide variety of
Python objects (in some cases, any Python object). Thus it's easy to detect
and raise the TypeError.

Note that `[] in set()` also raises TypeError, so it's consistent: values
of types that are impossible for the container to contain generally raise
TypeError (in this case, an unhashable type and a container that only
accepts hashable types).

(I may be oversimplifying this a bit, but I think the basic idea is right.)
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