On Sun, Feb 23, 2020 at 10:17:03PM -0000, Alex Hall wrote:
> Steven D'Aprano wrote:
> > Conceptually, we should be able to reason that every object that
> > supports indexing should be iterable, without adding a special case
> > exception "...except for str".
>
> Strings already have an exception in this area. Usually `x in y` means
> `any(x == elem for elem in y)`.
I don't think that there is anything specific to the `in` operator that
demands that it is implemented that way. It is certainly a reasonable
default implementation (I think Ruby offers it as method on a base
class) but the `in` operator conceptually provides a containment test
which might be far more general than the above:
- number in interval
- location in region
- subset in set
- subtree in tree
- offence in criminal_record
- monster in room
just off the top of my head.
In the case of strings, if the argument is a length-1 string, then your
implementation works. If it isn't, a generalisation of it works: rather
than iterate over substrings of length 1, iterate over substrings of
the same length as the argument.
So conceptually we have the string `in` operator being equivalent to:
any(arg == sub for sub in thestring)
except that iteration is not necessarily in length-1 chunks.
(Note that for efficiency reasons, string containment testing may not
actually be implemented in that way.)
> Another somewhat related example: we usually accept that basically
> every object can be treated as a boolean, even more so of it has a
> `__len__`. But numpy and pandas break this 'rule' by raising an
> exception if you try to treat an array as a boolean
Yes, they do, and I think they are wrong to have done so. But they had
their reasons, and its hard to know whether any other alternative would
have been better.
--
Steven
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