Hm... with only a little bit of cooperation of the container class (e.g.
xylophone), you could implement this yourself:
class xylophone:
def __contains__(self, item):
if hasattr(item, '__lcontains__'):
return item.__lcontains__(self)
return False
On Tue, Nov 12, 2019 at 5:04 PM Samuel Muldoon <muldoonsam...@gmail.com>
wrote:
> *Currently, the `in` operator (also known as `__contains__`) always uses
> the rightmost argument's implementation.*
>
> *For example,*
>
>
>> * status = obj in "xylophone" *
>>
>
> *Is similar to:*
>
> * status = "xylophone".__contains__( obj )*
>
>
> *The current implementation of `__contains__` is similar to the way that
> `+` used to only look to the leftmost argument for implementation. *
>
> * total = 4 + obj*
>>
>> * total = int.__add__(4, obj)*
>>
>
> *However, these days, `__radd__` gives us the following:*
>
> * try:*
>> * total = type(4).__add__(4, obj)*
>> * except NotImplementedError:*
>>
>> * total = type(obj).__radd__(obj, 4) *
>>
>
> *We propose something similar for `__contains__`: That a new dunder/magic
> method `__lcontains__` be created and that the `in` operator be implemented
> similarly to the following:*
>
> * # IMPLEMENTATION OF*
>>
>> * # status = obj in "xylophone"`*
>> * try:*
>> * status = "xylophone".__contains__(obj)*
>> * except NotImplementedError:*
>>
>> * status = False *
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> * if not status: try: status =
>> obj.__lcontains__(“xylophone”) except AttributeError: # type(obj)
>> does not have an `__lcontains__` method with io.StringIO() as
>> string_stream: print( "unsupported operand
>> type(s) for `in`:", repr(type(4).__name__),
>> "and", repr(type(obj).__name__),
>> file=string_stream ) msg = string_stream.getvalue()
>> raise TypeError(msg) from None*
>>
>
>
>
> *The proposed enhancement would be backwards compatible except in the
> event that a user already wrote a class having an `__lcontains__` method.*
>
> * With our running example of the string “xylophone”, writers of
> user-defined classes would be able to decide whether their objects are
> elements of “xylophone” or not. Programmer would do this by writing an
> `__lcontains__` method.*
>
> *As an example application, one might develope a tree in which each node
> represents a string (the strings being unique within the tree). A property
> of the tree might be that node `n` is a descendant of node `m` if and only
> if `n` is a sub-string of `m`. For example the string "yell" is a
> descendant of "yellow." We might want the root node of the tree to be a
> special object, `root` such that every string is in `root` and that `root`
> is in no string. That is, the code `root in "yellow"` should return
> `False`. If ` __lcontains__ ` were implemented, then we could implement the
> node as follows:*
>
>>
>>
>> *class RootNode(Node): *
>>
>> * def __contains__(container, element):*
>>
>>
>> * return True *
>>
>> * def __lcontains__(element, container):*
>>
>> * return False*
>>
> _______________________________________________
> Python-ideas mailing list -- python-ideas@python.org
> To unsubscribe send an email to python-ideas-le...@python.org
> https://mail.python.org/mailman3/lists/python-ideas.python.org/
> Message archived at
> https://mail.python.org/archives/list/python-ideas@python.org/message/X2VCWCBJZABPWJH5LNMYPODZKNM7UZML/
> Code of Conduct: http://python.org/psf/codeofconduct/
>
--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/27AFGAUBCVK4B43XXFKLT2EA4WRBN52E/
Code of Conduct: http://python.org/psf/codeofconduct/
