[Python-Dev] boxing and unboxing data types

[Neil Girdhar]

On Mon, Mar 9, 2015 at 2:07 AM, Serhiy Storchaka <storchaka at gmail.com>
wrote:

> On 09.03.15 06:33, Ethan Furman wrote:
>
>> I guess it could boil down to:  if IntEnum was not based on 'int', but
>> instead had the __int__ and __index__ methods
>> (plus all the other __xxx__ methods that int has), would it still be a
>> drop-in replacement for actual ints?  Even when
>> being used to talk to non-Python libs?
>>
>
> If you don't call isinstance(x, int) (PyLong_Check* in C).
>
> Most conversions from Python to C implicitly call __index__ or __int__,
> but unfortunately not all.
>
> >>> float(Thin(42))
> 42.0
> >>> float(Wrap(42))
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: float() argument must be a string or a number, not 'Wrap'
>
> >>> '%*s' % (Thin(5), 'x')
> '    x'
> >>> '%*s' % (Wrap(5), 'x')
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: * wants int
>
> >>> OSError(Thin(2), 'No such file or directory')
> FileNotFoundError(2, 'No such file or directory')
> >>> OSError(Wrap(2), 'No such file or directory')
> OSError(<__main__.Wrap object at 0xb6fe81ac>, 'No such file or directory')
>
> >>> re.match('(x)', 'x').group(Thin(1))
> 'x'
> >>> re.match('(x)', 'x').group(Wrap(1))
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> IndexError: no such group
>
> And to be ideal drop-in replacement IntEnum should override such methods
> as __eq__ and __hash__ (so it could be used as mapping key). If all methods
> should be overridden to quack as int, why not take an int?
>
>
You're absolutely right that if *all the methods should be overrriden to
quack as int, then you should subclass int (the Liskov substitution
principle).  But all methods should not be overridden — mainly the methods
you overrode in your patch should be exposed.  Here is a list of methods on
int that should not be on IntFlags in my opinion (give or take a couple):

__abs__, __add__, __delattr__, __divmod__, __float__, __floor__,
__floordiv__, __index__, __lshift__, __mod__, __mul__, __pos__, __pow__,
__radd__, __rdivmod__, __rfloordiv__, __rlshift__, __rmod__, __rmul__,
__round__, __rpow__, __rrshift__, __rshift__, __rsub__, __rtruediv__,
__sub__, __truediv__, __trunc__, conjugate, denominator, imag, numerator,
real.

I don't think __index__ should be exposed either since are you really going
to slice a list using IntFlags?  Really?


>
>
> _______________________________________________
> Python-Dev mailing list
> Python-Dev at python.org
> https://mail.python.org/mailman/listinfo/python-dev
> Unsubscribe: https://mail.python.org/mailman/options/python-dev/
> mistersheik%40gmail.com
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-dev/attachments/20150309/5010b6b6/attachment-0001.html>