[Python-Dev] Semantics of int(), index() (original) (raw)

Oscar Benjamin oscar.j.benjamin at gmail.com
Thu Apr 4 16:23:38 CEST 2013

On 4 April 2013 10:39, Hrvoje Niksic <hrvoje.niksic at avl.com> wrote:

On Wed, Apr 3, 2013 at 6:47 AM, Hrvoje Niksic <hrvoje.niksic at avl.com> wrote:

It seems like a good feature that an int implementation can choose to return an int subclass with additional (and optional) information. After all, int subclass instances should be usable everywhere where ints are, including in C code. [SNIP] The typical reason to subclass int is to add more information or new methods on the instance, not to break basic arithmetic. Doing anything else breaks subtitutability and is well outside the realm of "consenting adults". Someone who wants to change basic arithmetic is free to implement an independent int type.

The reason for calling int(obj) is to get an object that is precisely of type int. When I call this I do not want any modified or additional methods or data attached to the resulting object.

The example given at the start of the thread makes minimal modifications in an int subclass but still allows the result of int(obj) to be unpickleable:

class Int1(int): ... def int(self): ... return self ... n = Int1(4) n 4 import pickle ni = int(n) pickle.dumps(ni) ... snip ... File "q:\tools\Python27\lib\pickle.py", line 562, in save_tuple save(element) File "q:\tools\Python27\lib\pickle.py", line 331, in save self.save_reduce(obj=obj, *rv) File "q:\tools\Python27\lib\pickle.py", line 401, in save_reduce save(args) File "q:\tools\Python27\lib\pickle.py", line 286, in save f(self, obj) # Call unbound method with explicit self File "q:\tools\Python27\lib\pickle.py", line 562, in save_tuple save(element) File "q:\tools\Python27\lib\pickle.py", line 291, in save issc = issubclass(t, TypeType) RuntimeError: maximum recursion depth exceeded while calling a Python object

I don't know whether that's a bug in pickle, but I think it's fair to say that there are times when someone wants an object that is precisely of type int. They should be able to rely on int(obj) returning an int or raising an error.

This is true similarly for index. The entire purpose of index is to permit APIs like list.getitem to work extensibly with any int-like object. This is achieved by allowing any object to advertise its convertibility from an int-like object to an int with the same numeric value. Anyone who calls operator.index(obj) is explicitly stating that they do not want any property of obj apart from its integer value and that they want that as an int. That it should be an object of type int is explicitly stated in PEP 357 (http://www.python.org/dev/peps/pep-0357/): """ 2) The index special method will have the signature

   def __index__(self):
       return obj

   where obj must be either an int or a long.

"""

Oscar

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