On 9/29/2010 8:34 AM, Hrvoje Niksic wrote:
Steven D'Aprano<st...@remove-this-cybersource.com.au> writes:
On Wed, 29 Sep 2010 02:20:55 +0100, MRAB wrote:
On 29/09/2010 01:19, Terry Reedy wrote:
A person using instances of a class should seldom use special names
directly. They are, in a sense, implementation details, even if
documented. The idiom "if __name__ == '__main__':" is an exception.
__file__ is another exception.
As are __iter__, __next__, __add__, __dict__, and, yes, __class__, to
say nothing of all the other special methods.
I strongly disagree.
I think by "person using instances of a class" Terry referred to the
user of a class as opposed to the implementor.
Yes.
In that sense the user
should be calling iter(foo) instead of foo.__iter__(), next(foo) instead
of foo.__next__(), and foo+bar instead of foo.__add__(bar).
Yes. Guido added iter() and next() to the list of built-in functions,
even though they seem reduncant.. I believe it is his intention that the
use of special names outside of class statements should be fairly rare.
If I remember right, in the old, pre-2.2 system that separated built-in
types and user-written classes, the builtins did not have accessible
special method attributes. They are only for customizing user classes.
So one could not have generically written foo.__add__(bar).
Special-method attribute were added to builtins so that they could be
inherited (or replaced) by user-written subclasses, not so that one
could replace normal syntax.
> Direct
calls to special-name methods, such as __len__, often indicate that the
programmer hasn't grasped how those methods are intended to be used in
Python.
Right. That fact that *Python* interpreters implement syntax with
special methods is an implementation detail of the *language*. The
importance is that it allow *writers* of new classes to rather easily
imitate built-in classes so that their classes seamlessly plug into the
syntax.
--
Terry Jan Reedy
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