Why Do We Use This?

[Ken Seehof]

The "long" versions are not used in the way that you describe.  They are not
generally called directly, by name (e.g. x.__len__()), although they can be.

The missing concept here is called "operator overloading".  Operator
overloading gives you the ability to define the meaning of various
operations (such as length) by defining a function that will be called to
evaluate the operation.

Suppose you want to create a new class that behaves like a sequence.  For
example:

>>> x = MyClass()
>>> x[4] = 34.5
>>> n = len(x)

How would you go about writing MyClass?

The answer is that you define the "magic" functions that get called for
various sequence operations.

class MyClass:
    . . .
    def __len__(self):
        # . . . return the length
    def __setitem(self, i, v):
        # . . . set the value at position i

So __len__ gets called to evaluate len(x).

Operator overloading also is used for numeric classes, mappings (like
dictionaries), and also let you define the string representation and other
special effects.

--
Ken Seehof
kens at sightreader.com
starship.python.net/crew/seehof
Hi! I'm a .signature virus! copy me into your .signature file to help me
spread!

Akira Kiyomiya wrote:

> Okay, what is the difference between these?
>
> If these are same, whey do we bother using the "long" version such as
> __len__(a)??
>
> Sorry, the books I am reading do not explain very well about this issue.
>
> a = [1,2,3,4,5,6]
> len(a)                                # __len__(a)
> x = a[2]                            # __getitem__(a,2)
> a[1] = 7                            # __setitem__(a,1,7)
> del a[2]                            # __delitem__(a,2)
> x = a[1:5]                         #__getslice__(a,1,5)
> a[1:3] = [10,11,12]         # __setslice__(a,1,3,[10,11,12])
> del a[1:4]                        # __delslice__(a,1,4)
>
> Thanks
>
> Akira
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-list/attachments/20000606/f4bf4932/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 12klat.vcf
Type: text/x-vcard
Size: 343 bytes
Desc: Card for Ken Seehof
URL: <http://mail.python.org/pipermail/python-list/attachments/20000606/f4bf4932/attachment.vcf>