[Tutor] Question on implmenting __getitem__ on custom classes

[Steven D'Aprano]

On Tue, Apr 23, 2019 at 11:46:58AM +0530, Arup Rakshit wrote:
> Hi,
> 
> I wrote below 2 classes to explore how __getitem__(self,k) works in 
> conjuection with list subscriptions. Both code works. Now my questions 
> which way the community encourages more in Python: if isinstance(key, 
> slice): or if type(key) == slice: ?

In general, we should normally use `isinstance`, because it works with 
subclasses.

But `slice` can't be subclassed:

py> class S(slice):
...     pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: type 'slice' is not an acceptable base type


so there is no advantage to using `isinstance`. (There is no 
disadvantage either.)

I would use `type(key) is slice` to guarantee[1] that the key is 
certainly a slice.

Why use `is` instead of `==`?

The `is` operator will be a tiny bit faster than `==`, but more 
importantly, you could have a class designed to pretend to be a slice. 
It isn't easy to do (you would have to write a metaclass, which makes it 
an advanced technique) but by using `is` we can eliminate even that slim 
chance.


> How should I implement this if I 
> follow duck typing, because none of the code currently I wrote using 
> duck typing techiniqe IMO.


Why bother? Duck-typing is good for *data* values, but a slice is not a 
data value, it is a way of specifying a range of indexes.


> class MyCustomList:
>     def __init__(self, list = []):
>         self.list = list

Watch out here, you have a mutable default value, that probably doesn't 
work the way you expect. The default value is created ONCE, and then 
shared, so if you do this:

a = MyCustomList()  # Use the default list.
b = MyCustomList()  # Shares the same default list!
a.append(1)
print(b.list)
# prints [1]

You probably want:

     def __init__(self, list=None):
         if list is None:
             list = []
         self.list = list


>     def __getitem__(self, key):
>         if isinstance(key, slice):
>             return self.list[key]
>         else:
>             return self.list[key]


The "isinstance" check is useless, because you do precisely the same 
thing in both branches.

    def __getitem__(self, key):
         return self.list[key]


will do exactly the same, and more efficiently.






[1] Not actually a guarantee.


-- 
Steven