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 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  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.  Not actually a guarantee. -- Steven _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor