How to implement slice in Python 3?

Question

I read something about slice in Python 3. Then I wrote a program, tried to implement __getitem__(self, slice(s)). Code goes below:

class NewList:
    def __init__(self, lst):
        print('new list')
        self._list = lst
    def __getitem__(self, x):
        if type(x) is slice:
            return [ self._list[n] for n in range(x.start, x.stop, x.step) ]  #error?
        else:
            return self._list[x]
    ...

nl1 = NewList([1,2,3,4,5])
nl1[1:3]  #error occurs

Then I found out x.step is None, which made range raise an exception. So, how should I implement the __getitem__ method?

@JBernardo --Do u mean that I should avoid using slice objects explicitly? — Alcott
– Alcott, Commented Aug 17, 2011 at 3:59

Eryk Sun · Accepted Answer · 2011-08-17 04:20:37Z

6

You need to use the slice.indices method. Given the length of your sequence, it returns a tuple of start, stop, step:

>>> s = slice(2, 5, None)
>>> s.indices(10)
(2, 5, 1)

>>> [x for x in range(*s.indices(10))]
[2, 3, 4]

>>> s.indices(3)
(2, 3, 1)

>>> s.indices(0)
(0, 0, 1)

answered Aug 17, 2011 at 4:20

Eryk Sun

34.5k7 gold badges97 silver badges113 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Moberg Over a year ago

Where can we find documentation om the slice class?

sammywemmy Over a year ago

slice.indices docs

rwst · Accepted Answer · 2014-01-28 16:59:41Z

4

In the case where you don't know the length of your object there is an obvious trick to circumvent this mandatory parameter. For example an infinite sequence's getitem can look like this:

  def __getitem__( self, key ) :
    if isinstance( key, slice ) :
       m = max(key.start, key.stop)
       return [self[ii] for ii in xrange(*key.indices(m+1))]
    elif isinstance( key, int ) :
       #Handle int indices

It will only fail if you don't give start and stop but with checking for None this could be handled too.

answered Jan 28, 2014 at 16:59

rwst

2,7052 gold badges34 silver badges38 bronze badges

1 Comment

sazary Over a year ago

xrange doesnt exist in python3, instead we should use range. also using this implementation you can't have slices like [:4] because max can't handle None type. Instead you should check if start and stop are not None, and if they are, assign them to zero and len of items, respectively. Also, it's good to include key.step in range, too.

Mark Ransom · Accepted Answer · 2011-08-17 03:47:06Z

1

If x is a slice, you can do the same as the other condition:

return self._list[x]

answered Aug 17, 2011 at 3:47

Mark Ransom

310k44 gold badges423 silver badges660 bronze badges

3 Comments

Alcott Over a year ago

absolutely yes, but I just want to know how to do it myself

Mark Ransom Over a year ago

@Alcott, not sure what you mean by "do it myself".

Alcott Over a year ago

sorry, I mean I want to know how the __getitem__(self,slice) implemented in list.

jcomeau_ictx · Accepted Answer · 2011-08-17 03:46:56Z

0

how about x.step or 1?

class NewList:
    def __init__(self, lst):
        print('new list')
        self._list = lst
    def __getitem__(self, x):
        if type(x) is slice:
            return [ self._list[n] for n in range(x.start, x.stop, x.step or 1) ]  #error?
        else:
            return self._list[x]

nl1 = NewList([1,2,3,4,5])
nl1[1:3]

answered Aug 17, 2011 at 3:46

jcomeau_ictx

38.6k7 gold badges98 silver badges114 bronze badges

1 Comment

Alcott Over a year ago

ya, it's way circumventing the problem, but according to what I read, no.

Collectives™ on Stack Overflow

How to implement slice in Python 3?

4 Answers 4

2 Comments

1 Comment

3 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

1 Comment

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related