Problem with list slice syntax in python

Question

The extended indexing syntax is mentioned in python's doc.

slice([start], stop[, step])

Slice objects are also generated when extended indexing syntax is used. For example: a[start:stop:step] or a[start:stop, i]. See itertools.islice() for an alternate version that returns an iterator.

a[start:stop:step] works as described. But what about the second one? How is it used?

`a` is not necessarily a sequence of one of the default primitive types. — Amber, May 03 '10 at 20:41
Beats the heck out of me... it raises a TypeError if I try something like foo[3:7, 2] — Josh Wright, May 03 '10 at 20:41
The documentation gives me the impression that a[start:stop, i] should work just like a[start:stop:step]. But it doesn't. — Dingle, May 03 '10 at 20:45

score 12 · Accepted Answer · edited May 14 '19 at 11:51

12

a[start:stop,i] calls the method a.__getitem__((slice(start,stop,None), i)).

This raises a TypeError if a is a list, but it is valid and useful notation if a is a numpy array. In fact, I believe the developers of Numpy asked the developers of Python to extend valid Python slicing notation precisely so that numpy array slicing notation could be implemented more easily.

For example,

import numpy as np
arr=np.arange(12).reshape(4,3)
print(arr)
# [[ 0  1  2]
#  [ 3  4  5]
#  [ 6  7  8]
#  [ 9 10 11]]

1:3 selects rows 1 and 2, and the 2 selects the third column:

print(arr[1:3,2])
# [5 8]

PS. To experiment with what slice is getting sent to __getitem__, you can play around with this toy code:

class Foo(list):
    def __getitem__(self,key):
        return repr(key)

foo=Foo(range(10))
print(foo[1:5,1,2])
# (slice(1, 5, None), 1, 2)

edited May 14 '19 at 11:51

TrebledJ

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answered May 03 '10 at 20:47

unutbu

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I see. Basically, a[start:stop, i] only works with multidimensional arrays as implemented in numpy. – Dingle May 03 '10 at 20:53
@Dingle: Or, you can define your own classes with `__getitem__` and assign your own meaning and behavior! – unutbu May 03 '10 at 20:55
The question is similar to http://stackoverflow.com/questions/752602/slicing-in-python-expressions-documentation, but your example and the toy code are much easier to follow. – Dingle May 03 '10 at 21:09

score 4 · Answer 2 · edited Jan 12 '23 at 19:01

4

The notation [:,:] is used to slice multidimensional arrays. Python doesn't have any multi-dimensional arrays by default, but the syntax supports it and numpy for example takes advantage of this syntax.

edited Jan 12 '23 at 19:01

Glorfindel

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answered May 03 '10 at 20:34

Mark Byers

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2

And any other custom object which has potentially implemented multi-dimensional slicing via `__getitem__()`. – Amber May 03 '10 at 20:45

Problem with list slice syntax in python

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