Problem with list slice syntax in python

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)

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.