Slicing in Sequence

The Pythonic convention of excluding the last item in slices and ranges works well with the zero-based indexing used in Python, C and many other languages.

# Given the list below
l  =  [10,20,30,40,50,60]
# index 0  1  2  3  4  5
print(l[:2]) # start at index 0, stop at index 2
>>> 10, 20
print(l[2:]) # start at index 2, no stop -> retrieve all, starting from index 2
>>> 30, 40, 50, 60
print(l[:3]) # start at index 0, no stop and step is 3: 0, 0+3, 3+3 etc.
>>> 10, 40
print(l[3::2]) # start at index 3, no stop and step is 2: 3, 3+2 etc.
>>> 40, 60

The stride/step can also be negative, returning items in reverse

s = "bicycle"
print(s[::3])
>>> 'bye'
print(s[::-1]) # negative step: bicycle -> elcycib
>>> 'elcycib'
print(s[::-2]) # negative step: bicycle -> elcycib, and step 2 -> eccb
>>> 'eccb'

# start at 3: bic<-y->cle
# No stop
# step in negative 2, ycib, index 0, 0+2, i.e. y then i
print(s[3::-2])
>>> 'yi'

# start at 5: bicyc<-l->e
# stop at 2: cycl
# step in negative 2 (reverse search and skip 2): cycl -> lcyc ->index 0, 0+2, ly
# step in negative 2, bic<-y, i.e. y then i
print(s[5:2:-2]) 
>>> 'ly'

To evaluate the expression seq[start:stop:step], Python calls seq.__getitem__(slice(start, stop, step))

Slice Object contains indice, start, stop, step

The slice() constructor creates a Slice object representing the set of indices specified by range(start, stop, step)

For example, slice(1, 5, 2) contains the indice with (1, 3) –Reference

The [ ] operator can also take multiple indexes or slices separated by commas. The __getitem__ and __setitem__ special methods that handle the [] operator simply receive the indices in a[i, j] as a tuple. In other words, to evaluate a[i, j], Python calls a.__getitem__((i, j)).

The built-in sequence types in Python are one-dimensional, so they support only one index or slice, and not a tuple of them.

Multidimensional Slicing and Ellipsis