List Slicing

In
Python, expressions can be written that select subsections of a sequence, known
as slices. A slice is a span of items that are taken from a sequence.

To
get a slice of a list, you write an expression in the following general format:

list_name[start : end]

In
the general format, start is the index of the first element in the
slice, and end is the index marking the end of the slice. The expression
returns a list containing a copy of the elements from start up to (but
not including) end.

>>>
numbers = [1, 2, 3, 4, 5]

>>>
print(numbers)

[1,
2, 3, 4, 5]

 

>>>
print(numbers[1:3])

[2,
3]

 

>>>
print(numbers[:3])

[1,
2, 3]

 

>>>
print(numbers[2:])

[3,
4, 5]

 

>>>
print(numbers[:])

[1,
2, 3, 4, 5]

 

Slicing
expressions can also have step value, which can cause elements to be skipped in
the list.

The
following interactive mode session shows an example of a slicing expression
with a step value:

>>>
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

>>>
print(numbers)

[1,
2, 3, 4, 5, 6, 7, 8, 9, 10]

 

>>>
print(numbers[1:8:2])

[2,
4, 6, 8]

 

In
the slicing expression, the third number inside the brackets is the step value.
A step value of 2, as used in this example, causes the slice to contain every
second element from the specified range in the list.

Negative
numbers are used as indexes in slicing expressions to reference positions relative
to the end of the list.

Note:
Invalid
indexes do not cause slicing expressions to raise an exception.

For
example:

• If the end index specifies a
position beyond the end of the list, Python will use the

length of the list instead.

• If the start index specifies a
position before the beginning of the list, Python will

use 0 instead.

• If the start index is greater
than the end index, the slicing expression will return

an empty list.