Python list slice
last modified January 29, 2024
In this article we show how to create list slices in Python.
A list is an mutable, ordered collection of values. The list elements can be accessed by zero-based indexes.
A list slice is a portion of elements of a list.
Python slice syntax
List slicing is an operation that extracts certain elements from a list and forms them into another list. Possibly with different number of indices and different index ranges.
The indexes are zero-based. They can be negative.
The syntax for list slicing is as follows:
[start:end:step]
The start, end, step parts of the syntax are integers. Each of them is optional. They can be both positive and negative. The value having the end index is not included in the slice.
We include all elements up to the end index, not including the element with index end.
Python list slice start/end
The start index is the beginning index; the end index is the ending index of a slice.
#!/usr/bin/python vals = [-2, -1, 0, 1, 2, 3, 4, 5, 6] last = len(vals) s1 = vals[0:5] print(s1) s2 = vals[2:last] print(s2)
The program creates two slices.
last = len(vals)
With the len function, we get the size of the list. Since the end
index of a slice is not included, it can be used in the slice syntax.
s1 = vals[0:5]
We create a list slice with start=0 and end=5. The elements with indexes 0, 1, 2, 3 and 4 are included in the slice.
s2 = vals[2:last]
The second slice has elements with indexex 2..last-1.
$ ./main.py [-2, -1, 0, 1, 2] [0, 1, 2, 3, 4, 5, 6]
Python list slice omit indexes
All three indexes of the slice syntax can be ommitted. If we omit the start index, the slice is created from the first element. If we omit the end index, the slice is generated up to the last element. If we omit the step, than the default step is 1.
#!/usr/bin/python vals = [-2, -1, 0, 1, 2, 3, 4, 5, 6] s1 = vals[:3] print(s1) s2 = vals[3:] print(s2) s3 = vals[:] print(s3) s4 = vals[::] print(s4)
In the program, we omit the indexes.
s1 = vals[:3]
We create a slice from the beginning up to the third element.
s2 = vals[3:] print(s2)
A slice from the fourth element to the last element is created.
s3 = vals[:]
Here we create a copy of the list.
s4 = vals[::]
Here we also create a copy of the list.
$ ./main.py [-2, -1, 0] [1, 2, 3, 4, 5, 6] [-2, -1, 0, 1, 2, 3, 4, 5, 6] [-2, -1, 0, 1, 2, 3, 4, 5, 6]
Python slice negative indexes
Indexes can be negative numbers. Negative indexes refer to values from the end of the list. The last element has index -1, the last but one has index -2 etc. Indexes with lower negative numbers must come first in the syntax. This means that we write [-6, -2] instead of [-2, -6]. The latter returns an empty list.
#!/usr/bin/python vals = [1, 2, 3, 4, 5, 6, 7, 8] print(vals[-4:-1]) print(vals[:-2]) print(vals[-1:-4])
The program uses negative start/end indexes.
print(vals[-4:-1])
We create a slice from the fourt index from the end to the penultimate index.
$ ./main.py [5, 6, 7] [1, 2, 3, 4, 5, 6] []
Python list slice step
The third index in a slice syntax is the step. It allows us to take every n-th value from a list.
#!/usr/bin/python vals = [-2, -1, 0, 1, 2, 3, 4, 5, 6] print(vals[1:9:2]) print(vals[::2]) print(vals[::1]) print(vals[1::3])
We form four new lists using the step value.
print(vals[1:9:2])
Here we create a slice having every second element from the n list, starting from the second element, ending in the eighth element. The new list has the following elements: [-1, 1, 3, 5].
print(vals[::2])
Here we build a slice by taking every second value from the beginning to the end of the list.
print(vals[::1])
This creates a copy of a list.
print(vals[1::3])
The slice has every third element, starting from the second element to the end of the list.
$ ./main.py [-1, 1, 3, 5] [-2, 0, 2, 4, 6] [-2, -1, 0, 1, 2, 3, 4, 5, 6] [-1, 2, 5]
Python list slice negative step
With a negative step, we take every nth element from the end. The start index must be greater than the end index.
#!/usr/bin/python vals = [-2, -1, 0, 1, 2, 3, 4, 5, 6] print(vals[5:0:-2]) print(vals[::-2]) print(vals[::-1])
The program uses negative step.
print(vals[5:0:-2])
We create a slice from the sixth element to the first element, skipping every second element.
print(vals[::-2])
This creates a slice of every second element from the end to the beginning.
print(vals[::-1])
This line creates a copy of the list with reversed elements.
$ ./main.py [3, 1, -1] [6, 4, 2, 0, -2] [6, 5, 4, 3, 2, 1, 0, -1, -2]
Python list slice assignments
The slice syntax can be used with assignments.
#!/usr/bin/python vals = [1, 2, 3, 4, 5, 6, 7, 8] vals[0] = 10 vals[1:3] = 20, 30 vals[3::1] = 40, 50, 60, 70, 80 print(vals)
We have a list of eight integers. We use the slice syntax to replace the elements with new values.
$ ./main.py [10, 20, 30, 40, 50, 60, 70, 80]
Source
Python datastructures - language reference
In this article we have covered list slice operations in Python.
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