How to Slice a List in Python?
Slicing lets us extract or modify a subsequence of a list. In Python, we use the [start:end:step] syntax or the slice() function for this.
Contents
Basic usage of slices
list[start:stop]
In Python slicing, [start:stop] extracts elements from the start index up to, but not including, the stop index. The range follows the pattern start <= x < stop, where the element at start is included but the element at stop is excluded.
fruits = ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]
print(fruits[2:5])
>>> ["cherry", "date", "elderberry"]
In the Example 1, fruits[2:5] extracts ["cherry", "date", "elderberry"] from the list. Notice that the element at index 2 is included and the element at index 5 is excluded.
Omitting the start or stop index
If you leave out the start value, Python takes elements from the beginning. If you leave out the stop value, it goes to the end. Leaving out both gives you the whole sequence.
fruits = ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]
print(fruits[:5])
>>> ["apple", "banana", "cherry", "date", "elderberry"]
print(fruits[5:])
>>> ["fig", "grape"]
print(fruits[:])
>>> ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]
In the Example 2, fruits[:5] extracts ["apple", "banana", "cherry", "date", "elderberry"] from the list, fruits[5:] extracts ["fig", "grape"] from the list, and fruits[:] extracts the whole list.
list[start:stop:step]
The step parameter sets how many elements you skip in a slice. Used with start and stop, it looks like this: [start:stop:step]. For example, step 2 picks every second element, and step 3 picks every third element and so on.
fruits = ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]
print(fruits[::2])
>>> ["apple", "cherry", "elderberry", "grape"]
print(fruits[1::2])
>>> ["banana", "date", "fig"]
In the Example 3, fruits[::2] extracts ["apple", "cherry", "elderberry", "grape"] from the list by skipping every second element. on the other hand, fruits[1::2] extracts ["banana", "date", "fig"] from the list by skipping every second element starting from the second element.
Error Handling
If you try to access an index that is out of range, Python will raise an IndexError exception. But if you use slicing, it will return an empty list instead of raising an error. This is useful because it allows you to handle out-of-range indices gracefully without crashing your program.
fruits = ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]
print(fruits[100:200])
>>> []
Also, if the start index is greater than the stop index, Python will return an empty list without raising an error.
fruits = ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]
print(fruits[5:2])
>>> []
Working with negative indices
Negative start and stop values
Negative indices count backward from the end of a sequence, with -1 representing the last element, -2 the second-to-last, and so on.
fruits = ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]
print(fruits[-2:])
>>> ["fig", "grape"]
print(fruits[:-2])
>>> ["apple", "banana", "cherry", "date", "elderberry"]
In the Example 6, fruits[-2:] extracts ["fig", "grape"] from the list by starting from the second last element and going to the end. On the other hand, fruits[:-2] extracts ["apple", "banana", "cherry", "date", "elderberry"] from the list by starting from the beginning and going up to the second last element.
Negative step value
Negative step values reverse the traversal direction, extracting elements from right to left. When using a negative step, extraction begins at the start position. If start comes before stop in the sequence order, the result is an empty list.
fruits = ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]
print(fruits[::-1])
>>> ["grape", "fig", "elderberry", "date", "cherry", "banana", "apple"]
In the Example 7, fruits[::-1] reverses the list by extracting elements from right to left.
When step is negative and you omit start, Python begins from the end. Omitting stop continues to the beginning. Omitting both traverses the entire sequence. Setting step to -1 with both start and stop omitted creates a reversed copy of the sequence. Here is an example:
fruits = ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]
print(fruits[::-1])
>>> ["grape", "fig", "elderberry", "date", "cherry", "banana", "apple"]
print(fruits[3::-1])
>>> ["date", "cherry", "banana", "apple"]
print(fruits[:-3:-1])
>>> ["fig", "elderberry", "date"]
In the Example 8, fruits[::-1] reverses the whole list by extracting elements from right to left.
On the other hand, fruits[3::-1] reverses the list by extracting elements from the third element to the beginning. Similarly, fruits[:-3:-1] reverses the list by extracting elements from the third last element to the end.
For reversing sequences, Python also provides the reverse() and reversed() methods, which offer alternative approaches depending on your needs.
Creating reusable slice objects with slice()
Python's built-in slice() function creates slice objects that can be stored and reused. This is useful when you need to apply the same slice operation multiple times across different sequences.
The syntax slice(start, stop, step) is functionally equivalent to [start:stop:step].
fruits = ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]
sl = slice(2, 5, 2)
print(fruits[sl])
>>> ["cherry", "elderberry"]
In the Example 9, slice(2, 5, 2) creates a slice object that can be used to extract ["cherry", "elderberry"] from the list.
Using slice() with two arguments
When only two arguments are provided, step defaults to None, which behaves the same as start:stop.
sl = slice(2, 5)
print(sl)
>>> slice(2, 5, None)
print(l[sl])
>>> ["cherry", "elderberry"]
In the Example 10, slice(2, 5) creates a slice object that can be used to extract ["cherry", "elderberry"] from the list.
Using slice() with one argument
With a single argument, both start and step default to None, equivalent to :stop.
sl = slice(2)
print(sl)
>>> slice(None, 2, None)
print(l[sl])
>>> ["apple", "banana"]
In the Example 11, slice(2) creates a slice object that can be used to extract ["apple", "banana"] from the list.
Creating an empty slice
Calling slice() without arguments raises a TypeError. To create a slice equivalent to : (selecting all elements), explicitly pass None for all parameters.
sl = slice()
>>> TypeError: slice expected at least 1 arguments, got 0
sl = slice(None)
print(sl)
>>> slice(None, None, None)
print(fruits[sl])
>>> ["apple", "banana", "cherry", "date", "elderberry", "fig", "grape"]