slice() function returns a new slice object you can use to slice over sequences such as lists, strings, or tuples.
Read more about lists in our full tutorial about Python Slicing.
Example 1: Slice with Stop Argument
One of the best way to learn is by example! Here are some examples of how to use the
slice() built-in function.
First, let’s create a list of 20 elements using the
>>> lst =list(range(20)) >>> lst [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
The list consists of all elements from 0 to 19 (included). You can now access the first ten elements of this list by creating a new slice object with stop index 10, and pass the newly-created slice into the indexing scheme using the square bracket notation
>>> s = slice(10) >>> lst[s] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Example 2: Slice with Start and Stop Arguments
You can also give the start argument to start from any other position in the sequence to be sliced:
>>> lst =list(range(20)) >>> s = slice(5, 10) >>> lst[s] [5, 6, 7, 8, 9]
Example 3: Extended Slice with Start, Stop, and Step Arguments
The most advanced way is called extended slicing, where you define the start index, the stop index, and the step size:
>>> lst =list(range(20)) >>> s = slice(2, 10, 2) >>> lst[s] [2, 4, 6, 8]
You can use the
slice() method with up to three arguments:
Syntax: There are three ways of using the constructor: # 1. Create new slice that goes from start index 0 (default) to stop using default step size 1. slice(stop) # 2. Create new slice that goes from start index to stop using default step size 1. slice(start, stop) # 3. Create new slice that goes from start to stop using step as step size. slice(start, stop, step)
What’s the Purpose of slice() vs Python’s slicing notation?
Python also allows a more concise way of slicing sequences: the default slicing notation. Let’s quickly recap it:
Slicing is a concept to carve out a substring from a given string. Use slicing notation
s[start:stop:step] to access every
step-th element starting from index
start (included) and ending in index
stop (excluded). All three arguments are optional, so you can skip them to use the default values (
step=1). For example, the expression
s[2:4] from string
'hello' carves out the slice
'll' and the expression
s[:3:2] carves out the slice
You can either create a slice object explicitly using
slice() or you create it implicitly using the slice notation. Both variants are semantically identical:
>>> customers = ['Alice', 'Bob', 'Carl', 'Dave', 'Elena', 'Frank'] >>> customers[1:5:2] ['Bob', 'Dave'] >>> customers[slice(1, 5, 2)] ['Bob', 'Dave']
There’s only one difference: if you use
slice() explicitly to create a slice object, you can reuse it for further slicing operations.
Performance Evaluation: Slicing vs slice()
This has some performance benefits, for example, when using the same slicing operation on different lists in a loop:
import time lst = [list(range(10000)), list(range(20000)), list(range(30000))] # Method 1: Slicing Notation start = time.time() for l in lst: print('Size of slice: ', len(l[5:-5:4])) print(time.time() - start) # Method 2: slice() Object start = time.time() s = slice(5, -5, 4) for l in lst: print('Size of slice: ', len(l[s])) print(time.time() - start)
Both methods have the same output:
Size of slice: 2498 Size of slice: 4998 Size of slice: 7498 0.03080606460571289 Size of slice: 2498 Size of slice: 4998 Size of slice: 7498 0.015621423721313477
However, the performance difference in this example is significant.
- Method 1 takes 0.03 seconds to complete.
- Method 2 takes 0.015 seconds to complete.
Using the function
slice() to create a slice object once in advance results in a 50% runtime performance reduction compared to using standard slicing. The reason is that standard slicing will internally recreate the same slice object in each iteration which is inefficient.
Interactive Shell Exercise: Understanding slice()
Consider the following interactive code:
Exercise: Guess the output before running the code.
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slice(stop): create a new slice that goes from start index 0 (default) to stop using default step size 1.
slice(start, stop): create a new slice that goes from start index to stop using default step size 1.
slice(start, stop, step): create a new slice that goes from start to stop using step as step size.
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