Summary: The slice notation
list[::-1] with default
stop indices and negative step size
-1 reverses a given
Problem: Given a list of elements. How to reverse the order of the elements in the list.
Example: Say, you’ve got the following list:
['Alice', 'Bob', 'Carl', 'Dora']
Your goal is to reverse the elements to obtain the following result:
['Dora', 'Carl', 'Bob', 'Alice']
Slicing with Default Start and Stop Values
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
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
'hl'. Note that slicing works the same for lists and strings.
You can use a negative step size (e.g., -1) to slice from the right to the left in inverse order. Here’s how you can use this to reverse a list in Python:
# Reverse a List with Slicing names = ['Alice', 'Bob', 'Carl', 'Dora'] names = names[::-1] print(names) # ['Dora', 'Carl', 'Bob', 'Alice']
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Alternatives Reversing List
Alternatively, you can also use other methods to reverse a list.
list.reverse()— Best if you want to reverse the elements of list in place.
list[::-1]— Best if you want to write concise code to return a new list with reversed elements.
reversed(list)— Best if you want to iterate over all elements of a list in reversed order without changing the original list.
list.reverse() can be 37% faster than
reversed(list) because no new object has to be created.
Try it yourself in our interactive Python shell:
Exercise: Run the code. Do all methods result in the same reversed list?
Where to Go From Here?
Enough theory. Let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
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