Summary: The slice notation list[::-1] with default start and stop indices and negative step size -1 reverses a given list.
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 stop (excluded).
All three arguments are optional, so you can skip them to use the default values (start=0, stop=len(lst), 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.
The method 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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The post Python Reverse List with Slicing — An Illustrated Guide first appeared on Finxter.
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