Sometimes you need to reverse a range in Python to iterate over a list of items in reverse order. This quick guide explores two popular methods of reversing ranges (or any iterator) in Python.
Reverse a range using
reversed()Generate a reversed range
✋ Update: This post was originally published on my blog decodingweb.dev, where you can read the latest version for a 💯 user experience. ~reza
Before we start, let’s quickly review Python’s range() function. A range object in Python is a sequence of numbers you can use with a for loop to repeat a set of instructions a specific number of times.
Python’s range() function makes it super easy to generate range objects. The range function accepts three parameters: start, stop, and step.
With these parameters, you can define where the sequence begins and ends. And thanks to the step parameter, you can even choose how big the difference will be between one number and the next.
For instance, to generate a range from 0 to 10 with a step of 2, you call the range function like so:
items = range(0, 10, 2)
print(items)
# output: range(0, 10, 2)
# Convert them into a list
print(list(items))
# output: [0, 2, 4, 6, 8]
Using ranges with for loops is straightforward:
for i in range(0, 10, 2):
print(i)
The above code generates the following output:
0
2
4
6
8
Reverse a range using reversed()
Alright, now let's see how we can reverse a range in Python to iterate over it in reverse order.
Luckily, Python makes it easy to do this with the reversed() function.
The reversed() function takes a range (or any iterator supporting the sequence protocol) and returns a reversed-iterator object.
You can use reversed() function in conjunction with the range() function in for loops to iterate over a sequence in reverse order:
for i in reversed(range(0, 10, 2)):
print(i)
And the output would be:
8
6
4
2
0
You can also use the reversed() function with a list. In this case, the returned value will be a list_reverseiterator object:
my_list = [1, 2, 3, 4, 5]
for i in reversed(my_list):
print(i)
Generate a reversed range
You can also use the range() function to generate a sequence of numbers in reverse order.
In this case, the start argument is the highest number in the range, and the stop parameter is the lowest. Obviously, the step parameter should be a negative value like -1.
To create a range from 10 to 0 with a step of 2, we can do it like so:
for i in range(10, 0, -2):
print(i)
And to create a reversed list with negative numbers:
for i in range(-10, 0, 2):
print(i)
Output:
-10
-8
-6
-4
-2
Alright, I think that does it! I hope you found this quick guide helpful.
Thanks for reading
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