How to Reverse a Range in Python: A Step-by-Step Guide

Overview of Reversing Ranges in Python

Here are some key points we’ll cover:

• Basics of Python’s range() function for generating numeric ranges
• Using reversed() to reverse any sequence including ranges
• Passing a negative step to range() to count backwards
• Sorting ranges in reverse order with sorted()
• Pros and cons of each method for reversing ranges

By the end of this guide, you’ll understand multiple techniques you can use to iteratively go through range values in reverse.

The Basics of Python’s range() Function

The range() function is used to generate a sequence of numeric values. For example:

for n in range(5):
    print(n)

# Output: 
0
1  
2
3
4

As you can see, range(5) generates the values 0 up to 5 (exclusive).

Some key properties of range():

• By default starts at 0
• Stop value is exclusive (not included in range)
• Default step is 1 (no values skipped)

You can also specify start, stop, and step values:

range(2, 8, 3) # start=2, stop=8, step=3

This covers the basics of creating ranges with range(). Next let’s look at ways to reverse ranges.

Using reversed() to Reverse a Range

One of the simplest ways to reverse a range is using the reversed() function:

for n in reversed(range(5)):
    print(n)

# Output:  
4
3
2
1
0

reversed() takes any sequence as input and iterates over it in reverse order.

To get the actual reversed values from a reversed() object, you need to cast it to a list:

reversed_range = reversed(range(5)) 

print(list(reversed_range))
# [4, 3, 2, 1, 0]

reversed() creates a memory-efficient reversible iterator, without creating an actual reversed copy of the range.

Using a Negative Step with range()

Another way to reverse a range is by passing a negative step value to range().

For example:

for n in range(5, 0, -1):
    print(n)

# Output:
5  
4
3
2
1

Here we start at 5, end at 0, and decrement by -1 each iteration.

Key points:

• Start must be greater than end value
• Step is negative, so range counts backwards
• Allows skipping values by adjusting step amount

This approach performs an actual reversed copy of the range.

Reversing with sorted()

The sorted() function can also reverse ranges by passing reverse=True:

r = range(5)

sorted_r = sorted(r, reverse=True) 

print(sorted_r)
# [4, 3, 2, 1, 0]

Since ranges are already sorted, this simply reverses their order.

However, it’s not very efficient since it makes a full copy of the range.

Summary: Best Practices for Reversing Ranges

To recap, here are some guidelines on the best approach for reversing ranges in Python:

• reversed() – Best for simple iteration in reverse. Memory efficient.
• Negative step – Best for producing an actual reversed range object.
• sorted() – Avoid for reversing ranges specifically (inefficient).

The reversed() function is great for straightforward iteration over a range in reverse. But if you need the actual reversed range values, use a negative step with range().

Hopefully this guide gave you a solid understanding of techniques for reversing numeric ranges in Python!