Backward iteration in Python

Sometimes we need to iterate through a list in reverse order − reading the last element first, then the second-to-last, and so on. Python provides three common approaches: range() with negative step, slice notation [::-1], and the reversed() built-in function.

Using range() with Negative Step

Start from the last index and step backwards by -1 until index 0 ?

days = ['Mon', 'Tue', 'Wed', 'Thu']

for i in range(len(days) - 1, -1, -1):
    print(days[i])

Thu
Wed
Tue
Mon

range(3, -1, -1) generates indices 3, 2, 1, 0. This gives you access to the index, which is useful when you need both the position and the value.

Using Slice [::-1]

Slice notation creates a reversed copy of the list and iterates over it ?

days = ['Mon', 'Tue', 'Wed', 'Thu']

for item in days[::-1]:
    print(item)

Thu
Wed
Tue
Mon

[::-1] means start from the end, go to the beginning, stepping -1. This creates a new reversed list in memory.

Using reversed()

The reversed() built-in returns a reverse iterator without creating a copy ?

days = ['Mon', 'Tue', 'Wed', 'Thu']

for item in reversed(days):
    print(item)

Thu
Wed
Tue
Mon

reversed() is memory-efficient because it returns an iterator that traverses the original list backwards without creating a new list.

Comparison

Conclusion

Use reversed() for the most Pythonic and memory-efficient backward iteration. Use [::-1] for short, readable code when a copy is acceptable. Use range() with a negative step when you need access to the index during iteration.