Iterate backwards through a sequence by index.

To traverse a sequence backwards, often to safely modify the list (e.g., delete items) while iterating.

When you need to process elements in reverse order, especially when removing or inserting elements during iteration to avoid index shift errors.

Used in loops over lists, strings, or any sequence where reverse index access is needed, such as in-place removal, reversing, or processing from end to start.

for i in range(len(seq)-1, -1, -1):

for i in range(start, stop, step): where start = len(seq)-1, stop = -1, step = -1

seq (any sequence), i (loop variable name can be changed)

• len
• range
• -1
• -1 step
• list
• string

• i can be idx
• j
• etc.
• seq can be any list
• string
• tuple
• step -1 can be written as -1

off-by-one errors (using len(seq) as start), forgetting the -1 stop, using a positive step, confusing with forward loop

forward iteration 'for i in range(len(seq)):', 'for i, item in enumerate(seq):', 'for item in reversed(seq):'

Confusing reverse index loop with forward loop can lead to index-out-of-range errors; forgetting to decrement step causes an infinite loop.

• reverse iteration idiom

Allows safe in-place modification while iterating because indices remain valid after deletions or insertions.

Signals the intent to mutate the sequence while iterating in reverse order.

Think 'count down from last index to zero'.

Often preferred over reversed(seq) when you need to mutate the list during iteration.

Consider using reversed(seq) for readability when mutation is not required.