The list.reverse() Method

The reverse() method belongs to the list object. It performs an in-place modification, meaning it alters the original sequence directly without creating a new collection. The memory footprint remains minimal since no secondary list is instantiated, but the initial ordering is permanently lost.

items = ['alpha', 'beta', 'gamma', 'delta']
items.reverse()
print(items)  # Output: ['delta', 'gamma', 'beta', 'alpha']

The reversed() Built-in Function

Unlike the list method, reversed() is a built-in function that accepts any sequence (not just lists) and returns a reverse iterator. It does not mutate the source data. To materialize the elements, the iterator must be consumed, typically by passing it to a constructor like list() or using it in a loop.

values = [10, 20, 30, 40]
backward_iter = reversed(values)
print(list(backward_iter))  # Output: [40, 30, 20, 10]
print(values)  # Output: [10, 20, 30, 40]

Memory and Performance Implications

Because reverse() modifies the list in place, it requires O(1) additional space. It is highly efficient when the original order is no longer needed. However, if other parts of a program rely on the initial sequence, this mutation can introduce bugs.

reversed() generates an iterator, which also operates in O(1) space initially. It evaluates elements lazily, making it suitable for massive datasets where creating a full reversed copy in memory is impractical. The trade-off is that accessing elements via the iterator incurs a slight overhead compared to direct indexing on a list.

Selecting the Appropriate Tool

Use reverse() when you want to permanently flip the order of an existing list and memory conservation is critical. Opt for reversed() when you need to iterate over a sequence backwards temporarily, when working with immutable sequences like tuples or strings, or when preserving the original data structure is necessary.