appendleft

Appendleft is a method used by double-ended queue (deque) data structures to insert a new element at the left end of the sequence. A deque supports insertion and removal at both ends, allowing efficient addition at either the front or the back.

In Python's standard library, the collections.deque type provides appendleft, which inserts the element on the left side, i.e., the front. The counterpart append adds to the right side. The operation runs in O(1) time on average. The method mutates the deque in place and returns None. If the deque has a maximum length (maxlen) and is full, appending to either end will discard an element from the opposite end to make room.

Common use cases include maintaining a dynamic front-loaded sequence, implementing a queue where elements may need to be prepended, or supporting algorithms that require inserting at the front while maintaining fast access to both ends. It can be convenient for building a sequence in reverse order without reversing at the end, or for certain cache or streaming scenarios where new items arrive at the front.

Other libraries and languages implement an equivalent operation with different names. In many languages that provide a generic deque, the method is called push_front or push_left; appendleft is a naming convention most closely associated with Python's collections.deque.