LPUSH & RPUSH — Building Lists

Redis lists are doubly-linked lists (or more precisely, quicklists — linked lists of ziplist pages). They support O(1) push/pop from both ends.

Commands

• LPUSH key val [val ...] — push to head, return new length
• RPUSH key val [val ...] — push to tail, return new length

Multi-value Push

LPUSH mylist a b c pushes left-to-right: c ends up at the head. The list becomes: [c, b, a]. Length returned: :3

Type Checking

If a key already exists as a string (or any non-list type), return: -WRONGTYPE Operation against a key holding the wrong kind of value\r\n

This is Redis's type safety — each key has exactly one type.

Implementation

Use your language's deque/linked-list:

• Python: collections.deque
• JavaScript: Array (push/unshift)
• Go: slice or container/list
• C++: std::deque

Track key types in a separate map: key_types[key] = "list".