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Redis · Command
LPUSH
LPUSH prepends one or more elements to a Redis list, creating the list when the key is absent. It returns the list's length after insertion.
- Since
- 1.0.0
How it works
LPUSH prepends one or more values to the list at the specified key. Redis creates the key as a list when it is absent; multiple arguments are inserted sequentially, so the rightmost supplied value becomes the first list item.
Syntax
LPUSH key element [element ...]Parameters
| Name | Type | Description |
|---|---|---|
key | key | Name of the key that holds the list. |
element [element ...] | one or more values | One or more values to prepend to the list. |
Returns
Integer reply: the length of the list after the push operation.
Key points
- LPUSH prepends every supplied element to the target list.
- An absent key is initialized as an empty list before insertion.
- The result is the list length after all insertions complete.
- Each individual element insertion has constant-time complexity.
- Redis classifies LPUSH as a write command for list data structures and marks it fast.
Common mistakes
- The command fails if the key already contains a non-list value.
- With multiple elements, insertion order is reversed in the resulting list: the last argument ends up at the head.
Example: Prepend values to a list
LPUSH mylist "world"
LPUSH mylist "hello"
LRANGE mylist 0 -1Adds two values at the list head, then reads the complete list to show the newest value first.
When to use it
Queue production
Place work identifiers onto a list for later consumption by a worker.
LIFO stacks
Build stack-like behavior by pairing head insertion with head removal.
Interview questions
QWhat does LPUSH do and what does it return?
LPUSH adds one or more elements to the head of a list and returns the list's length afterward. If the key is absent, Redis creates the list automatically.
Follow-up: How does LPUSH mylist a b c order the values? The resulting order is c, b, a because arguments are inserted one at a time from left to right at the head.
QWhat is the syntax and argument structure of LPUSH?
Its syntax is LPUSH key element [element ...]. The command accepts one or more elements, and the key identifies the target list.
Follow-up: What happens when the key contains a string? Redis returns a type error because the existing value is not a list.
QWhat is LPUSH's complexity?
LPUSH has constant-time complexity for each element added, so a call adding multiple elements takes time proportional to the number of supplied elements. It is categorized as a write operation on a list.
Follow-up: Which command provides the create-only-if-present behavior? LPUSHX prepends values only when the key already exists and contains a list.
Related
LPUSHXRPUSHLPOPLRANGELLEN