LlenCommand
Time complexity: O(1)Return the length of the list stored at the specified key. If thekey does not exist zero is returned (the same behaviour as forempty lists). If the value stored at key is not a list an error is returned.
Integer reply, specifically:
The length of the list as an integer `>=` 0 if the operation succeeded
-2 if the specified key does not hold a list value
Note that library clients should raise an error if -2 is returned by the Redis server instead to pass the negative value back to the caller.
