Manipulating Redis lists in a Python Program using Redis-Py

Overview:

Redis is an in memory data-structure store, and it supports several data structures and list is one of them.
The list data structure supported by Redis is implemented as a Linked List. This differs from the Python's implementation of the list.
In a linked list, operations of reading an element, insertion, deletion in the beginning or at the end takes constant time or in Big O notation O(1), as the head and tail addresses are always available prior to the operation.
To access an element positioned at N_th location in the list it, takes O(N) time.
Redis list is a list of strings.
From a Python program, a Redis list can be manipulated using Redis-Py, which is the Python interface for Redis.

Adding an element to Redis List using the Python API Redis-Py:

There are several ways a value can be added to a Redis List.
The LPUSH command adds a value to the head of a Redis list. The time complexity is O(1).
The RPUSH command adds a value to the tail of a Redis list. The time complexity is O(1).
The LINSERT command adds a value before or after the first occurence of a specific value. The time complexity is O(N) except for the head element and the tail element.
The LSET command sets a value for the element specified by the index. The time complexity is O(N) except for the head element and the tail element.

Example 1:

# Sample Python program to demonstrate List operations of Redis and

# redis-py

import redis

# Create a redis client

redisClient = redis.StrictRedis(host='localhost',

port=6379,

db=0)

# Add values to the Redis list through the HEAD position of the list

redisClient.lpush('LanguageList', "Kotlin")

redisClient.lpush('LanguageList', "Python")

# Push multiple values through the HEAD of the list

redisClient.lpush('LanguageList', "Java", "C++")

# Print the contents of the Redis list

while(redisClient.llen('LanguageList')!=0):

print(redisClient.lpop('LanguageList'))

Output:

b'C++'

b'Java'

b'Python'

b'Kotlin'

Example 2:

# Sample Python program to demonstrate List operations of Redis

# and redis-py

import redis

# Create a redis client

redisClient = redis.StrictRedis(host='localhost',

port=6379,

db=0)

# Add values to the Redis list through the tail position of the list

noSQLList = "NoSQLStores"

redisClient.rpush(noSQLList, "Redis")

redisClient.rpush(noSQLList, "MongoDB")

redisClient.rpush(noSQLList, "Neo4J")

redisClient.rpush(noSQLList, "HBase")

redisClient.rpush(noSQLList, "Cassandra")

# Add more than one value through the tail of the Redis list

redisClient.rpush(noSQLList, "Riak", "CouchDB")

# Print the contents of the Redis list

while(redisClient.llen(noSQLList)!=0):

print(redisClient.rpop(noSQLList))

Output:

b'CouchDB'

b'Riak'

b'Cassandra'

b'HBase'

b'Neo4J'

b'MongoDB'

b'Redis'

Get the value of an element from Redis List using the Python API Redis-Py:

LINDEX command returns a value of an element from a Redis list as defined by the index.
LINDEX does not remove the element from the Redis list.
The LPOP command removes an element from the head of a Redis list and returns the value of it.
The RPOP command removes an element from the tail of a Redis list and returns the value of it .
The LREM command removes an element from a Redis list as specified the index.

Example:

# Sample Python program to demonstrate List operations of Redis

# and redis-py

import redis

# Create a redis client

redisClient = redis.StrictRedis(host='localhost',

port=6379,

db=0)

# Create a Redis list with all zeros

numberList = "numbers"

redisClient.lpush(numberList, 0,0,0,0,0)

# Print the Redis list before modification

print("Contents of the list before modification:")

for i in range(0, redisClient.llen(numberList)):

print(redisClient.lindex(numberList, i))

# Modify Redis list contents

for i in range(0, redisClient.llen(numberList)):

redisClient.lset(numberList, i, i*2)

print("Contents of the list after modification:")

# Print the Redis list after modification

for i in range(0, redisClient.llen(numberList)):

print(redisClient.lindex(numberList, i))

# Empty the Redis list

for i in range(0, redisClient.llen(numberList)):

redisClient.lpop(numberList)

# Print the length of the list after removing all its elements

print("Length of the Redis list after removing all elements:")

print(redisClient.llen(numberList))

Output:

Contents of the list before modification:

b'0'

Contents of the list after modification:

b'0'

b'2'

b'4'

b'6'

b'8'

Length of the Redis list after removing all elements:

Other list operations on a Redis list using the Python API Redis-Py:

The LLEN command returns the number of elements present in the Redis list.
The LTRIM command keeps only a specified range of indexes of a Redis list and removes other elements. Remember 0 is the beginning of the list and -1 is the beginning of the list in reverse order.

Example:

# Sample Python program to demonstrate List operations of Redis

# and redis-py

import redis

# Create a redis client

redisClient = redis.StrictRedis(host='localhost',

port=6379,

db=0)

# Create a Redis list with few even numbers

numberList = "numbers"

redisClient.rpush(numberList, 2,4,6,8,10,12)

# Trim the list to have only single digit elements

startIndex = 0

endIndex = 3

newList = redisClient.ltrim(numberList, startIndex, endIndex)

# Print the Redis list after trimming

print("Single digit even numbers:")

for i in range(0, redisClient.llen(numberList)):

print(redisClient.lindex(numberList,i))

# Clear the Redis list

for i in range(0, redisClient.llen(numberList)):

redisClient.lpop(numberList)

Output:

Single digit even numbers:

b'2'

b'4'

b'6'

b'8'