Python Memcached Prepend (Detailed Guide w/ Code Examples)

Use Case(s)

The prepend operation in Memcached is used to add some data at the beginning of an existing key's value. This can be useful in scenarios where you want to update logs, messaging systems, or any other data structure where updates need to be made in a prepend (add at the beginning) manner.

Code Examples

1. Basic Example of Prepend:

import memcache

# Connect to Memcached
mc = memcache.Client(['127.0.0.1:11211'], debug=0)

# Set a Key
mc.set('key', 'world')

# Prepend to that Key
mc.prepend('key', 'Hello ')

# Get Key Value to see output
print(mc.get('key'))  # Outputs: 'Hello world'

In this example, we first create a connection to the Memcached server. Then we set a key named 'key' with the value 'world'. Using the prepend command, we then prepend the string 'Hello ' to our existing value. When we fetch the value of 'key', it returns 'Hello world'.

2. Error Handling in Prepend:

import memcache

# Connect to Memcached
mc = memcache.Client(['127.0.0.1:11211'], debug=0)

# Try to prepend a non-existent key
try:
    mc.prepend('non_existent_key', 'Hello ')
except Exception as e:
    print(f"Failed to prepend: {str(e)}")

In this example, we try to prepend to a key that does not exist. Since the key must be pre-existing in Memcached for the prepend operation, this will throw an error.

Best Practices

• Ensure the key exists before using prepend, as Memcached does not create a new key-value pair if the key does not exist. You can use add or set to ensure the existence of the key.
• Use prepend operations sparingly, as they are slower compared to normal set/get operations because they require modification and expansion of existing data.

Common Mistakes

• Trying to prepend to a non-existent key. Always ensure the key exists in the cache before invoking a prepend operation.

FAQs

1. Can I prepend on a non-existent key in Memcached?

   No, the prepend function only works with existing keys. If you try to prepend to a non-existent key, it will result in an error.

2. How fast is the prepend operation in Memcached?

   The prepend operation in Memcached is slower compared to normal set/get operations since it requires alteration of existing data.