memoization.md

Automatically Cache

This guide addresses how to use the panel.cache decorator to memoize (i.e., cache the output of) functions automatically.

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The pn.cache decorator provides an easy way to cache the outputs of a function depending on its inputs and/ or Parameter dependencies (i.e., memoization).

If you've ever used the Python @lru_cache decorator, you will be familiar with this concept. However, the pn.cache functions support additional cache policys apart from LRU (least-recently used), including LFU (least-frequently-used) and 'FIFO' (first-in-first-out). This means that if the specified number of max_items is reached, Panel will automatically evict items from the cache based on this policy. Additionally, items can be deleted from the cache based on a ttl (time-to-live) value given in seconds.

Caching Functions

The pn.cache decorator can easily be combined with the different Panel APIs, including pn.bind and pn.depends, providing a powerful way to speed up your applications.

@pn.cache(max_items=10, policy='LRU')
def load_data(path):
    return ... # Load some data

Once you have decorated your function with pn.cache, any call to load_data will be cached in memory until the max_items value is reached (i.e., you have loaded 10 different path values). At that point, the policy will determine which item is evicted.

The pn.cache decorator can easily be combined with pn.bind to speed up the rendering of your reactive components:

import pandas as pd
import panel as pn

pn.extension('tabulator')

DATASETS = {
    'Penguins': 'https://raw.githubusercontent.com/mwaskom/seaborn-data/master/penguins.csv',
    'Diamonds': 'https://raw.githubusercontent.com/mwaskom/seaborn-data/master/diamonds.csv',
    'Titanic': 'https://raw.githubusercontent.com/mwaskom/seaborn-data/master/titanic.csv',
    'MPG': 'https://raw.githubusercontent.com/mwaskom/seaborn-data/master/mpg.csv'
}

select = pn.widgets.Select(options=DATASETS)

@pn.cache
def fetch_data(url):
    return pd.read_csv(url)

pn.Column(select, pn.bind(pn.widgets.Tabulator, pn.bind(fetch_data, select), page_size=10))

Caching Functions with Dependencies

The pn.cache decorator can easily be combined with pn.depends to speed up the rendering of your reactive components:

select = pn.widgets.Select(options=DATASETS)

@pn.cache
@pn.depends(select)
def fetch_data(url):
    return pd.read_csv(url)

pn.Column(select, pn.widgets.Tabulator(fetch_data, page_size=10))

Caching Methods with Dependencies

import param

class DataExplorer(pn.viewable.Viewer):
    dataset = param.Selector(objects=DATASETS)

    @pn.cache
    @param.depends("dataset")
    def fetch_data(self):
        return pd.read_csv(self.dataset)

    def __panel__(self):
        return pn.Column(self.param.dataset, pn.widgets.Tabulator(self.fetch_data, page_size=10))

DataExplorer().servable()

Disk Caching

Disk backed caching persists cached results across server restarts, making it ideal for expensive operations like data loading or model inference.

Prerequisites

First, install the diskcache library:

pip install diskcache

Basic Disk Caching

To enable disk caching, set to_disk=True when decorating your function:

import panel as pn
import time
from datetime import datetime

@pn.cache(to_disk=True)
def expensive_computation(n):
    # Simulate expensive operation
    time.sleep(2)
    return datetime.now()

# First call takes 2 seconds
result1 = expensive_computation(5)

# Subsequent calls are instant, even after server restart
result2 = expensive_computation(5)

pn.Column(result1, result2).servable()

By default, cached values are stored in a ./cache directory relative to your application.

Configuring the Cache Path

You can customize where cached values are stored in three ways:

1. Inline configuration:

@pn.cache(to_disk=True, cache_path='./cache1')
def expensive_computation(n):
    ...

2. Global configuration with pn.extension:

pn.extension(cache_path='./cache2')

3. Direct configuration:

pn.config.cache_path = './cache3'

Clearing the Cache

Once a function has been decorated with pn.cache, you can easily clear the cache by calling .clear() on that function, e.g., in the example above, you could call load_data.clear(). If you want to clear all caches, you may also call pn.state.clear_caches().

Per-session Caching

By default, any functions decorated or wrapped with pn.cache will use a global cache that will be reused across multiple sessions, i.e., multiple users visiting your app will all share the same cache. If instead, you want a session-local cache that only reuses cached outputs for the duration of each visit to your application, you can set pn.cache(..., per_session=True).

Related Resources

• Manually Cache