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Python caching library with tag-based invalidation and dogpile effect prevention

Project description

Pipeline status Test code coverage PyPI RTFD


Hermes is a Python caching library. It was designed to fulfil the following requirements:

  • Tag-based O(1) cache invalidation
  • Dogpile effect (cache stampede) mitigation
  • Support for multi-threaded, multi-process and multi-machine operation
  • Cache compression
  • Modular design (pluggable backends, compressors, serialisers, etc.)
  • Simple yet flexible decorator API

Implemented backends: redis, memcached, dict.


Hermes 0.8+ is Python 3-only. Previous versions supported Python 2.


pip install HermesCache

For Redis and Memcached it has the following extra dependencies.

HermesCache[redis] Pure Python Redis client
HermesCache[redis-ext] Pure Python Redis client & C extension parser
HermesCache[memcached] Pure Python Memcached client
HermesCache[memcached-ext] C extension Memcached client


The following demonstrates the most of the package’s API.

import hermes.backend.redis

cache = hermes.Hermes(
  ttl = 600,
  host = 'localhost',
  db = 1,

def foo(a, b):
  return a * b

class Example:

  @cache(tags = ('math', 'power'), ttl = 1200)
  def bar(self, a, b):
    return a ** b

  @cache(tags = ('math', 'avg'), key = lambda fn, a, b: f'avg:{a}:{b}')
  def baz(self, a, b):
    return (a + b) / 2

print(foo(2, 333))

example = Example()
print(, 10))
print(example.baz(2, 10))

foo.invalidate(2, 333), 10)
example.baz.invalidate(2, 10)

cache.clean(['math']) # invalidate entries tagged 'math'
cache.clean()         # flush cache


The API encourages import-time instantiation of Hermes facade to allow decoration of existing classes and functions, to make caching transparent to them. The instantiation has no side-effects. Underlying backend connections are lazy.

Moreover, if backend configuration is only available at runtime, Hermes.backend instance can be replaced at runtime.

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