A simple python hook system that doesn't mutate global variables and tries to be as side-effect free as possible

_utils.py

from __future__ import print_function

import sys
from collections import namedtuple
from imp import load_module, find_module

from pathlib import Path

Hooks = namedtuple('Hooks', 'preactivate postactivate get_env_details postrmvirtualenv')
_HOOKS = Hooks._fields

def hook(f):
    fname = f.__name__
    if fname not in _HOOKS:
        msg = '{0} is not a valid hook. Possible hooks are: {1}'.format(fname, _HOOKS)
        raise ImportError(msg)
    return f

paths = ['.', 'inner']

def select_hooks(module):
    for _hook in _HOOKS:
        loaded_hook = getattr(module, _hook, None)
        if loaded_hook:
            yield _hook, loaded_hook

def load_hooks(paths):
    for path in paths:
        if Path(path).exists():
            try:
                module = load_module('<TEMP>', *find_module('hooks', [path]))
                for hooktuple in select_hooks(module):
                    yield hooktuple # this would be a 'yield from' in Python3 only
                del module
            except ImportError as err:
                print(err, file=sys.stderr)

def discover_hooks(paths):
    hooks = {h:[] for h in _HOOKS}
    for hookname, hook in load_hooks(paths):
        hooks[hookname].append(hook)

    return Hooks(**hooks)

if __name__ == '__main__':
    hooks = discover_hooks(paths)
    for h in hooks.preactivate:
        h('fakeenv')

    for h in hooks.get_env_details:
        h('fakeenv')

hooks.py

from _utils import hook

@hook
def preactivate(env): # This will fail due to the extra "e"
    print('preactivating', env, '!')

# This would fail due to the extra "e"
# @hook
# def preeactivate(env):
#     print('preactivating', env, '!')

# The hook decorator is completely optional
def get_env_details(env):
    print("here I'd need to supply you some details about", env)