Post import hooks in Python 3

Question

I would like to have some callback run whenever a particular module is imported. For example (using a fake @imp.when_imported function that does not really exis

Answer 1

I would be shocked to find out that this is the best way to do this ... However, since early python2.x versions, monkey patching __import__ has been supported. We can take advantage of that here:

try:
    import builtins  # python3.x
except ImportError:
    import __builtin__ as builtins  # python2.x
import sys
import collections

_builtin_import = builtins.__import__

def _my_import(name, globals=None, locals=None, fromlist=(), level=0):
    already_imported = name in sys.modules

    mod = _builtin_import(
        name,
        globals=globals,
        locals=locals,
        fromlist=fromlist,
        level=level)

    if not already_imported and name in _post_import_hooks:
        for hook in _post_import_hooks[name]:
            hook()
    return mod

builtins.__import__ = _my_import

_post_import_hooks = collections.defaultdict(list)

def on_import(name):
    def decorator(func):
        _post_import_hooks[name].append(func)
        return func
    return decorator

@on_import('numpy')
def print_hi():
    print('Hello Numpy')

print('before numpy')
import numpy
print('after numpy')

This answer makes a super simple registry for registering callbacks. The decorator just registers the function and then returns it. It doesn't do any fancy checking (for whether the module is already loaded, for example), but could easily be extended to do that.

Obviously the downside is if some other module decides to monkey patch __import__, then you're out of luck -- Either this module or the other one is likely to end up broken.

I've tested this and it seems to work on both python2.x and python3.x.