108 – Dynamic module attribute look-up

Modules can implement the dunder method __getattr__, which can be used to dynamically load names into a module. This can be useful to create lazy imports or to issue warnings when certain things are imported.

Suppose HeavyClass is available from the module my_module, but suppose that HeavyClass takes a while to initialise/import and therefore you want to be able to import it lazily. Then, you’d define this __getattr__ in my_module:

def __getattr__(name):
    if name == "HeavyClass":
        print("Lazy importing...")
        from _module import HeavyClass
        globals()[name] = HeavyClass
        return HeavyClass
        
    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")

Then, you can use HeavyClass elsewhere. The first time you use it, you’ll trigger __getattr__, but the second time you won’t because the lazy import is then saved in the global namespace of the module.

import my_module

obj1 = my_module.HeavyClass()
# Lazy importing...

obj2 = my_module.HeavyClass()

This is used in the modules typing and collections, for example!