Python factory class with dynamic imports

Question

I'm trying to write a factory class that essentially provides a user-friendly frontend to creating objects of different kinds depending on keywords. So the user will just have to import this module, and then use module.factory.make_new with the relevant keywords.

The directory structure will be pretty flat: this file will sit in a directory along with SubclassA.py which defines the class SubclassA likewise for SubclassB etc. That is, each subclass is in its own module of the same name.

If the user wants to dig a bit deeper, they can provide their extension to the code by writing SubclassC.py which provides class SubclassC and then add the line factory.register_format("subclassc", "SubclassC") to the bottom of this file (or, eventually, this will be handled by a config file). Here's what I have so far:

import importlib


class TrackerFactory:
    def __init__(self):
        self.subclass_dict = {}

    def register_format(self, subclass_name, subclass_package):
        fmt = importlib.import_module(subclass_package)
        self.subclass_dict[subclass_name] = getattr(fmt, subclass_package)

    def make_new(self, subclass, **kwargs):
        return self.subclass_dict[subclass](subclass, **kwargs)


factory = TrackerFactory()
factory.register_format("subclassA", "SubclassA")
factory.register_format("subclassB", "SubclassB")

I'm just wondering what the possible downsides of this sort of approach are, and what kind of gotchas I might have to look out for. Is there a better way to do this? I get the feeling I can achieve what I want with class methods and a class variable dictionary, and thus avoid hardcoding an instance of the factory, but I couldn't get it to work...

Some things I'm not bothered by:

• sneaky hidden imports not at the top of the file (the advantages of dynamically loading the dependencies of only the registered formats outweigh the poor style, for me). And in any case, this is only used to load a very specific type to module. The modules themselves being loaded in this way (SubclassA etc) could have lots of unusual dependencies.

Some things I've already thought of:

• extend the register_format method to allow the module name and class name to differ
• use some try/except logic
• since, in my use case, there will actually be only one or a few objects actually instantiated, I could juggle things around so that the import is actually in make_new (and subclass_dict just holds a string rather than the function), and then registering a format you won't use that you don't have the dependencies for wouldn't choke.

Answer 1

If a factory pattern is indeed called for - much of the time it is not - there is an easier way that needs neither importlib nor explicit registration calls.

If you follow something like this layout:

mypackage/
mypackage/__init__.py
mypackage/tracker_factory.py
mypackage/trackers/__init__.py
mypackage/trackers/type_a.py
mypackage/trackers/type_b.py
...

then in mypackage/trackers/__init__.py:

from .type_a import TypeA
from .type_b import TypeB
...

then your factory can import mypackage.trackers as all_trackers, do a dir(all_trackers), and every subtype that you care about will be listed. Instead of a dynamic import plus runtime registration calls, this uses traditional imports and will look up the individual type using getattr() on the module object. The class and module names are automatically able to differ though it would not be a good idea to do so. The subtype modules would still be able to import, as you put it, "unusual dependencies".

If you are concerned about the performance impact of initializing modules that you may not end up using, I think this concern is unwarranted - though of course you haven't shown any code as evidence one way or the other. A properly-written Python module should be fast and safe to load, and should only incur execution expenses when something is called.