Python dependency injection

Question

I have 3 modules: A, B, C

A contains a set of classes that B has fetchers for.

B contains a bunch of singletons that just deal with caching created objects and providing them when requested. Essentially just fetchers.

C is a list of functions that requires instances of A.

The operation that I need to do is something along the lines of:

C::SomeFunc():
   B.getInstance("instance ID")


B::getInstance(someID: str) -> A:
   -look at cache, which is either [] or {}
   -if it is in cache, return that, else do:  A(someID)

My question is, how would you pass around the instances of these modules around? This question is primarily motivated by my lack of understanding of Python's memory allocation system.

I could do something along the lines of constructor-based dependency injection to get an instance of A,B,C where they need to go, and then have some "master/god/controller" object that just passes things where they need to go -

eg:

class god(object):
    def __init__(self):
        a = A()
        b = B()
        c = C(b)
.....
.....
class C(object):
    def __init__(self, B_instance):
         self.B = B_instance
    def SomeFunc(self, instanceID):
         self.B.getInstance(instanceID)

but this seems like a hack.

Any suggestions?

Thanks!

Aigrefin · Accepted Answer · 2015-12-18 00:31:45Z

3

I've seen some packages that offer an instance provider on there on PyPI if that's what you are looking for. Personally, I didn't want to deal with another object to obtain instances. So I created my own library for this (Python 3 only). You only need @inject and annotations.

from py3njection import inject
from some_package import ClassToInject

class Demo:
    @inject
    def __init__(self, object_to_use: ClassToInject):
        self.dependency = object_to_use

demo = Demo()

You can find it here. It basically makes a new instance each time the method is called.

Testing is easy with this since you only have to pass mocks as arguments to unittest your function/method.

I've added @singleton too.

If you need even more instantiation ways, like the cache system you're describing, the docs explain how to easily implement your own. Everything cache related would be taken care of in a factory, and the objects that depends on them wouldn't have to know how to get them.

But if you're on Python 2 or if you need to have more control on injections, this could work too. But you still have to manipulate a provider.

edited Dec 18, 2015 at 0:31

answered Dec 18, 2015 at 0:19

Aigrefin

1258 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

ecoe Over a year ago

I am intrigued by how you're storing a reference to the singleton instance inside a classmethod. I've never seen a dependency injected through a static method. For the sake of ensuring finite lifetime and reducing global access, do you think it is possible to store the singleton in a container, rather than in a static method? For instance, would it be a trivial change storing it in a separate class that more traditionally manages the dependencies?

Rodrigo Oliveira · Accepted Answer · 2020-04-13 16:41:14Z

2

You can use injectable for that. It's a dependency injection autowiring micro-framework.

This is what your code would look like:

@injectable(singleton=True)
class MySingleton:
    ...

@autowired
def func_that_needs_my_singleton(my_singleton: Autowired(MySingleton)):
    ...

edited Apr 13, 2020 at 16:41

answered Feb 5, 2018 at 3:17

Rodrigo Oliveira

1,6024 gold badges21 silver badges37 bronze badges

Comments

Sune Andreas Dybro Debel · Accepted Answer · 2018-02-01 21:04:24Z

1

I recently released a small library that might help you achieve this. Feedback is very welcome :D

https://github.com/suned/serum

answered Feb 1, 2018 at 21:04

Sune Andreas Dybro Debel

5132 gold badges5 silver badges13 bronze badges

Collectives™ on Stack Overflow

Python dependency injection

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related