__init__

The dependencies of a class are injected before calling it’s __init__ method. It’s perfectly fine to use these dependencies during object’s initialization.

Example #1:

example1.py

from injectme import inject, register


class Dependency:
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return f"Dependency: {self.name}"


@inject
class Service:
    dep: Dependency

    def __init__(self):
        print("In Service __init__")
        print(self.dep)


register(Dependency, Dependency("Debra"))

service = Service()

$ python3 example1.py

In Service __init__
Dependency: Debra

If you decide to use inheritance and implement custom __init__ methods you have to call parent’s __init__ before you can use dependencies declared there. If you do not call super().__init__() you will get an error when trying to access them.

Example #2:

example2.py

from injectme import inject, register


class DependencyA:
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return f"DependencyA: {self.name}"


class DependencyB:
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return f"DependencyB: {self.name}"


register(DependencyA, DependencyA("Alice"))
register(DependencyB, DependencyB("Bob"))


@inject
class BaseService:
    dep_a: DependencyA


# after calling super().__init__() you can use deps without
# any issue
@inject
class AdvancedService(BaseService):
    dep_b: DependencyB

    def __init__(self):
        super().__init__()
        print("AdvancedService __init__")
        print(self.dep_a)
        print(self.dep_b)


advanced_service = AdvancedService()

# if you try to access parent's dependency before calling its __init__
# you'll get AttributeError
@inject
class AdvancedServiceBrokenInit(BaseService):
    dep_b: DependencyB

    def __init__(self):
        print("AdvancedService __init__")
        print(self.dep_a)
        print(self.dep_b)
        super().__init__()


try:
    AdvancedServiceBrokenInit()
except AttributeError as e:
    print(e)


# you'll get same result if you completly forget about initializing parent
@inject
class AdvancedServiceMissingSuperInit(BaseService):
    dep_b: DependencyB

    def __init__(self):
        print("AdvancedServiceMissingSuperInit __init__")

    def print_deps(self):
        print("AdvancedServiceMissingSuperInit deps:")
        print(self.dep_a)
        print(self.dep_b)


try:
    service = AdvancedServiceMissingSuperInit()
    service.print_deps()
except AttributeError as e:
    print(e)

$ python3 example2.py

AdvancedService __init__
DependencyA: Alice
DependencyB: Bob

AdvancedServiceBrokenInit __init__
'AdvancedServiceBrokenInit' object has no attribute 'dep_a'

AdvancedServiceMissingSuperInit __init__
AdvancedServiceMissingSuperInit deps:
'AdvancedServiceMissingSuperInit' object has no attribute 'dep_a'