How to use dependency injection to inject object into module?

Question

This is my scenario. I have a package that consists of several modules. They all import from settings.py. Some of the variable, though, depend on user input.

...
# some CONSTANTS
...
PROJECT_DIR = Path(os.path.abspath(__file__)).parent.ancestor(1)
SCRIPT_DIR = PROJECT_DIR.child('scripts')
data_input = DATA_ROOT.child('input')
input_root = data_input.child(options.root_input) # the options object holds some user input

# then use input_root to get an instance of class Countries
from countries import Countries
country_object = Countries(input_root)

there are several modules that need the country_object. So importing them from settings would be the cleanest solutions.

So I was reading up on dependency injection and I think this is what comes in handy here. But I find it difficult to wrap my around it so how would one use dependency injection to inject the options object into a module?

cchristelis · Accepted Answer · 2014-06-25 09:11:39Z

1

When it comes to patterns there are two philosophies, make your problem fit the pattern and make the pattern fit your problem. I follow the latter. So this would be my adaption of the dependency injection pattern to your problem:

class UserCountry(object):
     def __init__(self):be populated by user data
         self.Country = None

     def set_input_root(self, input_root):
         self.input_root = input_root # <-- this is a list/dict etc that I assume will 

     def __call__(self):
         if self.Country:
             return self.Country
         else:
             # Select country
             self.Country = Country
             return self.Country

in settings.py:

 user_country = UserCountries()

when input_root is defined:

settings.user_country.set_input_root(input_root)

in other modules:

 settings.user_country() # gives you the Country object

edited Jun 25, 2014 at 9:11

answered Jun 25, 2014 at 8:26

cchristelis

2,0351 gold badge14 silver badges17 bronze badges

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

5 Comments

LarsVegas Over a year ago

But settings.py does not know anything about input_root. So I can't call UserCountries() from within settings. I could call it from my main program but the problem remains as other modules also don't know about root_input.

cchristelis Over a year ago

so when is input_root set?

cchristelis Over a year ago

@LarsVegas I updated my answer, but you do realize that you won't know input_root, before you know input root...

LarsVegas Over a year ago

it depends on user input, so when the main program is invoked. But the main program already imports from settings. Other modules than the main program will import settings as well. A module might use another module of the package as well outside of the main program. So input_root and country-instance must be valid and accessible globally during execution of the main program.

cchristelis Over a year ago

Have you seen my updated version? As long as set_input_root is set in the main program before the actual country is needed anywhere you should be fine. You may want to add a default and also save it to the database, if sessions are persistent.

Collectives™ on Stack Overflow

How to use dependency injection to inject object into module?

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related