Conside the code below which represents the following conditions. We have a class A which needs an instance of another class B to carry out some computation. There is something clearly wrong with it.
class B:
def doSomething(self):
return "hello"
class A:
def __init__(self, foo, bar):
self.foo = foo
self.bar = bar
self.b = B()
def makeCalculations(self):
return self.foo + self.bar + self.b.doSomething()
if name ==__main__:
a = A(foo="some text", bar="some other text")
a.makeCalculations()The two major problems with the above code are: 1) Both classses are tightly coupled together, Suppose if later on there is a change to the constructor of class B, class A has to explicitly changed to reflect the new requirements.
2)We cannot test class A in isolation.
To imporve the above code , we can rewrte it as:
class B:
def doSomething(self):
return "hello"
class A:
def __init__(self, foo, bar, B):
self.foo = foo
self.bar = bar
self.b = B
def makeCalculations(self):
return self.foo + self.bar + self.b.doSomething()
if name ==__main__:
b = B()
a = A(foo="some text", bar="some other text", B=b)
a.makeCalculations()Now if there is any change is there in class B it can simple be paseed to class A without changing the class A itself,
The advantages are:
We can now test both classes independent of each other.(to test class A we can simply pass a
mock of class B) & We also gained he ability to control functionality from a singl eplace insted of spreading it throughout the program.
Consider the case when class B needs some parameter in its constructor, class A has to know about all parameters that class B accepts, which is something completely unrelated to class A.it never has/need to know about what class A needs.
Passing an already constructed instance of class B Bexplicitly to A’s constructor solves the above problema and make A depend only on what A needs.
Looking at the above example we are free to conclude that dependency injection is a way to create your dependencies outside of the class that uses it. Inject them from the outside, and take control about their creation away from the inside of your class.