Dependency injection example in python

dependency-injection.py

class MySqlDb(object):
  def run(self, query):
    # whatever...


# ----------------------------------------------------------------------
# Here, Foo can get bars, AND knows exactly what db implementation to use:

class Foo:
  def __init__(self):
    self.db = MySqlDb()
    
  def get_bars(self):
    return self.db.run("select * from bars")

foo = Foo()
bars = foo.get_bars()


# ----------------------------------------------------------------------
# Here, Foo can only get bars, you have to give it a db to use.
# Makes it more flexible (and easier to test).

class Foo:
  def __init__(self, db):
    self.db = db
    
  def get_bars(self):
    return self.db.run("select * from bars")

db = MySqlDb()
foo = Foo(db)
bars = foo.get_bars()