SOLID principles using Python

1. S.O.L.I.D

2. S.O.L.I.D
• Single Responsibility
• Open/Closed
• Liskov Substitution
• Interface Segregation
• Dependency Inversion

3. Why?

4. • Create easy to maintain systems
• Improve reusability
• Easy testing

6. It’s all about the money

7. Always try to test
everything

8. “I don’t need to test this.”

9. –Michael C. Feathers, Working Effectively with Legacy Code
“Code without tests is bad code. It doesn't
matter how well written it is; it doesn't matter
how pretty or object-oriented or well-
encapsulated it is. With tests, we can change
the behavior of our code quickly and verifiably.
Without them, we really don't know if our code
is getting better or worse.”

11. Single Responsibility
Principle

12. - Robert Martin
“There should never be more than one reason
for a class to change”

13. class CarWashService(object):
def __init__(self, sms_sender):
self.persistence = {}
self.sms_sender = sms_sender
def require_car_wash(self, car, customer):
service_id = uuid.uuid4().hex
self.persistence[service_id] = (car, customer)
return service_id
def wash_completed(self, service_id):
car, customer = self.persistence[service_id]
self.sms_sender.send(mobile_phone=customer.mobile_phone,
text='Car %{car.plate} washed'.format(car=car))

14. class CarWashService(object):
def __init__(self, sms_sender):
self.persistence = {}
self.sms_sender = sms_sender
def require_car_wash(self, car, customer):
service_id = uuid.uuid4().hex
self.persistence[service_id] = (car, customer)
return service_id
def wash_completed(self, service_id):
car, customer = self.persistence[service_id]
self.sms_sender.send(mobile_phone=customer.mobile_phone,
text='Car %{car.plate} washed'.format(car=car))

15. class CarWashService(object):
def __init__(self, sms_sender):
self.persistence = {}
self.sms_sender = sms_sender
def require_car_wash(self, car, customer):
service_id = uuid.uuid4().hex
self.persistence[service_id] = (car, customer)
return service_id
def wash_completed(self, service_id):
car, customer = self.persistence[service_id]
self.sms_sender.send(mobile_phone=customer.mobile_phone,
text='Car %{car.plate} washed'.format(car=car))

17. class CarWashService(object):
def __init__(self, notifier, repository):
self.repository = repository
self.notifier = notifier
def enter_in_the_car_wash(self, car, customer):
job = CarWashJob(car, customer)
self.repository.put(job)
return job
def wash_completed(self, service_id):
car_wash_job = self.repository.find_by_id(service_id)
self.notifier.job_completed(car_wash_job)

18. Dependency Inversion
Principle

19. class CarWashService(object):
def __init__(self, repository):
self.repository = repository
def enter_the_car_wash(self, car, customer):
job = CarWashJob(car, customer)
self.repository.put(job)
return job
def wash_completed(self, service_id):
car_wash_job = self.repository.find_by_id(service_id)
SmsNotifier.send_sms(car_wash_job)

20. Would you solder a lamp
directly to the wiring in a
wall?

21. Depend upon
abstraction. Do not
depend upon concretion

22. class CarWashService(object):
def __init__(self, notifier, repository):
self.repository = repository
self.notifier = notifier
def enter_the_car_wash(self, car, customer):
job = CarWashJob(car, customer)
self.repository.put(job)
return job
def wash_completed(self, service_id):
car_wash_job = self.repository.find_by_id(service_id)
self.notifier.job_completed(car_wash_job)

23. Open Closed Principle

24. Open chest surgery isn’t
needed when putting a
coat

25. class JobRepository(object):
def __init__(self, type):
self.type = type
def put(self, job):
if(type == ‘in_memory'):
# WOW, SUCH CODE
self.memory = {}
elif(type == 'database'):
self.memory = Database.connect()
# MUCH SOURCE

27. Ifs and else’s are not a sign of problem
complexity. They are a sign of code complexity

28. class IJobRepository(object):
def put(self, job):
raise NotImplementedError()
class InMemoryJobRepository(IJobRepository):
def __init__(self):
self._storage = {}
def put(self, job)
self._storage[job.service_id] = job

29. Liskov Substituion
Principle

31. - Barbara Liskov
“Let phi(x) be a property provable about
objects x of type T. Then phi(y) should be true
for objects y of type S where S is a subtype of
T.”

32. class Rectangle(object):
def __init__(self, width, height):
self.width = width
self.height = height
def area(self):
return self.width * self.height
class Square(Rectangle):
def area(self):
return self.height * self.height
def much_code():
my_shape = factory.rectangle(10, 5)
my_shape.area() # I expect this to be 50

33. If you change the references
from base classes to child
classes, the tests should pass.

35. class Rectangle(object):
def __init__(self, width, height):
self.width = width
self.height = height
def area(self):
return self.width * self.height
class Square(Rectangle):
def area(self):
return self.height * self.height
def much_code():
my_shape = factory.rectangle(10, 5)
my_shape.area() # I expect this to be 50

36. Square is not a subtype of Rectangle

37. Interface Segregation
Principle

39. Always prefer small
interfaces (contracts)

40. Several interfaces are
better than one big one