python_introduction/Intro_to_Solid.md

Object-Oriented class design

SOLID

SOLID coding is a principle created by Robert C.Martin, he is a famous computer scientist. SOLID is an acronym for his five conventions of coding. With their conventions, you can improve the structure of your code, reduce time to implement changes and technical debts, etc. It is a collection of best practices. And it was developed through this decade. Principles of SOLID acronym are:

• The Single-Responsibility Principle (SRP)
• The Open-Closed Principle (OCP)
• The Liskov Substitution Principle (LSP)
• The Interface Segregation Principle (ISP)
• The Dependency inversion Principle (DIP)

The first convention is SRP, that means that all classes of your code must do one thing. That is an important principle. That is the best way to work with others people in the same project. Version control is easier, You will never have Merge conflicts, because other people work in other operations. So, he will never have two same things in the code.

Single-Responsibility

Let's start something ! We will make common mistake that violate SRP and correct them. Let's code a bookstore invoice.

class Book(object):
    def __init__(self, name, authorName, year, price, isbn):
        self.name = name
        self.authorName = authorName
        self.year = year
        self.price = price
        self.isbn = isbn

As you can see, there is a class named Book with some fields. This fields are public and characterize a book.

OK ! Now we can start the invoice class. This class will calculate the final price for a customer.

class Invoice:
    def __init__(self, book, quantity, discountRate, taxRate, total):
        self.book = book
        self.quantity = quantity
        self.discountRate = discountRate
        self.taxRate = taxRate
        self.total = total


    def calculateTotal(self):
        self.price = ((self.book.price - self.book.price * self.discountRate)*self.quantity)

        self.priceWithTaxes = self.price * (1 + self.taxRate)

        return self.priceWithTaxes

    def printInvoice(self):
        print(self.quantity, "x", self.book.name,"", self.book.price, "$");
        print("Discount Rate: ", self.discountRate)
        print("Tax Rate: ", self.taxRate)
        print("Total: ", self.total)


    def saveToFile(self, fileName):
        pass

Alright, now we have the Invoice class, he had 3 methods (calculateTotal, printInvoice, saveToFile) and some fields too. Why this code violate the first convention of SOLID ?

The printInvoice method violate this one because the SRP told us to make just one thing per classes. Here, our printing logic is in the same class than calculateTotal method. So, the printing logic is mixed with business logic in the same class. As you think, the saveToFile method violate this convention too.

Let's correct this example.

class InvoicePrinter(object):
    def __init__(self, invoice):
        self.invoice = invoice
    def printInvoice(self):
        print(self.invoice.quantity, "x", self.invoice.book.name,"", self.invoice.book.price, "$");
        print("Discount Rate: ", self.invoice.discountRate)
        print("Tax Rate: ", self.invoice.taxRate)
        print("Total: ", self.invoice.total)

class InvoicePersistence(object):
    def __init__(self, invoice):
        self.invoice = invoice

    def saveToFile(self):
        pass

We have now two others classes, InvoicePrinter class and InvoicePersistence class. The InvoicePrinter is used to print information. And the InvoicePersistence is used to save information.

With these three classes, we respect the first principle of SOLID.

Open-Closed Principle

This principle says that classes are open for extension and closed to modification. Extension mean news functionality and modification mean modifying your code. If you want to add new functionalities, you are able to add it without manipulating the existing program. If you touch the existing code, you have a risk to have news bugs. So, if you want to add something else, you can use abstract classes and help of interface.

Ok so, let's add new functionality in the InvoicePersistence class.

class InvoicePersistence(object):
    def __init__(self, invoice):
        self.invoice = invoice

    def saveToFile(self):
        pass

    def saveToDataBase(self):
        pass

The saveToDataBase method is used to save information in a Data Base. We have modified the InvoicePersistence class. This class will be more difficult to make easily extendable.