186 lines
6.1 KiB
Markdown
186 lines
6.1 KiB
Markdown
# Object-Oriented class design
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## SOLID
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SOLID coding is a principle created by Robert C.Martin, he is a famous computer scientist.
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SOLID is an acronym for his five conventions of coding.
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With their conventions, you can improve the structure of your code, reduce time to implement changes and technical debts, etc.
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It is a collection of best practices.
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And it was developed through this decade.
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Principles of SOLID acronym are:
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* The Single-Responsibility Principle (**SRP**)
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* The Open-Closed Principle (**OCP**)
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* The Liskov Substitution Principle (**LSP**)
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* The Interface Segregation Principle (**ISP**)
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* The Dependency inversion Principle (**DIP**)
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The first convention is **SRP**, that means that all classes of your code must do one thing.
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That is an important principle.
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That is the best way to work with others people in the same project.
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Version control is easier,
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You will never have _Merge conflicts_, because other people work in other operations.
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So, he will never have two same things in the code.
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### Single-Responsibility
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Let's start something !
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We will make common mistake that violate **SRP** and correct them.
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Let's code a bookstore invoice.
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```python
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class Book(object):
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def __init__(self, name, authorName, year, price, isbn):
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self.name = name
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self.authorName = authorName
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self.year = year
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self.price = price
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self.isbn = isbn
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```
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As you can see, there is a class named Book with some fields.
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This fields are public and characterize a book.
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OK ! Now we can start the invoice class.
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This class will calculate the final price for a customer.
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```python
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class Invoice(object):
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def __init__(self, book, quantity, discountRate, taxRate, total):
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self.book = book
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self.quantity = quantity
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self.discountRate = discountRate
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self.taxRate = taxRate
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self.total = total
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def calculateTotal(self):
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self.price = ((self.book.price - self.book.price * self.discountRate)*self.quantity)
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self.priceWithTaxes = self.price * (1 + self.taxRate)
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return self.priceWithTaxes
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def printInvoice(self):
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print(self.quantity, "x", self.book.name,"", self.book.price, "$");
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print("Discount Rate: ", self.discountRate)
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print("Tax Rate: ", self.taxRate)
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print("Total: ", self.total)
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def saveToFile(self, fileName):
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pass
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```
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Alright, now we have the _Invoice_ class, he had 3 methods (calculateTotal, printInvoice, saveToFile) and some fields too.
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Why this code violate the first convention of **SOLID** ?
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The _printInvoice_ method violate this one because the **SRP** told us to make just one thing per classes.
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Here, our printing logic is in the same class than _calculateTotal_ method.
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So, the printing logic is mixed with business logic in the same class.
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As you think, the _saveToFile_ method violate this convention too.
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Let's correct this example.
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```python
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class InvoicePrinter(object):
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def __init__(self, invoice):
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self.invoice = invoice
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def printInvoice(self):
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print(self.invoice.quantity, "x", self.invoice.book.name,"", self.invoice.book.price, "$");
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print("Discount Rate: ", self.invoice.discountRate)
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print("Tax Rate: ", self.invoice.taxRate)
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print("Total: ", self.invoice.total)
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```
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```python
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class InvoicePersistence(object):
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def __init__(self, invoice):
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self.invoice = invoice
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def saveToFile(self):
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pass
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```
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We have now two others classes, _InvoicePrinter_ class and _InvoicePersistence_ class.
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The _InvoicePrinter_ is used to print information.
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And the _InvoicePersistence_ is used to save information.
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With these three classes, we respect the first principle of **SOLID**.
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### Open-Closed Principle
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This principle says that classes are open for extension and closed to modification.
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Extension mean news functionalities and modification mean modifying your code.
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If you want to add new functionalities, you are able to add it without manipulating the existing program.
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If you touch the existing code, you have a risk to have news bugs.
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So, if you want to add something else, you can use abstract classes and help of interface.
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Ok so, let's add new functionality in the _InvoicePersistence_ class.
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```python
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class InvoicePersistence(object):
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def __init__(self, invoice):
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self.invoice = invoice
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def saveToFile(self):
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pass
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def saveToDataBase(self):
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pass
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```
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The _saveToDataBase_ method is used to save information in a Data Base.
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We have modified the _InvoicePersistence_ class.
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And this class will be more difficult to make easily extendable.
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So, we violate the **OCP** convention.
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If you want to respect this principle, you have to create a new class.
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```python
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class InvoicePersistence(abc.ABC):
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@abstractmethod
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def save(self, invoice):
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pass
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```
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The new _InvoicePersistence_ class has an abstract method.
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So, if a class inherits the _InvoicePersistence_ class, you have to implement the _save_ method.
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And for example, we will create a _DataBasePersistence_ class, and this class inherits the abstract _InvoicePersistence_ class.
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```python
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class DatabasePersistence(InvoicePersistence):
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def __init__(self, invoice):
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self.invoice = invoice
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self.save(self.invoice)
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def save(self, invoice):
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print("Save in database ...")
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```
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Let's do the same thing with _FilePersistence_ class.
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```python
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class FilePersistence(InvoicePersistence):
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def __init__(self, invoice):
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self.invoice = invoice
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self.save(self.invoice)
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def save(self, invoice):
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print("Save to file ...")
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```
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Ok, we do a lot of things, let's make a UML to represent our class structure.
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<center>
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<img src="./assets/UML_Solid" alt="" width="400"/>
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</center>
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That will be more simple to do extension.
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### Liskov Substitution Principle
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This convention tells us to create a substitutable subclass for their parents.
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So, if you want to create an object in the subclass, you have to be able to pass it in the interface.
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### Interface Segregation Principle
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The _Interface Segregation Principle_ means that all interfaces have to be separated.
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That means that clients has not functions that they do not need.
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### Dependency Inversion Principle
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