Обектно-орентирано програмиране, част 1

„ Програмиране с Python“, ФМИ

Стефан Кънев && Николай Бачийски && Точо Точев && Димитър Димитров

23.03.2009г.

Python и Зен философията

>>> import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

Класовете, обектите и питоните

Основи на класовете (1)

Основи на класовете (2)


class Person:
    """Represents a person."""
    people = 0

    def __init__(self, name):
        """Constructs a new person."""
        self.name = name
        Person.people += 1

    def say_hi(self):
        """Presents one's self."""
        print("Hello, I'm {0}!".format(self.name))


>>> mityo = Person("Mityo the Python")
>>> mityo.say_hi()
Hello, I'm Mityo the Python!
>>> guido = Person("Guido")
>>> guido.say_hi()
Hello, I'm Guido!
>>> print(Person.people)
2

Полета (1)


class Spam:
    def __init__(self, arg):
        self.stored = arg

>>> spam = Spam(42)
>>> print(spam.stored)
42
>>> spam.stored = 60
>>> print(spam.stored)
60
>>> spam.foo = 10
>>> print(spam.foo)
10

Полета (2)

По този начин може да използвате класовете като структури:

class Student: pass

mityo = Student()
mityo.name = "Mityo the Python"
mityo.age = 22
mityo.faculty_number = 42424

Методи


class Person:
    def __init__(self, name):
        self.name = name

    def greet(self, somebody):
        print("Hello {0}, I'm {1}!".format(somebody, self.name))

>>> mityo = Person("Mityo the Python")
>>> mityo.greet('Stefan')
Hello Stefan, I'm Mityo the Python!
>>> Person.greet(mityo, 'Stefan')
Hello Stefan, I'm Mityo the Python!

Методи (2)

Интересен страничен (или не толкова страничен) ефект е следното:

>>> person = Person("Mityo the Python")
>>> greet_someone = person.greet
>>> greet_someone("Stefan")
Hello Stefan, I'm Mityo the Python!

Обаче:

>>> greeter = Person.greet
>>> greeter(mityo, "Stefan")
Hello Stefan, I'm Mityo the Python!

Статични методи

При статичните методи положението е малко странно:

class Person:
    people = []
    def register(name):
        Person.people.append(name)
        print(len(Person.people), "people are registered now")
    register = staticmethod(register)

>>> Person.register("Mityo the Python")
1 people are registered now
>>> Person.register("Pooh")
2 people are registered now

Класови методи

В Python има "класови" методи, които вземат класът на който са извикани като първи аргумент. Понякога е полезно при наследяване:

class Something:
    def greet(cls, someone):
        print(someone, "was greeted from", cls)
    greet = classmethod(greet)

>>> Something.greet("Mityo")
Mityo was greeted from <class '__main__.Something'>

Конструкция


class Person:
    def __init__(self, name, age = 20, location = "Sofia"):
        self.name = name
        self.age = age
        self.location = location

>>> person = Person("Mityo the Python", 30, "Great Tarnovo")

Наследяване

Проста хватка:

class Something:
    def __init__(self, name): self.name = name
    def introduce(self): "This is something called", self.name

class Vegetable(Something):
    def eat(self): print(self, "was eaten")

class Animal(Something):
    def notice(self): print("Look! This is a", self.name)
    def introduce(self): print("This is an animal called", self.name)

>>> snake = Animal("python")
>>> snake.introduce()
This is an animal called python
>>> snake.notice()
Look! This is a python

Методи и променливи на родителите

Стандартното клас.метод би трябвало да ви е достатъчно:

class PopularPerson(Person):
    def greet(self, somebody):
        Person.greet(self, somebody)
        print("Do you want my autograph?")

>>> pop = PopularPerson("Mityo the Python")
>>> pop.greet("Dim")
Hello Dim, I'm Mityo the Python!
Do you want my autograph?