1.1   定制类

1.1.1   __str__

>>> class Student(object):

...    def __init__(self, name):

...        self.name = name

...

>>> s = Student('daidai')

>>> s.name

'daidai'

>>> Student('daidai').name

'daidai'

>>> print(Student('daidai'))     --发现这样打印并不好看

<__main__.Student object at0x2ac5ebc99128>

>>> class Student(object):

...    def __init__(self, name):

...        self.name = name

...    def __str__(self):

...         return 'Student object (name: %s)' %self.name

...

>>> print(Student('daidai'))

Student object (name: daidai)

>>> s = Student('daidai')

>>> s         --指向变量后，仍然不好看

<__main__.Student object at0x2ac5ebc99160>

>>> s.name

'daidai'

将__str__同时指向__repr__

>>> class Student(object):

...    def __init__(self, name):

...        self.name = name

...    def __str__(self):

...        return 'Student object (name: %s)' % self.name

...    __repr__ = __str__

...

>>> print(Student('daidai'))

Student object (name: daidai)

>>> s = Student('daidai')

>>> s.name

'daidai'

>>> s

Student object (name: daidai)

1.1.2   __iter__

如果一个类想被用于for ... in循环，类似list或tuple那样，就必须实现一个__iter__()方法，该方法返回一个迭代对象，然后，Python的for循环就会不断调用该迭代对象的__next__()方法拿到循环的下一个值，直到遇到StopIteration错误时退出循环。

以斐波那契数列为例。

>>> class Fib(object):

...    def __init__(self):

...        self.a, self.b = 0, 1     #初始化 a和b

...    def __iter__(self):

...        return self               #返回迭代对象，实例本身就是迭代对象返回自 己

...    def __next__(self):

...        self.a, self.b = self.b, self.a + self.b #计算下一个值

...        if self.a > 10000:            #退出循环条件

...             raiseStopIteration();

...        return self.a              #返回下一个值

...

>>> for n in Fib():

...    print(n)

...

1

1

2

3

5

8

13

21

34

55

89

144

233

377

610

987

1597

2584

4181

6765

1.1.3   __getitem__

上述Fib实例感觉和list类似，但是并不能通过下标来取元素。

通过__getitem__方法就可以。

>>> class Fib(object):

...    def __getitem__(self, n):

...        a, b = 1, 1

...        for x in range(n):

...             a, b = b, a + b

...        return a

...

>>> f = Fib()

>>> f[0]

1

>>> f[4]

5

>>> f[88]

1779979416004714189

1.1.4   __getattr__

正常情况下，当我们调用类的方法或属性时，如果不存在，就会报错。

>>> class Student(object):

...    def __init__(self):

...        self.name = 'daidai'

...

>>> s = Student()

>>> print(s.name)

daidai

>>> print(s.score)

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

AttributeError: 'Student' object has noattribute 'score'

加上__getattr__动态返回属性值

>>> class Student(object):

...    def __init__(self):

...        self.name = 'daidai'

...    def __getattr__(self, attr):       --当输入score不存在时，python动态调用这个方法

...         if attr == 'score':

...             return 98

...

>>> s = Student()

>>> s.name

'daidai'

>>> s.score

98

__getattr__返回函数也是可行的

>>> class Student(object):

...    def __getattr__(self, attr):

...         if attr == 'age':

...             return lambda : 24

...

>>> s = Student()

>>> s.age

<functionStudent.__getattr__.<locals>.<lambda> at 0x2ac5ebc8e730>

>>> s.age()

24

只有在没有找到属性的情况下，才调用__getattr__，已有的属性，比如name，不会在__getattr__中查找。

>>> class Student(object):

...    def __getattr__(self, attr):

...        if attr == 'age':

...             return lambda : 24

...        raise AttributeError('\'Student\' object has no attribute \'%s\'' %attr)

...

>>>

>>> s = Student()

>>> s.name

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

File "<stdin>", line 5, in __getattr__

AttributeError: 'Student' object has noattribute 'name'

>>> s.age

<functionStudent.__getattr__.<locals>.<lambda> at 0x2ac5ebc8e488>

>>> s.age()

24

1.1.5   __call__

__call__方法能实现直接在实例本身上调用。

>>> class Student(object):

...    def __init__(self, name):

...        self.name = name

...    def __call__(self):

...         print('My name is %s.' % self.name)

...

>>> s = Student('daidai')

>>> s

<__main__.Student object at0x2ac5ebc99748>

>>> s()      --直接实例对象调用

My name is daidai.

可通过callable判断对象是否可调用

>>>callable(Student('daidai'))

True

>>> callable(max)

True

>>> callable((1, 2, 3))

False

>>> callable('str')

False

__call__也可传入参数

>>> class Student(object):

...    def __init__(self, name):

...        self.name = name

...    def __call__(self, others):

...         print('My name is %s, %s' %(self.name, others))

...

>>>

>>> s = Student('daidai')

>>> s('so handsome.')

My name is daidai, so handsome.