【 python 基础系列 】 - python 魔法方法

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# -*- coding:utf-8 -*-
from os.path import join


class FileObject(object):
    """给文件对象进行包装从而确认在删除时文件流关闭"""

    def __init__(self, filepath='', filename='sample.txt'):
        # 读写模式打开一个文件
        self.file = open(join(filepath, filename), 'r+')
        print("打开文件")

    def __del__(self):
        self.file.close()
        del self.file
        print('销毁文件')


class Word(str):
    """存储单词的类，定义比较单词的几种方法"""

    def __new__(cls, word):
        # 注意我们必须要用到__new__方法，因为str是不可变类型
        # 所以我们必须在创建的时候将它初始化
        if ' ' in word:
            print("Value contains spaces. Truncating to first space.")
            word = word[:word.index(' ')]  # 单词是第一个空格之前的所有字符
        return str.__new__(cls, word)

    def __init__(self, word):
        self.word = word

    def __gt__(self, other):
        return len(self) > len(other)

    def __lt__(self, other):
        return len(self) < len(other)

    def __ge__(self, other):
        return len(self) >= len(other)

    def __le__(self, other):
        return len(self) <= len(other)

    def __str__(self):
        print('call str func ')
        return "Word: %s" % self.word

    def __repr__(self):
        print('class repr func')
        return "<Word:%s>" % self.word

    def __unicode__(self):
        print('call unicode')
        return 'Word:%s' % self.word


class MyCounter(object):
    """一个包含计数器的控制权限的类每当值被改变时计数器会加一"""

    def __init__(self, val):
        # 使用 super 因为不是所有的类都有 __dict__ 属性
        super(MyCounter, self).__setattr__('counter', 0)
        super(MyCounter, self).__setattr__('value', val)

    def __getattr__(self, item):
        print('call  getattr ')
        return super(MyCounter, self).__getattr__(item)

    def __getattribute__(self, item):
        print('call getattribute')
        return super(MyCounter, self).__getattribute__(item)

    def __setattr__(self, name, value):
        # self.key = value  # 会隐式的再次调用self.__setattr__ 引起递归
        # self.__dict__[key] = value
        print('call setattr')
        if name == 'value':
            super(MyCounter, self).__setattr__('counter', self.counter + 1)
            # 如果你不想让其他属性被访问的话，那么可以抛出 AttributeError(name) 异常
        super(MyCounter, self).__setattr__(name, value)

    def __delattr__(self, name):
        print('call del attr')
        if name == 'value':
            super(MyCounter, self).__setattr__('counter', self.counter + 1)
        super(MyCounter, self).__delattr__(name)


class Container(object):
    """ 容器基类 """

    def __len__(self):
        return len(self)

    def __setitem__(self, key, value):
        pass

    def __getitem__(self, item):
        pass


class DynamicContainer(Container):
    """ 可变容器 """

    def __delitem__(self, key):
        pass

    def __iter__(self):
        pass

    def __reversed__(self):
        pass

    def __contains__(self):
        pass


class NoDynamicContainer(Container):
    """ 不可变容器 """

    def __iter__(self):
        pass

    def __contains__(self):
        pass


class MyList(Container):
    """ 一个封装了一些附加魔术方法比如 head, tail, init, last, drop, 和take的列表类。 """

    def __init__(self, values=None):
        if values is None:
            self.values = []
        else:
            self.values = values

    def __len__(self):
        return len(self.values)

    def __getitem__(self, key):
        # 如果键的类型或者值无效，列表值将会抛出错误
        return self.values[key]

    def __setitem__(self, key, value):
        self.values[key] = value

    def __delitem__(self, key):
        del self.values[key]

    def __iter__(self):
        return iter(self.values)

    def __reversed__(self):
        return reversed(self.values)

    def append(self, value):
        self.values.append(value)

    def head(self):
        return self.values[0]

    def tail(self):
        return self.values[1:]

    def init(self):
        # 返回一直到末尾的所有元素
        return self.values[:-1]

    def last(self):
        # 返回末尾元素
        return self.values[-1]

    def drop(self, n):
        # 返回除前n个外的所有元素
        return self.values[n:]

    def take(self, n):
        # 返回前n个元素
        return self.values[:n]


class Entity(object):
    """调用实体来改变实体的位置。"""

    def __init__(self, size, x, y):
        self.x, self.y = x, y
        self.size = size

    def __call__(self, x, y):
        """改变实体的位置"""
        self.x, self.y = x, y


class Closer:
    """通过with语句和一个close方法来关闭一个对象的会话管理器"""

    def __init__(self, obj):
        self.obj = obj

    def __enter__(self):
        print('call enter')
        return self.obj  # bound to target

    def __exit__(self, exception_type, exception_val, trace):
        print(exception_type, exception_val, trace)
        try:
            self.obj.close()
        except AttributeError:  # obj isn't closable
            print('Not closable.')
            return True  # exception handled successfully


class Meter(object):
    """Descriptor for a meter."""

    def __init__(self, value=0.0):
        self.value = float(value)

    def __get__(self, instance, owner):
        return self.value

    def __set__(self, instance, value):
        self.value = float(value)


class Foot(object):
    """Descriptor for a foot."""

    def __get__(self, instance, owner):
        return instance.meter * 3.2808

    def __set__(self, instance, value):
        instance.meter = float(value) / 3.2808


class Distance(object):
    meter = Meter()
    foot = Foot()


if __name__ == '__main__':
    # fileobj = FileObject()
    #
    # foo = Word('foo')
    # bara = Word('bara')
    # print(foo > bara)  # False
    # print(foo < bara)  # True
    # print(str('foo') == str('bar'))  # False 利用了 string 内置魔法函数 __eq__

    # print(str(foo))  # Word: foo
    # print(repr(foo))  # <Word:foo>
    # # print(unicode(foo))  # only in python2

    # inst = MyCounter('Test')
    # inst.value = 'test'
    # print(inst.counter)  # 1
    # print(inst.value)  # test
    # inst.value = 'test1'
    # print(inst.counter)  # 2
    # print(inst.value)  # test1  属性存在,只有__getattribute__调用
    # try:
    #     inst.value2  # 属性不存在, 先调用__getattribute__, 后调用__getattr__
    # except AttributeError:
    #     pass
    # del inst.value  # call del attr

    # mylist = MyList([1, 2, 3, 4, 5, 6])
    # print(mylist.head())
    # print(list(mylist.__reversed__()))

    # entiry = Entity(10, 1, 2)
    # entiry(2, 1)
    # print(entiry.x, entiry.y)  # 2 1

    # with Closer(int(5)) as i:
    #     i += 1
    # print(i)
    # with Closer(open('sample.txt')) as file:
    #     print('file>>>>:', file.read())

    d = Distance()
    print(d.meter, d.foot)  # 0.0, 0.0
    d.meter = 1
    print(d.meter, d.foot)  # 1.0 3.2808
    d.meter = 2
    print(d.meter, d.foot)  # 2.0 6.5616