对python new()和init()的理解

最新推荐文章于 2025-10-23 15:36:56 发布

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本文深入解析Python中__new__与__init__的区别与联系，阐述__new__作为对象创建的构造函数，__init__作为初始化函数的角色。通过具体示例展示__new__如何创建对象并返回self，以及如何自动调用__init__进行初始化。同时，探讨子类如何调用父类的__new__方法。

要点

__new__()是创建对象的构造函数，而__init()__是初始化函数；
__new__()返回的对象即为self；
__new__()会自动调用__init()__。

简单例子

class Person(object):
    def __new__(cls, name, age):
        print(cls)
        print('__new__ is called')
        # test = super().__new__(cls)
        # return test
        return super().__new__(cls) # object.__new__() receives only one para

    def __init__(self, name, age):
        print('__init__ is called')
        self.name = name
        self.age = age

    def __str__(self):
        return('<Person: %s(%s)>' % (self.name, self.age))

name = Person('Tom', 24)
print(name)

__new__()创建了Person实例，其返回值其赋值给self。然后自动调用__init()__，且参数self为__new__()返回值，name和age来自于__new__()的参数。注意object.__new()__只接受cls参数，而cls参数是由解释器自动赋值的。

子类调用父类`new()`

import datetime


class MfDateSub(datetime.date):
    def __init__(self, year, month, day):
        super().__new__(datetime.date, year, month, day)

    def next_day(self):
        result = datetime.date(self.year, self.month, self.day+1)
        return result

mfdate = MfDateSub(2018, 11, 4)
print(mfdate.next_day())

而datetime包中date类并无__init()__，只有__new__()：

class date:
    def __new__(cls, year, month=None, day=None):
        """Constructor.

        Arguments:

        year, month, day (required, base 1)
        """
        if month is None and isinstance(year, bytes) and len(year) == 4 and \
                1 <= year[2] <= 12:
            # Pickle support
            self = object.__new__(cls)
            self.__setstate(year)
            self._hashcode = -1
            return self
        year, month, day = _check_date_fields(year, month, day)
        self = object.__new__(cls)
        self._year = year
        self._month = month
        self._day = day
        self._hashcode = -1
        return self

MfDateSub的作用是扩展date类为其增加next_day功能。MfDateSub的__init()__只能调用date的__new__()，而date的__new__()调用object的__new__()将其赋值给date的self。MfDateSub继承了date，也继承了date的self。所以，MfDateSub的__init()__执行后，就有了date的self。