__init__和__call__有什么区别？

Question

我想知道__init__和__call__方法之间的区别.

例如:

class test:

  def __init__(self):
    self.a = 10

  def __call__(self): 
    b = 20

Answer 1

第一个用于初始化新创建的对象,并接收用于执行此操作的参数:

class Foo:
    def __init__(self, a, b, c):
        # ...

x = Foo(1, 2, 3) # __init__

第二个实现函数调用操作符.

class Foo:
    def __call__(self, a, b, c):
        # ...

x = Foo()
x(1, 2, 3) # __call__

Answer 2

__call__()在元类中定义自定义方法允许将类的实例作为函数调用,而不是始终修改实例本身.

In [1]: class A:
   ...:     def __init__(self):
   ...:         print "init"
   ...:         
   ...:     def __call__(self):
   ...:         print "call"
   ...:         
   ...:         

In [2]: a = A()
init

In [3]: a()
call

Answer 3

在Python中,函数是第一类对象,这意味着:函数引用可以在输入中传递给其他函数和/或方法,并从它们内部执行.

类的实例(也称为对象)可以被视为它们是函数:将它们传递给其他方法/函数并调用它们.为了实现这一点,__call__类功能必须是专用的.

def __call__(self, [args ...]) 它将可变数量的参数作为输入.假设x是Class的一个实例X,x.__call__(1, 2)类似于调用x(1,2)或实例本身作为一个函数.

在Python中,__init__()被正确定义为类构造函数(以及__del__()类析构函数).因此,存在之间的净区别__init__()和__call__():第一建立类的实例时,该第二使得这样的实例可调用一个函数将不会影响物体本身的生命周期(即__call__不影响施工/销毁生命周期),但它可以修改其内部状态(如下所示).

例.

class Stuff(object):

    def __init__(self, x, y, range):
        super(Stuff, self).__init__()
        self.x = x
        self.y = y
        self.range = range

    def __call__(self, x, y):
        self.x = x
        self.y = y
        print '__call__ with (%d,%d)' % (self.x, self.y)

    def __del__(self):
        del self.x
        del self.y
        del self.range

>>> s = Stuff(1, 2, 3)
>>> s.x
1
>>> s(7, 8)
__call__ with (7,8)
>>> s.x
7

Answer 4

__call__使类的实例可调用.为什么需要它？

技术上在创建对象时__init__调用一次__new__,以便可以初始化它.

但是在很多场景中你可能想要重新定义你的对象,比如你完成了对象,并且可能需要一个新的对象.有了__call__你可以重新定义相同的对象,就好像它是新的.

这只是一种情况,还有更多.

Answer 5

__init__将被视为构造函数,其中__call__可以使用对象多次调用方法.两者__init__和__call__函数都采用默认参数.

Answer 6

>>> class A:
...     def __init__(self):
...         print "From init ... "
... 
>>> a = A()
From init ... 
>>> a()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: A instance has no __call__ method
>>> 
>>> class B:
...     def __init__(self):
...         print "From init ... "
...     def __call__(self):
...         print "From call ... "
... 
>>> b = B()
From init ... 
>>> b()
From call ... 
>>> 

Answer 7

我将尝试用一个例子来解释这个,假设你想从斐波那契系列中打印一定数量的术语.请记住,斐波纳契系列的前两个项是1.例如:1,1,3,3,5,8,13 ....

您希望包含斐波那契数字的列表仅初始化一次,之后应该更新.现在我们可以使用这个__call__功能了.阅读@mudit verma的回答.这就像你希望对象可以作为函数调用,但是在你调用它时不会重新初始化.

例如:

class Recorder:
    def __init__(self):
        self._weights = []
        for i in range(0, 2):
            self._weights.append(1)
        print self._weights[-1]
        print self._weights[-2]
        print "no. above is from __init__"

    def __call__(self, t):
        self._weights = [self._weights[-1], self._weights[-1] + self._weights[-2]]
        print self._weights[-1]
        print "no. above is from __call__"

weight_recorder = Recorder()
for i in range(0, 10):
    weight_recorder(i)

输出是:

1
1
no. above is from __init__
2
no. above is from __call__
3
no. above is from __call__
5
no. above is from __call__
8
no. above is from __call__
13
no. above is from __call__
21
no. above is from __call__
34
no. above is from __call__
55
no. above is from __call__
89
no. above is from __call__
144
no. above is from __call__

如果您观察到输出__init__仅在第一次实例化类时被调用,则稍后在没有重新初始化的情况下调用该对象.

Answer 8

__call__允许返回任意值，而__init__作为构造函数隐式返回类的实例。正如其他答案正确指出的那样，__init__只调用一次，但可以__call__多次调用，以防初始化的实例分配给中间变量。

>>> class Test:
...     def __init__(self):
...         return 'Hello'
... 
>>> Test()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
TypeError: __init__() should return None, not 'str'
>>> class Test2:
...     def __call__(self):
...         return 'Hello'
... 
>>> Test2()()
'Hello'
>>> 
>>> Test2()()
'Hello'
>>> 

Answer 9

因此，__init__在您创建任何类的实例并初始化实例变量时调用。

例子：

class User:

    def __init__(self,first_n,last_n,age):
        self.first_n = first_n
        self.last_n = last_n
        self.age = age

user1 = User("Jhone","Wrick","40")

而__call__当你调用像任何其他函数的对象被调用。

例子：

class USER:
    def __call__(self,arg):
        "todo here"
         print(f"I am in __call__ with arg : {arg} ")


user1=USER()
user1("One") #calling the object user1 and that's gonna call __call__ dunder functions

Answer 10

您还可以使用__call__方法来实现装饰器。

本示例摘自Python 3 Patterns，Recipes和Idioms

class decorator_without_arguments(object):
    def __init__(self, f):
        """
        If there are no decorator arguments, the function
        to be decorated is passed to the constructor.
        """
        print("Inside __init__()")
        self.f = f

    def __call__(self, *args):
        """
        The __call__ method is not called until the
        decorated function is called.
        """
        print("Inside __call__()")
        self.f(*args)
        print("After self.f( * args)")


@decorator_without_arguments
def sayHello(a1, a2, a3, a4):
    print('sayHello arguments:', a1, a2, a3, a4)


print("After decoration")
print("Preparing to call sayHello()")
sayHello("say", "hello", "argument", "list")
print("After first sayHello() call")
sayHello("a", "different", "set of", "arguments")
print("After second sayHello() call")

输出：

Answer 11

情况1：

class Example:
    def __init__(self, a, b, c):
        self.a=a
        self.b=b
        self.c=c
        print("init", self.a, self.b, self.c)

跑步：

Example(1,2,3)(7,8,9)

结果：

- init 1 2 3
- TypeError: 'Example' object is not callable

案例2：

class Example:
    def __init__(self, a, b, c):
        self.a=a
        self.b=b
        self.c=c
        print("init", self.a, self.b, self.c)
    def __call__(self, x, y, z):
        self.x=x
        self.y=y
        self.z=z
        print("call", self.x, self.y, self.z)

跑步：

Example(1,2,3)(7,8,9)

结果：

- init 1 2 3
- call 7 8 9

Answer 12

上面已经提供了简短而甜蜜的答案。我想提供一些与Java相比的实际实现。

 class test(object):
        def __init__(self, a, b, c):
            self.a = a
            self.b = b
            self.c = c
        def __call__(self, a, b, c):
            self.a = a
            self.b = b
            self.c = c


    instance1 = test(1, 2, 3)
    print(instance1.a) #prints 1

    #scenario 1
    #creating new instance instance1
    #instance1 = test(13, 3, 4)
    #print(instance1.a) #prints 13


    #scenario 2
    #modifying the already created instance **instance1**
    instance1(13,3,4)
    print(instance1.a)#prints 13

注意：场景 1 和场景 2 在结果输出方面似乎相同。但在场景1中，我们再次创建另一个新实例instance1。在场景2中，我们只需修改已经创建的instance1。__call__在这里是有益的，因为系统不需要创建新实例。

Java 中的等效项

public class Test {

    public static void main(String[] args) {
        Test.TestInnerClass testInnerClass = new Test(). new TestInnerClass(1, 2, 3);
        System.out.println(testInnerClass.a);

        //creating new instance **testInnerClass**
        testInnerClass = new Test().new TestInnerClass(13, 3, 4);
        System.out.println(testInnerClass.a);

        //modifying already created instance **testInnerClass**
        testInnerClass.a = 5;
        testInnerClass.b = 14;
        testInnerClass.c = 23;

        //in python, above three lines is done by testInnerClass(5, 14, 23). For this, we must define __call__ method

    }

    class TestInnerClass /* non-static inner class */{

        private int a, b,c;

        TestInnerClass(int a, int b, int c) {
            this.a = a;
            this.b = b;
            this.c = c;
        }
    }
}

Answer 13

__init__()能：

• 初始化类的实例。
• 被叫很多次。
• 只能返回None。

__call__()可以像实例方法一样自由使用。

例如，Person类有__init__()and ，__call__()如下所示：

class Person:
    def __init__(self, f_name, l_name):
        self.f_name = f_name
        self.l_name = l_name
        print('"__init__()" is called.')
        
    def __call__(self, arg):
        return arg + self.f_name + " " + self.l_name

现在，我们创建并初始化Person类的实例，如下所示：

    # Here
obj = Person("John", "Smith")

然后，__init__()调用如下所示：

"__init__()" is called.

接下来我们__call__()通过如下2种方式调用：

obj = Person("John", "Smith")
print(obj("Hello, ")) # Here
print(obj.__call__("Hello, ")) # Here

然后，__call__()调用如下所示：

"__init__()" is called.
Hello, John Smith # Here
Hello, John Smith # Here

并且，__init__()可以多次调用，如下所示：

obj = Person("John", "Smith")
print(obj.__init__("Tom", "Brown")) # Here
print(obj("Hello, "))
print(obj.__call__("Hello, "))

然后，__init__()被调用，Person类的实例被重新初始化并None返回，__init__()如下所示：

"__init__()" is called.
"__init__()" is called. # Here
None # Here
Hello, Tom Brown
Hello, Tom Brown

并且，如果__init__()没有返回None，我们调用__init__()如下所示：

class Person:
    def __init__(self, f_name, l_name):
        self.f_name = f_name
        self.l_name = l_name
        print('"__init__()" is called.')
        return "Hello" # Here
        
    # ...

obj = Person("John", "Smith") # Here

出现以下错误：

类型错误： __init__() 应该返回 None，而不是 'str'

并且，如果类__call__中没有定义Person：

class Person:
    def __init__(self, f_name, l_name):
        self.f_name = f_name
        self.l_name = l_name
        print('"__init__()" is called.')
        
    # def __call__(self, arg):
    #     return arg + self.f_name + " " + self.l_name

然后，我们调用obj("Hello, ")如下所示：

obj = Person("John", "Smith")
obj("Hello, ") # Here

出现以下错误：

类型错误：“Person”对象不可调用

然后我们再次调用，obj.__call__("Hello, ")如下所示：

obj = Person("John", "Smith")
obj.__call__("Hello, ") # Here

出现以下错误：

AttributeError：“Person”对象没有属性“__call__”