>>> 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 ... 
>>> 

__init__将被视为构造函数，其中__call__方法可以被对象调用任意次数。__init__和__call__函数都接受默认参数。

上面已经给出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是相同的。 但是在场景1中，我们再次创建另一个新实例instance1。在scenario2, 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的实例。

在Java中等价

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;
        }
    }
}

__init__()可以:

初始化类的实例。 被叫了很多次。 只返回None。

__call__()可以像实例方法一样自由使用。

例如，Person类有__init__()和__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")

然后调用__init__()，如下所示:

"__init__()" is called.

接下来，我们以如下所示的2种方式调用__call__():

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

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

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

并且，如果__init__()不返回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:

__init__()应该返回None，而不是'str'

并且，如果Person类中没有定义__call__:

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:

'Person'对象不可调用

同样，我们调用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__'

第一个用于初始化新创建的对象，并接收用于初始化的参数:

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

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

第二部分实现函数调用操作符。

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

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

因此，当你创建任何类的实例并初始化实例变量时，__init__也会被调用。

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")

当你像调用其他函数一样调用对象时，会调用__call__。

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