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
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
当前回答
我想提供一些捷径和语法糖,以及一些可以使用的技术,但我在当前的答案中没有看到它们。
实例化类并立即调用它
在很多情况下,例如当需要做一个APi请求时,逻辑被封装在一个类中,我们真正需要的只是把数据给那个类,并立即作为一个单独的实体运行它,实例化类可能不需要。这就是
instance = MyClass() # instanciation
instance() # run the instance.__call__()
# now instance is not needed
相反,我们可以这样做。
class HTTPApi:
def __init__(self, val1, val2):
self.val1 = val1
self.val2 = val2
def __call__(self, *args, **kwargs):
return self.run(args, kwargs)
def run(self, *args, **kwargs):
print("hello", self.val1, self.val2, args, kwargs)
if __name__ == '__main__':
# Create a class, and call it
(HTTPApi("Value1", "Value2"))("world", 12, 213, 324, k1="one", k2="two")
给调用另一个现有的方法
我们也可以向__call__声明一个方法,而不需要创建一个实际的__call__方法。
class MyClass:
def __init__(self, val1, val2):
self.val1 = val1
self.val2 = val2
def run(self, *args, **kwargs):
print("hello", self.val1, self.val2, args, kwargs)
__call__ = run
if __name__ == '__main__':
(MyClass("Value1", "Value"))("world", 12, 213, 324, k1="one", k2="two")
这允许声明另一个全局函数而不是一个方法,无论出于什么原因(可能有一些原因,例如你不能修改该方法,但你需要由类调用它)。
def run(self, *args, **kwargs):
print("hello",self.val1, self.val2, args, kwargs)
class MyClass:
def __init__(self, val1, val2):
self.val1 = val1
self.val2 = val2
__call__ = run
if __name__ == '__main__':
(MyClass("Value1", "Value2"))("world", 12, 213, 324, k1="one", k2="two")
其他回答
我们可以使用调用方法来使用其他类方法作为静态方法。
class _Callable:
def __init__(self, anycallable):
self.__call__ = anycallable
class Model:
def get_instance(conn, table_name):
""" do something"""
get_instance = _Callable(get_instance)
provs_fac = Model.get_instance(connection, "users")
我将尝试用一个例子来解释这一点,假设您想从斐波那契数列中输出固定数量的项。记住斐波那契数列的前两项都是1。例:1,1,2,3,5,8,13 ....
您希望包含斐波那契数的列表只初始化一次,然后更新。现在我们可以使用__call__函数了。阅读@mudit verma的回答。这就像你希望对象作为函数可调用,但不是每次调用时都重新初始化。
Eg:
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)
输出结果为:
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__
如果你观察到输出__init__只被调用了一次,那就是类第一次实例化的时候,后来对象被调用而没有重新初始化。
__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。
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
方法用于使对象像函数一样工作。
>>> 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
<*There is no __call__ method so it doesn't act like function and throws error.*>
>>>
>>> class B:
... def __init__(self):
... print "From init ... "
... def __call__(self):
... print "From call it is a function ... "
...
>>> b = B()
From init ...
>>> b()
From call it is a function...
>>>
<* __call__ method made object "b" to act like function *>
我们也可以把它传递给一个类变量。
class B:
a = A()
def __init__(self):
print "From init ... "