Python內置函數(9)——callable

英文文檔：

callable(object)

　　Return True if the object argument appears callable, False if not. If this returns true, it is still possible that a call fails, but if it is false, calling object will never succeed. Note that classes are callable (calling a class returns a new instance); instances are callable if their class has a __call__() method.

說明：

　　1. 方法用來檢測對象是否可被調用，可被調用指的是對象能否使用()括號的方法調用。

>>> callable(callable)
True
>>> callable(1)
False
>>> 1()
Traceback (most recent call last):
File "<pyshell#5>", line 1, in <module>
1()
TypeError: 'int' object is not callable
>>>

　　2. 可調用對象，在實際調用也可能調用失敗；但是不可調用對象，調用肯定不成功。

　　3. 類對象都是可被調用對象，類的實例對象是否可調用對象，取決於類是否定義了__call__方法。

>>> class A: #定義類A
    pass

>>> callable(A) #類A是可調用對象
True
>>> a = A() #調用類A
>>> callable(a) #實例a不可調用
False
>>> a() #調用實例a失敗
Traceback (most recent call last):
  File "<pyshell#31>", line 1, in <module>
    a()
TypeError: 'A' object is not callable


>>> class B: #定義類B
    def __call__(self):
        print('instances are callable now.')

        
>>> callable(B) #類B是可調用對象
True
>>> b = B() #調用類B
>>> callable(b) #實例b是可調用對象
True
>>> b() #調用實例b成功
instances are callable now.