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??；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，取決于類是否定義了 __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.