英文文档：
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.
说明：
- 方法用来检测对象是否可被调用，可被调用指的是对象能否使用()括号的方法调用。
>>> 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
- 可调用对象，在实际调用也可能调用失败；但是不可调用对象，调用肯定不成功。
- 类对象都是可被调用对象，类的实例对象是否可调用对象，取决于类是否定义了 __call__ 方法。
>>> class A: #定义类A
pass
>>> callable(A) #类A是可调用对象
True
>>> a = A() #调用类A
>>> callable(a) #实例a不可调用
False
>>> a() #调用实例a失败
Traceback (most recent call last):
File "<pyshell#31>", line 1, in <module>
a()
TypeError: 'A' object is not callable
>>> class B: #定义类B
def __call__(self):
print('instances are callable now.')
>>> callable(B) #类B是可调用对象
True
>>> b = B() #调用类B
>>> callable(b) #实例b是可调用对象
True
>>> b() #调用实例b成功
instances are callable now.
