From: https://blog.csdn.net/cs0301lm/article/details/6002504?utm_source=blogxgwz4
[ 先贴参考书籍原文（中文英文对照）]
__init__方法介绍:
If a base class has an __init__() method the derived class's __init__() method must explicitly call it to ensure proper initialization of the base class part of the instance; for example: "BaseClass.__init__(self, [args...])"
As a special contraint on constructors, no value may be returned; doing so will cause a TypeError to be raised at runtime.
如果其基类也具有__init__(), 必须显式地在__init__()调用它, 以保证能够适当地初始化它的基类部分;例如: "BaseClass.__init__(self, [args...])"作为构造器的特殊情况, 它没有值被返回, 如果返回某个值, 会在运行时抛出异常TypeError.
1.子类定义了__init__方法时若未显示调用基类__init__方法，python不会帮你调用,解释运行都Ok
class A():
def __init__(self):
print 'a'
class B(A):
def __init__(self):
print 'b'
if __name__=='__main__':
b=B()
>>>
b
2.子类未定义__init__方法时,python会自动帮你调用首个基类的__init__方法，注意是首个。即：子类继承自多个基类时，只有第一个基类的__init__方法会被调用到:
class A:
def __init__(self):
print 'a'
class B:
def __init__(self):
print 'b'
class C(B):
def __init__(self):
print 'c'
pass
class D1(A,B,C):
pass
class D2(B,A,C):
pass
class D3(C,B,A):
pass
if(__name__=='__main__'):
print 'd1------->:'
d1=D1()
print 'd2------->:'
d2=D2()
print 'd3------->:'
d3=D3()
>>>
d1------->:
a
d2------->:
b
d3------->:
c
3）基类未定义__init__方法时，若此时子类显示调用基类__init__方法时，python向上超找基类的基类的__init__方法并调用,实质同2
class A:
def __init__(self):
print 'a'
class B:
def __init__(self):
print 'b'
class C1(B,A):
pass
class C2(A,B):
pass
class D1(C1):
def __init__(self):
C1.__init__(self)
class D2(C2):
def __init__(self):
C2.__init__(self)
if(__name__=='__main__'):
print 'd1------->:'
d1=D1()
print 'd2------->:'
d2=D2()
>>>
d1------->:
b
d2------->:
a
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