当类有多个超类，而且超类中有相同的方法时，继承顺序就很重要，如下：
class A
(def save(self):...) # A 中有self方法
/ \
/ \
class B class C
(def save(self):...) # B 和 C 中都继承 A，只有C中重写save方法
\ /
\ /
\ /
class D # 继承 B 和 C
旧式类
在旧式类中，会根据深度优先(depth-first)的顺序来继承，由左至右依次查找save()方法，D -> B -> A ->C,当查到方法时，立刻放回，不在继续查找。
# 例：
class A:
def __init__(self):
print 'it is A'
def save(self):
print 'save method from A'
class B(A):
def __init__(self):
print 'it is B'
class C(A):
def __init__(self):
print 'it is C'
def save(self):
print 'save method from C'
class D(B, C):
def __init__(self):
print 'it is D'
d = D()
d.save()
# 结果 ==》
it is D
save method from A # 根据上边的顺序，查到A中的save方法，即返回。
新式类
新式类中会根据方法解析顺序（MRO）来进行搜索，广度优先，当查到对应方法属性时，立即返回，不再继续查找。
__mro__属性，标记了python继承层次中父类的查找顺序，Python的多重继承机制就是按照__mro__的顺序进行查找，一旦找到对应属性，则查找马上返回。
# 例 1
class A(object):
def __init__(self):
print 'it is A'
def save(self):
print 'save method from A'
class B(A):
def __init__(self):
print 'it is B'
class C(A):
def __init__(self):
print 'it is C'
def save(self):
print 'save method from C'
class D(B, C):
def __init__(self):
print 'it is D'
print D.__mro__
d = D()
d.save()
# 结果==》
(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <type 'object'>)
it is D
save method from C
# 例 2
class A(object):
def save(self):
print ('save A')
class B(object):
def save(self):
print ('save B')
class C(object):
def save(self):
print ('save C')
class D(B,C,A):
pass
print D.__mro__
cla = D()
cla.save()
# 结果
(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <type 'object'>)
save B
# 例 3
class B(object):
def foo(self):
print ('foo B')
def bar(self):
print 'bar B'
class A(object):
def foo(self):
print ('foo A')
class C1(A, B):
pass
class C2(A, B):
def bar(self):
print 'bar C2'
class D(C1, C2):
pass
print D.__mro__
d = D()
d.foo()
d.bar()
# 结果
(<class '__main__.D'>, <class '__main__.C1'>, <class '__main__.C2'>, <class '__main__.A'>, <class '__main__.B'>, <type 'object'>)
foo A
bar C2
