python类继承与子类实例初始化用法分析

本文实例讲述了python类继承与子类实例初始化用法。分享给大家供大家参考。具体分析如下：

[ 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

希望本文所述对大家的Python程序设计有所帮助。

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