python - 不同类实例的成员通过方法定义中的参数默认值引用同一对象

Question

Python 文档说明了关键字参数（词汇表）：

...变量名称指定函数中的局部名称，该值被分配给...

因此，除非明确说明，否则我认为一个类的不同实例是真正不同的。然而，下面的代码显然是通过给定的默认值将类的两个实例的一个成员变量引用到 /same/ 对象（好像方法的关键字字典是类变量（“静态”））。一旦修改了成员，一切都很好，但如果由于某种原因没有发生：

1. 我想知道代码的行为是否有充分的理由，因为这根本不是我所期望的（请参阅代码和输出中的标记）和

2. 问题是否有好的解决方案（除了使用“复制”）。谢谢。

   class C:
       def setParams(self):
           self.e = []
       def setParamsKw(self, kw=['default list']): # <----- kw arg.
           self.eKw = kw                           # <--- that member should
                                                   #      default to the
                                                   #      default argument
                                                   #      (use 'kw.copy()'?)
   
   def outputMember():
       print " c1.e=", c1.e
       print " c2.e=", c2.e
       print " type(c1.e)=", type(c1.e)
       print " type(c2.e)=", type(c2.e)
       print " c1.e == c2.e:", c1.e == c2.e
       print " c1.e is c2.e:", c1.e is c2.e
   
   def outputMemberKw():
       print " c1.eKw=", c1.eKw
       print " c2.eKw=", c2.eKw
       print " type(c1.eKw)=", type(c1.eKw)
       print " type(c2.eKw)=", type(c2.eKw)
       print " c1.eKw == c2.eKw:", c1.eKw == c2.eKw
       print " c1.eKw is c2.eKw:", c1.eKw is c2.eKw # <----- this result
                                                    #      is unexpected
   
   
   c1 = C()
   c2 = C()
   
   print " c1 == c2:",  c1 == c2
   print " c1 is c2:",  c1 is c2
   
   print "Calling setParams for both instances:"
   c1.setParams()
   c2.setParams()
   outputMember()
   
   print "Calling setParamsKw for both instances:"
   c1.setParamsKw()
   c2.setParamsKw()
   outputMemberKw()
   
   print "Now manually modifying members of c1:"
   c1.e = [1, 2, 3, 4, 5]
   c1.eKw = [1, 2, 3, 4, 5]
   print "e:"
   outputMember()
   print "eKw:"
   outputMemberKw()
   

输出：

c1 == c2: False
 c1 is c2: False
Calling setParams for both instances:
 c1.e= []
 c2.e= []
 type(c1.e)= <type 'list'>
 type(c2.e)= <type 'list'>
 c1.e == c2.e: True
 c1.e is c2.e: False
Calling setParamsKw for both instances:
 c1.eKw= ['default list']
 c2.eKw= ['default list']
 type(c1.eKw)= <type 'list'>
 type(c2.eKw)= <type 'list'>
 c1.eKw == c2.eKw: True
 c1.eKw is c2.eKw: True
Now manually modifying members of c1:
e:
 c1.e= [1, 2, 3, 4, 5]
 c2.e= []
 type(c1.e)= <type 'list'>
 type(c2.e)= <type 'list'>
 c1.e == c2.e: False
 c1.e is c2.e: False
eKw:
 c1.eKw= [1, 2, 3, 4, 5]
 c2.eKw= ['default list']
 type(c1.eKw)= <type 'list'>
 type(c2.eKw)= <type 'list'>
 c1.eKw == c2.eKw: False
 c1.eKw is c2.eKw: False

Answer 1

def setParamsKw(self, kw=None): # <----- kw arg.
            self.eKw = kw  if kw == None else ['default list']

应该管用