通过方法定义中的参数默认值引用同一对象的不同类实例的成员

Question

Python文档介绍了关键字参数（词汇表）：

...The variable name designates the local name in the function to which the value is assigned...

因此，我认为一个类的不同实例是完全不同的，除非另有说明 明确地。但是下面的代码引用了两个实例的一个成员变量 通过给定的默认值（好像 方法的关键字字典是一个类变量（“static”））。一旦有人修改了 会员：一切都很好，但如果因为某种原因没有发生：

1. 我想知道是否有一个很好的理由让代码像它那样运行， 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. 是否有解决问题的好方法（除了使用“copy”之外）。 谢谢。你知道吗

   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']

应该有用