Question

Python文档说明关键字参数（词汇表）：

...变量名称指定赋值的函数中的本地名称...

因此，除非另有说明，否则我认为一个班级的不同实例是真正不同的明确。然而，以下代码引用了两个实例的一个成员变量类似于/ same / object，显然是通过给定的默认值（就好像是该方法的关键字字典是一个类变量（“static”））。一旦修改了会员一切都很好，但如果由于某种原因不会发生：

我想知道代码的行为是否有充分的理由，因为它不是我所期望的（见代码和输出中的标记）和

是否有解决问题的好方法（除了使用'复制'）。感谢。

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']

应该有效

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

1 个答案: