python 默认参数问题的陷阱

def func(mylist = []):
  mylist.append(1)
  return mylist

print func()
print func()
print func()
print func(['a'])
print func()

[1]
[1, 1]
[1, 1, 1]
['a', 1]
[1, 1, 1, 1]

print func.func_default

In [30]: def func2(var = 1):
  ....:   var += 1
  ....:   return var
  ....: 

In [31]: func2()
Out[31]: 2

In [32]: func2()
Out[32]: 2

In [34]: func2.func_defaults
Out[34]: (1,)

In [35]: def func(mylist = []):
  ....:   mylist = []  #这里 创建了新的对象，
       mylist.append(1)
       return mylist

In [44]: func()
Out[44]: [1]

In [45]: func.func_defaults
Out[45]: ([],)  

def outer():
  res = []
  for i in range(4):
    def inner(j):
      return j * i
    res.append(inner)
  return res

print [m(2) for m in outer()]

def multipliers():
  return [lambda x : i * x for i in range(4)]
print [m(2) for m in multipliers()]

def outer():
  res = []
  for i in range(4):
    def inner(j, i = i):
      return j * i
    res.append(inner)
  return res

print [m(2) for m in outer()]

def multipliers():
  return [lambda x, i = i : i * x for i in range(4)]
print [m(2) for m in multipliers()]

def multipliers():
  return (lambda x, i = i : i * x for i in range(4)) #修改成生成器
print [m(2) for m in multipliers()]