学习日记

python 默认参数工作机制

Default values are computed once, then re-used.

默认参数只会计算一次，不会重复使用

class demo_list:
  def __init__(self, l=[]): 
    self.l = l 
  def add(self, ele): 
    self.l.append(ele)
def appender(ele): 
  obj = demo_list() 
  obj.add(ele) 
  print obj.l
if __name__ == "__main__": 
  for i in range(5): 
    appender(i)


[0] 
[0, 1]
[0, 1, 2]
[0, 1, 2, 3] 
[0, 1, 2, 3, 4]

函数也是对象，因此定义的时候就被执行，默认参数是函数的属性，它的值可能会随着函数被调用而改变。

Functions in Python are first-class objects, and not only a piece of code.

def a():
    print "a executed"
    return []

def b(x=a()):
    print "id(x): ", id(x)
    x.append(5)
    print "x: ", x

for i in range(2):
    print "-" * 15, "Call b()", "-" * 15
    b()
    print b.__defaults__
    print "id(b.__defaults__[0]): ", id(b.__defaults__[0])

for i in range(2):
    print "-" * 15, "Call b(list())", "-" * 15
    b(list())
    print b.__defaults__
    print "id(b.__defaults__[0]): ", id(b.__defaults__[0])


a executed
————— Call b() —————
id(x): 4316528512
x: [5]
([5],)
id(b.__defaults__[0]): 4316528512
————— Call b() —————
id(x): 4316528512
x: [5, 5]
([5, 5],)
id(b.__defaults__[0]): 4316528512
————— Call b(list()) —————
id(x): 4316684872
x: [5]
([5, 5],)
id(b.__defaults__[0]): 4316528512
————— Call b(list()) —————
id(x): 4316684944
x: [5]
([5, 5],)
id(b.__defaults__[0]): 4316528512