python tips - 注意函数参数的默认值

且看如下例子：

def f(a=[]):
    a.append(0)
    return a

print dir(f)
for i in range(3):
    print f()
    print f.__defaults__

结果：

['__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__', '__dict__', '__doc__', '__format__', '__get__', '__getattribute__', '__globals__', '__hash__', '__init__', '__module__', '__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'func_closure', 'func_code', 'func_defaults', 'func_dict', 'func_doc', 'func_globals', 'func_name']
([],)
[0]
([0],)
[0, 0]
([0, 0],)
[0, 0, 0]

函数参数的默认值会保存上次调用的修改么？

每运行一次，函数作为一个对象的内在属性的值发生了变化。导致每次运行的结果不一致

 

def f(num=0):
    num += 1
    return num

print dir(f)
for i in range(3):
    print f.__defaults__
    print f()

结果：

['__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__', '__dict__', '__doc__', '__format__', '__get__', '__getattribute__', '__globals__', '__hash__', '__init__', '__module__', '__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'func_closure', 'func_code', 'func_defaults', 'func_dict', 'func_doc', 'func_globals', 'func_name']
(0,)
1
(0,)
1
(0,)
1

函数参数的默认值怎么又没有保存上次调用的修改呢？

 

参考：python tips - Python可变对象和不可变对象：http://www.cnblogs.com/congbo/archive/2012/11/20/2777031.html

使用可变参数作为默认值可能导致意料之外的行为。为了防止出现这种情况，最好使用None值，并且在后面加上检查代码，如示例1改为：

def f(a=None):
    if a is None:
        a = []
    a.append(0)
    return a

 

 

查阅Python manual有如下的说法：

Default parameter values are evaluated when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that that same “pre-computed” value is used for each call. This is especially important to understand when a default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default value is in effect modified. This is generally not what was intended. A way around this is to use None as the default, and explicitly test for it in the body of the function, e.g.:

python中函数的默认值只会被执行一次，(和静态变量一样，静态变量初始化也是被执行一次）。Python可以通过函数的默认值来实现静态变量的功能。