關於Python預設引數，假如預設引數是可變（mutable）物件是會有副作用的，一個函式引數的預設值，僅僅在該函式定義的時候，被賦值一次。如此，只有當函式第一次被定義的時候，才將引數的預設值初始化到它的預設值（如一個空的列表）

如：

>>> def function(data=[]):
	data.append(1)
	return data

>>> function()
[1]
>>> function()
[1, 1]
>>> function()
[1, 1, 1]

當def function(data=[])執行完成，預先計算的值，即data就會被每次函式呼叫所使用，尤其是當預設引數是可變的物件時，引數可以"類比為全域性變數，當然這個全域性變數的範圍是在函式範圍內的"

引用其他博文所說：“

“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

def whats_on_the_telly(penguin=None):
    if penguin is None:
        penguin = []
    penguin.append("property of the zoo")
    return penguin

def f(x, I=[]):  # 函式初始化定義，I僅僅被賦值一次
    print('程式執行前I的值為: {}'.format(I))
    for i in range(x):
        I.append(i*i)
    print('I id is : {} '.format(id(I)))