例項變數的預設值

如果變數包含一個不可變型別的值（例如一個字串），那麼可以像這樣分配一個預設值

class Rectangle(object):
    def __init__(self, width, height, color='blue'):
        self.width = width
        self.height = height
        self.color = color
    
    def area(self):
        return self.width  * self.height 

# Create some instances of the class
default_rectangle = Rectangle(2, 3)
print(default_rectangle.color) # blue

red_rectangle = Rectangle(2, 3, 'red')
print(red_rectangle.color) # red

在初始化建構函式中的可變物件（如列表）時需要注意。請考慮以下示例：

class Rectangle2D(object):
    def __init__(self, width, height, pos=[0,0], color='blue'):  
        self.width = width
        self.height = height
        self.pos = pos
        self.color = color

r1 = Rectangle2D(5,3)
r2 = Rectangle2D(7,8)
r1.pos[0] = 4
r1.pos # [4, 0]
r2.pos # [4, 0] r2's pos has changed as well

這種行為是由於 Python 預設引數在函式執行時繫結而不是函式宣告。要獲取不在例項之間共享的預設例項變數，應該使用如下構造：

class Rectangle2D(object):
    def __init__(self, width, height, pos=None, color='blue'):  
        self.width = width
        self.height = height
        self.pos = pos or [0, 0] # default value is [0, 0]
        self.color = color

r1 = Rectangle2D(5,3)
r2 = Rectangle2D(7,8)
r1.pos[0] = 4
r1.pos # [4, 0]
r2.pos # [0, 0] r2's pos hasn't changed 

另請參見 Mutable Default Arguments 和 Least Astonishment 以及 Mutable Default Argument 。