1.函式的預設引數是可變資料型別時，要尤其注意：如果函式執行過程中預設引數被修改，則永久被修改（下次呼叫時還會使用上一次被改過的預設引數）
Default parameter values are evaluated from left to right when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the 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.
1 #示例 2 def whats_on_the_telly(penguin=[]): 3 l=[1,2,3,4] 4 penguin.append("property of the zoo") 5 l.append(666) 6 return penguin, l 7 a,c=whats_on_the_telly() 8 print(a,c) 9 b,d=whats_on_the_telly() 10 print(b,d) 11 #result 12 [`property of the zoo`] [1, 2, 3, 4, 666] 13 [`property of the zoo`, `property of the zoo`] [1, 2, 3, 4, 666] 14 #解決方案 15 def whats_on_the_telly(penguin=None):#修改為None 16 if penguin is None: #增加 17 penguin = [] #增加 18 l=[1,2,3,4] 19 penguin.append("property of the zoo") 20 l.append(666) 21 return penguin, l 22 a,c=whats_on_the_telly() 23 print(a,c) 24 b,d=whats_on_the_telly() 25 print(b,d) 26 #result 27 [`property of the zoo`] [1, 2, 3, 4, 666] 28 [`property of the zoo`] [1, 2, 3, 4, 666]