• In Python, functions are objects as well! Therefore, we can assign functions to another function.
• By knowing this, we should prevent variable assignment to the reserved words in Python. (Python won't complain about this, but we will see bugs in our code.)

# functions are all object (用function assign 到另外一個function)

def Addition(a, b):
    return a + b

def Substraction(a, b):
    return a - b

print(Addition(10, 5))  # 執行上面Addtion function

Addition = Substraction  # Addition 已經被 subtraction assign in (變成減法)

print(Addition(10, 5)) 

-----------------------------------
15
5 

**# 把保留字改成 function 的問題 object is not callable
# string case 已經改成def function 就沒有再用原本的str功能**

**str** = "this is my string"

x = 25

print("Hello !" + str(x))

--------------------------
print("Hello !" + str(x))
TypeError: 'str' object is not callable

**string** = "this is my string"

x = 25

print("Hello !" + str(x))
-------------------------------
Hello !25