To define a function in Python, we need to use the keyword "def". The basic syntax of defining a function is:

• def functionName(input1, input 2, …):

• function code here

  • Input 1 , Input2 = parameter (參數)
  • 類似數學中 function 的要代入未知數 x, y f(x) = 2n+1
  • 給定要代入的值 = argument

• Inputs are optional; if we don’t need inputs, then we just keep the empty parenthesis.

• Inputs are the local variables in the function; they are either copy by value or reference

  • local variable 只在function 中對function 有意義

• You can also use doctring (or documentation string) to give explanations or comments for a function.

#Parameter and argument

a = 30
b = 40

# x, y are parameters

def addition(x, y):
    print(x + y)

addition(15, 20)  # 15, 20 are arguments
addition(a, b)  # argument 也可以是variable

-------------------------------------------
35
70

# global variable / local variable

a = 20  # global variable 不同的function 皆可代入

def f1():
    x = 2  # local variable 只在function f1 內有效
    y = 3  # local variable 只在function f1 內有效
    print(x, y, a)  # a 會指向外層的global variable

f1()

def f2():
    m = 4 # local variable 只在function f2 內有效
    n = 5 # local variable 只在function f2 內有效
    print(m, n, a) # a is global variable 
    print(m, n, x) # **x is the f1 function local variable NOT callabl**e 

f2()

--------------------------------

2 3 20
4 5 20
Traceback (most recent call last):
  File "d:\\2022_python_\\u5168\\u653b\\u7565\\tempCodeRunnerFile.py", line 20, in <module>
    f2()
  File "d:\\2022_python_\\u5168\\u653b\\u7565\\tempCodeRunnerFile.py", line 17, in f2
    print(m, n, x)
NameError: name 'x' is not defined

#　change by value and by reference

a = 10

def change(num):  # input 是local variable in function

    num = 25  # num = a (copy by value a是int)

change(a)  # 執行def function
print(a)  # 預期 a 從 10 變成 25 但結果還是 10
------------------------
10

# --------------------
a = [1, 2, 3, 4]

def change(lst):
    lst[0] = 100  # copy by reference 會指向外層的 a

change(a)
print(a)
----------------------

[100, 2, 3, 4]

# can we change any copy by value global variables ?

a = 10

def change(num):  # input 是local variable in function
    global a  # global keyword 是專用字 直接宣告 a 會指向外層的a 

    num = 25  # num = a (copy by value)

change(a)  # 執行def function
print(a)  # 預期 a 從 10 變成 25 yes --> change to 25

---------
25