• In computer programming, the scope of a name binding—an association of a value to a variable—is the part of a program where the name binding is valid; that is, the variable can be referred to the specific value.

• In short, the scope is the area within which the variable is active.

• How does python determine the scope of variables? It uses LEGB rules.

  • The LEGB rule is a kind of name lookup procedure that determines the order in which Python looks up names.
  • If the name exists, we'll get the first occurrence of it.

    • 找scope 順序的方法 (LEGB)

    • LEGB分別為Local、Enclosing、Global、Built-in，python會遵循此規則進行變數搜尋，

       只要變數被找到搜尋行動就會停止，如果沒有找到就會出現Error
      

• • Local: names are assigned in any way within a function and not declared global in that function. (先由內部local variable 先找)
  • Enclosing function locals: check from inner to outer if the variable name exists. (由內往外一層一層找)
  • Global: names assigned at the top level of a module file or declared globally in a def within the file. (找到最外層 Global variable)
  • Built-in: Python built-in names. (Python 內建variable 也掃過一遍)

LEGB RULE

# scope variable / function block of code 影響範圍

def greet():
    name = "Albert"

    def hello():
        print("Hello, my name is" + name)

    def hello2():
        print("greeting from hello2.")

    hello()
    hello2()  # hello/hello2 function 只能在greet function 中執行

greet()
hello() # hello function 在外層是無法執行會show NameError: name 'hello' is not defined

-------------------------------------------------------
Hello, my name is Albert
greeting from hello2.

NameError: name 'hello' is not defined # hello function 在外層是無法執行會show NameError: name 'hello' is not defined