B14 - variable scopes and lookup

variable scope rules

b = 6

def f1(a):
    print(a)
    print(b)
    b = 9
    
f1(1)

• when the above code is run, print(a) is run, but an error is thrown for print(b)
• a variable assigned in the body of a function is local by feature
• global b needs to be added in the function to treat b as global

variable lookup logic

• if there is a global x declaration, x comes from and is assigned to the global variable

x = 10

def func():
	print(x)
	
func() ## 10

• if there is a nonlocal x declaration, x comes from and is assigned to the x local variable of the nearest surrounding function where x is defined

x = 10

def func():
	x = 5
	print(x)
  
func()    ## 5
print(x) ## 10

• if x is a parameter or is assigned a value in the function body, it is a local variable

x = 10

def func(x):
	print(x)
  
func(15)    ## 15

• if x is referenced, but not assigned or a parameter:
  • it will be looked up in the local scops of the surrounding function bodies
  • if not found in surrounding scopes, it will be read from the module global scope
  • if not found in the gloval scope, it will be read from `builtins.dict

def outer():
    a = 10 

    def inner():
        print(a)  ## a is not assigned in inner, so python looks up in outer
    inner()

outer() ## 10