Python

Python search variable name by a hierarchy levels

Local can be inside a function or class method

Enclosed can be its enclosing function

Global refers to the uppermost level of the executing script itself

Built-in are special names that Python reserves for itself

One sentence rule: the reference of a variable is visible to all its lower levels, thus mutable variables are visible and able to be modified in the lower levels, immutable variables are visible in the lower levels, but not able to be modified since the modification will create a local variable instead of modify the variable in the upper levels

dir() will give you the list of in scope variables

globals() will give you a dictionary of global variables

locals() will give you a dictionary of local variables

n = 10; #global variables

def f():
    m = 100;
    print(dir()) # ['m']
    print(globals()) # {'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <_frozen_importlib_external.SourceFileLoader object at 0x108df2860>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, '__file__': 's1.py', '__cached__': None, 'n': 10, 'f': <function f at 0x108cf52f0>}
    print(locals()) # {'m': 100}

print(dir()) # ['__annotations__', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'f', 'n']
f();

n = 10 # global variable
print(id(n), n) # 4405450128 10
 
# access local variable
def f1():
    n = 1;
    print(id(n), n) # 4450449840 1, reference local variable n
 
# access variable in enclosing function
def f2():
    n = 2;
    def f3():
        print(id(n), n) # 4405449872 2, reference n defined in f2
    f3();
 
# access global variable
def f4():
    def f5():
        print(id(n), n) # 4405450128 10, reference global variable
    f5();
 
f1();
f2();
f4();

a = 10 # global variable
print(id(a), a) # 27193472, 10
 
def f1():
	# global variable is visible to the lower level
	# globals: {'a': 10, ...}
	print(id(a), a) # 27193472, 10
 
def f2():
	# create a local variable, which screens the global variable
	# globals: {'a': 10, ...}
	a = 100
	# locals: {'a': 100}
	print(id(a), a) # 27195296 100
 
f1()
f2()
print(id(a), a) # 27193472, 10

l = list(range(10)) # global variable
print(id(l), l) # 140678046472600, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
 
def f3():
	# the reference of the gocal variable is visible to the lower level
	print(id(l), l) # 140678046472600, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
 
def f4():
	# modify an element of the global variable
	l[1] = 100
	print(id(l), l) # 140678046472600, [0, 100, 2, 3, 4, 5, 6, 7, 8, 9]
 
f3()
f4()
print(id(l), l) # 140678046472600, [0, 100, 2, 3, 4, 5, 6, 7, 8, 9]
 
def f5():
	# create a local variable, which screens the global variable
	l = [1, 2, 3, 4]
	print(id(l), l) # 139718240373432, [1, 2, 3, 4]
 
f5()
print(id(l), l) # 140678046472600, [0, 100, 2, 3, 4, 5, 6, 7, 8, 9]

t = 10
print(id(t), t) # 140417416870688, 10
 
def f8():
	global t # t is a global variable
	print(id(t), t) #140417416870688, 10
	t = 100 # modify the global variable t, create a new address, reference global variable to new address
	print(id(t), t) #140249594380832, 100
	print('globals: ', globals()) # globals: {'t': 100}
	print('locals: ', locals()) # locals: {}
 
f8()
print(id(t), t) # 140249594380832, 100

def f():
    import math; # import math in local namespace
    print(math.pi)
 
f();
#print(math.pi) # not import math error

global_list is imported once

global_list is shared by both module a and module b

# g.py
global_list = [1, 2]
print('g', id(global_list), global_list)

# a.py
print('a ...')

#import g
from g import global_list

def change():
    global_list.append(100)

change()
print('a', id(global_list), global_list)

# b.py
print('b ...')

#import g
from g import global_list
from a import *

def display():
    print('display', id(global_list), global_list)

print('b', id(global_list), global_list)

change()
display()

# python b.py
b ... # start module b
g 140597500568384 [1, 2] # import module g
a ... # import module a
a 140597500568384 [1, 2, 100] # global_list in a
b 140597500568384 [1, 2, 100, 100] # global_list in b

global_list is imported once, no need to import in b again

# g.py
global_list = [1, 2]

# a.py
print('a ...')

from g import global_list

print('a', id(global_list), global_list)

# b.py
print('b ...')

from a import *

print('b', id(global_list), global_list) # global_list is available in b.py