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

#!/usr/bin/python

n = 10; #global variables

def f():
    m = 100;
    print dir(); # ['m']
    print globals(); # ['f', '__builtins__', '__file__', 'n', '__name__', '__main__', '__package__', '__doc__']
    print locals(); {'m': 100}

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

#!/usr/bin/python

n = 10 # global variable
print id(n), n
 
# access local variable
def f1():
    n = 1;
    print id(n), n;

# access variable in enclosing function
def f2():
    n = 2;
    def f3():
        print id(n), n;
    f3();

# access global variable
def f4():
    def f5():
        print id(n), n;
    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 = 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
	print id(t), t #140249594380832, 100
	print 'globals: ', globals() # globals: {'t': 100}
	print 'locals: ', locals() # locals: {}

f8()
print id(t), t # 140249594380832, 100
			

#!/usr/bin/pyhton

def f():
    import math; # import math in local namespace
    print math.pi;

f();
print math.pi; # not import math error
			

# m1.py
a = 10
			

# m2.py
import m1

print m1.a
			

Beginner's Guide

Namespace at Python Course