Scopes

PEP 3104 - Access to Names in Outer Scopes. The specification for the nonlocal statement.

https://realpython.com/python-namespaces-scope/
Namespaces and Scope in Python

INTRODUCTION

A 'scope' defines the visibility of a name/variable within a block.

A 'namespace' is a collection of currently defined symbolic names along with information about the object that each name references. You can think of a namespace as a dictionary in which the keys are the object names and the values are the objects themselves.

There are four types of namespaces in a Python program (LEGB rule):
(1) Local
(2) Enclosing
(3) Global
(4) Builtin

def my_len(sequence):         # sequence is local
    """Return the length of a sequence."""
    length = 0                # length is local
    for item in sequence:     # item is local
        length += 1
    return length

X = 99   # global name

def func():   # local namespace
    #print(X)       # UnboundLocalError, X will be local
    X = 88          # local name
    print(X)        # 88, reference to the local X

func()              # 88
print(X)            # 99, X is global here

X = 99   # global name

def func():
    global Z        # global statement (declaration)
    print(X)        # reading global name
    Y = 88          # local name
    Z = 77          # Z is created

#print(Z)           # NameError, Z is not defined
X = 55              # global X changed
func()              # 55, here the function body is executed
#print(Y)           # NameError, Y is not defined
print(Z)            # 77, Z is global

X = 99              # global name

def func():
    global X        # global statement
    print(X)        # OK
    X = 88          # global X changed

func()              # 99
print(X)            # 88

# PEP 227 - Statically Nested Scopes

x = 1
def outer():
    #from math import *   # SyntaxError: import * is not allowed ...
    import math    # possible
    x = 2
    def inner():   # nested scope
        print(x)
    inner()

outer()   # print 2 from nested scope, not 1 from global scope
#print(math.sin(1))   # NameError: name 'math' is not defined

def make_score(score=0):
    for i in [1, 2, 3, 4]:
        def increment(step=i):
            #global score
#NameError: name 'score' is not defined

            nonlocal score   # if not, there is an exception (Py3)
#UnboundLocalError: local variable 'score' referenced before assignment
            # 'score' is not local and it is not global

            score = score + step
            print(score)
        increment()

make_score()   # 1 3 6 10