Critical difference between global variables and function parameters and local variables

scope.py

# Consider the following

# This global variable is defined outside the scope of all functions
global_variable = 1

# The parameter is the "input variable". Calling the function "sends in" a value, assigning it to parameter_variable
# The scope of the parameter variable is only inside of a function
def add_one(parameter_variable):
    return global_variable + 1
  
# This works because the global_variable + 1 is 2
print("The input argument is 1. One plus one is two. add_one(1) returns", add_one(1))

# This fails because the function is returning global_variable + 1.
print("The input argument is 2. Two plus one is three. add_one(2) returns,", add_one(2))

# This fails because the function is returning global_variable + 1 and argument is 100.
# 100 + 1 should be 101
print("The input argument is 100. One hundred plus one is a hundred and one. add_one(100) returns", add_one(100))

print()

# An appropriate add_one function depends on the input argument and is correct, predictable, and reusable.
def correct_add_one(parameter_variable):
    return parameter_variable + 1



print("The input argument is 1. One plus one is two. correct_add_one(1) returns", correct_add_one(1))

print("The input argument is 2. Two plus one is three. correct_add_one(2) returns,", correct_add_one(2))

print("The input argument is 100. One hundred plus one is a hundred and one. correct_add_one(100) returns", correct_add_one(100))