Scope

CS 65/STEM 280: Introduction to Computer Science I

Scope of a variable

A variable's scope is the part of the program that the variable is visible to.

In Python, a scope can be local to a function or global (visible everywhere).

Furthermore, scope may also be limited within a function - e.g., you can't use a variable before you've assigned something to it

Parameters are local to the function they're defined in.

import csv

def calculate_pay(wage,hours):
    if hours <= 40:
        pay = wage*hours
    else:
        overtime_hours = hours-40
        pay = (wage*40) + (wage*1.5*overtime_hours)

    return pay


def user_pay_calculator():
    hourly_wage = float(input("Enter your hourly wage: "))
    num_hours = float(input("Enter your number of hours worked: "))
    #calculate_pay(hourly_wage,num_hours)
    #print(pay) #won't work
    print("Your total pay is",calculate_pay(hourly_wage,num_hours))
    
user_pay_calculator()

Enter your hourly wage: 10
Enter your number of hours worked: 20
Your total pay is 200.0

wage, hours, overtime_hours, pay are all local to the calculate_pay() function

You can't use pay inside user_pay_calculator()

Similarly, hourly_wage couldn't be used in calculate_pay()

Global variables

Global variables are weird

num_students = 350 #this variable is global because it is defined outside of a function

def first_grade_classroom():
    print(num_students)
    
def second_grade_classroom():
    print(num_students)
    
first_grade_classroom()
second_grade_classroom()

350
350

num_students = 350 #this variable is global because it is defined outside of a function

def first_grade_classroom():
    num_students = 30  #this is actually now a local variable with the same name as the global variable!
    print(num_students)
    
def second_grade_classroom():
    print(num_students)
    
first_grade_classroom()
second_grade_classroom()

30
350

If you really want to change a global variable inside a function, you have to use the global keyword.

num_students = 350 #this variable is global because it is defined outside of a function

def first_grade_classroom():
    global num_students #here, we tell it to really use the global variable
    num_students = 30
    print(num_students)
    
def second_grade_classroom():
    print(num_students)
    
first_grade_classroom()
second_grade_classroom()

30
30

But, you really shouldn't do this. Changing global variables inside of functions makes programs extremely difficult to debug.

If you need the same variable in multiple functions, it is better to pass it as an argument or return it.

Constants

It's best to have all your code be in functions, except

• Constant values you might need that never change (by convention, use all caps for names)
• Maybe a single function call to launch the program.

import csv

OVERTIME_THRESHOLD = 40

def calculate_pay(wage,hours):
    if hours <= OVERTIME_THRESHOLD:
        pay = wage*hours
    else:
        overtime_hours = hours-OVERTIME_THRESHOLD
        pay = (wage*OVERTIME_THRESHOLD) + (wage*1.5*overtime_hours)

    return pay


def user_pay_calculator():
    hourly_wage = float(input("Enter your hourly wage: "))
    num_hours = float(input("Enter your number of hours worked: "))
    print("Your total pay is",calculate_pay(hourly_wage,num_hours))
    
user_pay_calculator()