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Introduction to Python Programming

6.3 Variable scope

Introduction to Python Programming6.3 Variable scope

Learning objectives

By the end of this section you should be able to

  • Identify the scope of a program's variables.
  • Discuss the impact of a variable's scope.

Global scope

A variable's scope is the part of a program where the variable can be accessed. A variable created outside of a function has global scope and can be accessed anywhere in the program. A Python program begins in global scope, and the global scope lasts for the entire program execution.

Checkpoint

Global variables in a program with a function

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Concepts in Practice

Global variables

1.
Which variables are global? 
num = float(input())
num_sq = num * num
print(num, "squared is", num_sq)
  1. num only
  2. num_sq only
  3. num and num_sq
2.
Which variables have global scope? 
def print_square():
  num_sq = num * num
  print(num, "squared is", num_sq)

num = float(input())
print_square()
  1. num only
  2. num_sq only
  3. num and num_sq
3.
Which functions can access num? 
def print_double():
  num_d = num * 2
  print(num, "doubled is", num_d)

def print_square():
  num_sq = num * num
  print(num, "squared is", num_sq)

num = float(input())
print_double()
print_square()
  1. print_double()
  2. print_square()
  3. print_double() and print_square()

Local scope

A variable created within a function has local scope and only exists within the function. A local variable cannot be accessed outside of the function in which the variable was created. After a function finishes executing, the function's local variables no longer exist.

Checkpoint

Global and local variables in a program with a function

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Concepts in Practice

Local variables

4.
Which variables are local? 
def print_time():
  out_str = "Time is " + str(hour) + ":" + str(min)
  print(out_str)

hour = int(input())
min = int(input())
print_time()
  1. hour and min
  2. out_str
  3. hour, min, and out_str
5.
Which variables are local? 
def print_greeting():
  print(out_str)

hour = int(input())
min = int(input())
if hour < 12:
  out_str = "Good morning"
else:
  out_str = "Good day"
print_greeting()
  1. hour and min
  2. out_str
  3. none
6.
Which functions directly access out_str? 
def print_greeting():
  print("Good day,")
  print_time()

def print_time():
  out_str = "Time is " + str(hour) + ":" + str(min)
  print(out_str)

hour = int(input())
min = int(input())
print_greeting()
  1. print_greeting()
  2. print_time()
  3. print_greeting() and print_time()

Using local and global variables together

Python allows global and local variables to have the same name, which can lead to unexpected program behavior. A function treats a variable edited within the function as a local variable unless told otherwise. To edit a global variable inside a function, the variable must be declared with the global keyword.

Checkpoint

Editing global variables in a program with a function

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Concepts in Practice

Using both local and global variables

Consider the following variations on the example program with the input 9.

7.
What is the output? 
def update_hour():
  tmp = hour
  if is_dst:
    tmp += 1
  else:
    tmp -= 1

is_dst = True
hour = int(input("Enter hour: "))
update_hour()
print("New hour:", hour)
  1. New hour: 9
  2. New hour: 10
  3. Error
8.
What is the output?
def update_hour():
  new_hour = hour
  if is_dst:
    new_hour += 1
  else:
    new_hour -= 1

is_dst = True
hour = int(input("Enter hour: "))
update_hour()
print("New hour:", new_hour)
  1. New hour: 9
  2. New hour: 10
  3. Error
9.
What is the output? 
Enter hour: "))
update_hour()
print("New hour:", new_hour)
def update_hour():
  global new_hour
  new_hour = hour
  if is_dst:
    new_hour += 1
  else:
    new_hour -= 1

is_dst = True
hour = int(input("Enter hour: "))
update_hour()
print("New hour:", new_hour)
  1. New hour: 9
  2. New hour: 10
  3. Error

Benefits of limiting scope

A programmer might ask, "Why not just make all variables global variables to avoid access errors?" Making every variable global can make a program messy. Ex: A programmer debugging a large program discovers a variable has the wrong value. If the whole program can modify the variable, then the bug could be anywhere in the large program. Limiting a variable's scope to only what's necessary and restricting global variable use make a program easier to debug, maintain, and update.

Try It

Battle royale game launch

Write a program that reads in a selected game mode and calls one of two functions to launch the game. If the input is "br", call battle_royale(). Otherwise, call practice().

battle_royale():

  • Reads in the number of players.
  • Computes the number of teammates still needed. A full team is 3 players.
  • Calls the function find_teammates() with the calculated number.
  • Prints "Match starting . . .".

practice():

  • Reads in a string representing the desired map.
  • Prints "Launching practice on [desired map]".

Note: find_teammates() is provided and does not need to be edited.

Given input:

    br
    1

The output is:

    Finding 2 players...
    Match starting...

Given input:

    p
    Queen's Canyon

The output is:

    Launching practice on Queen's Canyon
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