Introduction to Python Programming - Control Flow

At some point, while writing a program, you would need to make decisions either based on your business logic or based on user input from a client such as a shell or a webpage. Also, you may want some parts of your program to run repeatedly. This article shows you how you can control the flow of your program in Python.

Table of Contents

• Conditions
  • If-Elif-Else Statements
  • Nested If Statements
  • If-Else Expression
  • match statement
• Loops
  • For Loop
  • While Loop
  • Break and Continue Statements
• Pass statement
• Try-Except Statements

Conditions

If-Elif-Else Statements

Use an if or if-elif-else statement when you want to perform an operation or logic based on some conditions. The overall format is:

# Do not run this!
# It's just for illustration

if condition1:
    # Run if the condition is true, avoid the others if they specified
elif condition2:
    # Run here if condition1 is false and condition2 is true
else:
    # Do something else

Note that in Python, indentation matters. In the above statement, there are three blocks or indents - The if, elif, and the else blocks.
Among the three, only the if block is required. The elif and else parts are optional.

Let's say we run a restaurant that only allows people 18 years and above. We may set a variable called age_allowed to the number 18. Then we check if the customer's age matches our expectations. Let's set three variables associated with age. Also, add two string variables as messages to the customers.

# The allowed age
allowed_age = 18

# Age of the first customer
age_1 = 57

# Age of the second customer
age_2 = 17

# Some message to the customer depending on our criterium
not_ok_message = "You are not allowed to enter!"
ok_message = "Welcome to the DELISH haven!"

We do the tests:

# True: prints the ok_message
if age_1 >= allowed_age:
    print(ok_message)


# False: does not print the ok_message
if age_2 >= threshold_age:
    print(ok_message)

Print an "okay message" if the customer's age is higher or equals the allowed age. We used two if statements or blocks, and that's fine. However, what if the test fails? We could use another if block or add an else block. For example:

# True: prints the ok_message
if age_1 >= allowed_age:
    print(ok_message)
else:
    print(not_ok_message)

# False: prints the not_ok_message
if age_2 >= allowed_age:
    print(ok_message)
else:
    print(not_ok_message)

What if we have multiple conditions that are different? We test many conditions using elif blocks. Only the block whose condition passes executes. If available, an else block executes if all tests fail.

Let's say we give out separate seats to customers based on their age group. For customers between 18 and 27, we will give them the first-row seats, those between 28 and 37 will take the second-row seats, and so on. Let's assign a seat to a customer!

# Yeah, a_customer
a_customer = 45

# I like laziness. Let's save typing
message = "Please take the {} seat"

if a_customer < 18:
    print("Sorry, our DELISH only allows customers 18 and above!")

# checks between 18 and 27
elif age <= 27:
    print(message.format("first"))

# checks between 28 and 37
elif age <= 37:
    print(message.format("second"))

# checks between 38 and 47
elif age <= 47:
    print(message.format("third"))

# 81 and above
else:
    print("Well, this part is not always necessary!")

Wondering about the message variable in the above code block? Remember that Python has a string format function/method that lets you do something like this "My name is {}, and I am {} years old".format("Peter Griffin", 90).

One last example, just for fun!

# Reset age_1 and age_2

age_1 = 5
age_2 = 30

pro_footballer = True

if (age_2 > 18) and pro_footballer:
    # runs
    print("Seems appropriate, maybe! I guess you deserve more credit!")
else:
    print("Dude? why not!")

if (age_1 < 10) and pro_footballer:
    print("You are a prodigy! Have you seen the news?")
else:
    print("You probably read too much!")

Nested If Statements

An if statement (top-level) can contain other if statements (child-level). The child-level or contained if statements can contain other if statements, and so on. For example

# Do not run this!
# It's just for illustration
if condition:
    # top level if block
    if another_conditon:
        # second-level if block
        if yet_another_condition:
            # you get the gist
        else:
            # definitely!
    else:
        # second-level else block
else:
    # top/first-level else block

Heads up! Use nested ifs wisely. Nesting too deep is terrible for eggs! Sorry :)

If-Else Expression

An if-else expression has the pattern True if condition else False. This expression is "give me the True part if the condition is true or, give me the False part". For example, chosen_age is age_2 if we want a pro_footballer else it is age_1.

chosen_age = age_2 if pro_footballer else age_1

print("The chosen age is {} years old".format(chosen_age))
# The chosen age is 30 years old

Match Statement

You can use a match statement in place of an if-elif-else with multiple elif-blocks. The format is

match an_expression:
    case option_1:
        # do something if an_expression equals/matches option_1
    case option_2:
        # do something if an_expression equals/matches option_2
    ...
    case option_n:
        # do something if an_expression equals/matches option_n
    case _:
        # do something if an_expression does not match any of the provided options

Only the code in the first block that matches the expression executes. In the case of no match, the default case block case _ runs.

Let's say we have a list of fruits and would like to react to a customer's choice of the best fruit.

best_fruit = 'oranges'

match best_fruit:
    case 'apples':
        print('nice fruit..but not as nice as...whatever!')
    case 'oranges':
        print('even better...Lorem ipsum...Don\'t worry')
    case 'mangoes':
        print('You made a great choice!')
    case 'banana':
        print('We are quite far from the trees...Might you pick something else?')
    case _:
        print('Don\'t know what this is...Try again?')

The code above will print 'even better...Lorem ipsum...Don't worry' because best_fruit is oranges in the example.

If you want to react to multiple options, use the pipe (|) operator. For example

food = 'rice'

match food:
    case 'rice' | 'potatoes' | 'corn':
        print('We are rich in carbohydrates, and you can spend it!')
    case 'carrot' | 'cabbage' | 'lettuce':
        print('What a bunch of vegetables!')
    case _:
        print('Probably an Aliens\' diet!')

If a user selects any of rice or potatoes or corn then 'We are rich in carbohydrates, and you can spend it!' is printed. Conversely, the same code runs if the user chooses carrot, cabbage, or lettuce.

Loops

Using loops, we can repeat specific parts of our code. Or we can go through sequences or dictionaries one item at a time. : The for and the while loops are types of loops in Python.

For Loops

The for statement allows us to go through the elements of a collection (such as list, tuples, dictionary, string) one after the other (iteration). It follows the following pattern:

for element in collection:
    # Do something with the element

The element variable represents each item in the collection during iteration. Note that the element can take any name. Let's see some examples.

We start by creating four variables, my_list, a list of numbers. A list containing tuples list_of_tuples, menu a dictionary, and name a string.

my_list = [1, 2, 3, 4, 5]

list_of_tuples = [(1, 2), (3, 4), (9, 10), (11, 12)]

menu = { "rice": 45, "apples": 33, "garlic": 20, "pepper": 10 }

name = "John Doe"

Let's print two statements using the my_list elements.

for num in my_list:
    print("The number is {}".format(num))
    print(f"num times 2 is {num * 2}\n") # notice the \n

The number is 1
num times 2 is 2

The number is 2
num times 2 is 4

The number is 3
num times 2 is 6

The number is 4
num times 2 is 8

The number is 5
num times 2 is 10

\n is called a newline character. The pattern f"{num}" is equivalent to "{}".format(num).

Using tuple unpacking, we can retrieve the elements of the tuple in the list within the iteration.

for first, second in list_of_tuples:
    print(f"First is {first}, second is {second}")

First is 1, second is 2
First is 3, second is 4
First is 9, second is 10
First is 11, second is 12

First run: first, second = (1, 2). Second run: first, second = (3, 4). Third run: first, second = (9, 10). Fourth run: first, second = (11, 12).

We get a series of letters when we loop through a string. For example:

for letter in name:
    print(letter)

J
o
h
n

D
o
e

Looping Through a Dictionary

Iterating through a dictionary is not so straightforward. We can loop through the dictionary keys using the keys() method and through the values using the values() method, or both (items()).

# Loop through the keys only
for key in menu.keys():
    print(key)

rice
apples
garlic
pepper

Omitting the keys() method yields the same result.

# Same as above
for key in menu:
    print(key)

rice
apples
garlic
pepper

Use the values() method to retrieve only the values:

# Loop through the values only
for value in menu.values():
    print(value)

45
33
20
10

Use the items() method to retrieve the keys and values!

# loop through the keys and values
for key, value in menu.items():
    print("{}: {}".format(key, value))

rice: 45
apples: 33
garlic: 20
pepper: 10

While Loop

The template is:

while condition_is_true:
    # Keep doing whatever we put here

Let's say we want to print out some numbers 1 to 5. We could set a count variable and increment it within a loop.

count = 1

while count <= 5:
    print(count)
    count += 1  # increment the count

1
2
3
4
5

To avoid an infinite loop, the while condition must be evaluated as False at some point. We increment the count variable count += 1 to ensure that the count becomes greater than five eventually.

Operators like += -= \= *= allows us to perform arithmetics and assignment in a single step. For example,
a = a + 1 can be written as a += 1. So also a = a * 7 can be replaced with a *= 7 and so on.

Note that just like the if statement, loops can be nested.

Give me all even numbers between 1 and 20:

# We're starting from zero
count = 0

while count <= 20:
    if count % 2 == 0:
        print(count)
    count += 1  # increment the count

0
2
4
6
8
10
12
14
16
18
20

Break and Continue Statements

The break statement helps us control a loop's termination. The continue statement skips an iteration step.

In the example below, we want to print the multiples of 5, specifically from 5 to 20. We set the counter to 5 and then increment by 5. When the counter becomes 20, we exit the while loop using a break statement.

counter = 5

while True:
    print(f'counter is {counter}')
    if counter == 20:
        print("**** stopping here ****")
        break
    counter += 5

counter is 5
counter is 10
counter is 15
counter is 20
**** stopping here ****

To demonstrate the continue statement, create a numbers list like the one below.

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]

If we want the odd numbers between 1 and 11, we will write:

for n in numbers:
    if n % 2 == 0:
        continue # skip even numbers
    print(n)
    if n == 11:
        break  # stop at 11

1
3
5
7
9
11

In the code above, we skipped the even numbers with continue and then exited with break when the current number n was 11,

How do I choose between a for and a while loop? It largely depends on the situation. If you're dealing with a collection, a for loop is preferable most of the time. In other cases, consider a while loop.

Pass Statement

Like comments (#), the pass statement tells Python to ignore parts of a program. However, the pass statement is excellent for ignoring code blocks. This code block may be if-elif-else, a loop block, or a function or class block. For example:

# I'm still thinking of what to do
# So I'll just use a pass so Python won't complain
num_years = 45

if num_years > 80:
    pass
else:
    pass


# I'm not ready to work with num
for num in [1, 2, 3, 4, 5]:
    pass

Try-Except Statement

The try-except statement enables us to perform an alternative action if the intended action fails. For example, we may want to open a file, convert a string to an integer, and so on. These attempts might fail, resulting in an error that may crash our program. The try-except statement solves this problem. The format is:

try:
    # Do something that may fail
except:
    # Do something else. For example, handle the error and inform the user of your program

The example below demonstrates the many uses of try-except. Converting from integers may produce errors. To guarantee the continuation, use a try-except statement.

bad_integer_string = "aa12r"

good_integer_string = "12345"

try:
    good_integer = int(good_integer_string)
    print("Worked! The number is", good_integer)
except:
    print("Failed")

Worked! The number is 12345

try:
    bad_integer = int(bad_integer_string)
    print("Worked??? The number is", bad_integer)
except:
    print("Failed!!!")

Failed!!!

Conclusion

In this article, we looked at controlling program execution with if-elif-statements statements, match statements, for and while loops, pass statements, and so on. Next, we will write some programs. Thanks for reading!