Python Control Flows

Control flows in Python defines the order that the computer executes statements, instructions or function calls in a script.

They are used to write more complex and dynamic programs to answer different situations in different ways.

The control flow in Python is defined by the conditional statements, the loops and the function calls.

• if/elif/else statements
• for loops statements
• while loops statement
• break and continue statements
• pass statement
• try/except statements
• match statement

if/elif/else Statements

The if, elif and else keywords are used to apply conditions for the Python code execution.

(If condition, do something, else do something else.)

if condition:
    # Do something if condition is true
else:
    # Do something else if condition is false

If Statement

The if statement is the statement that starts the control flow and defines the condition to be tested on.

The if statement can be used by itself.

# If statement
if True:
    print('This is True')

This is True

Else Statement

The else statement is the optional control flow statement that defines the code to be executed when the prior condition(s) is not met.

# If/Else Statement
i = 1
if i > 10:
    print('i is greater than 10')
else:
    print('i is smaller than 10')

i is smaller than 10

Elif Statement

The elif statement is the optional control flow statement that defines additional conditions to be evaluated and the code to be executed in the conditional statement.

# Simple Elif Example
i = 12
if i < 2:
    print('i is less than 2')
elif i > 10:
    print('i is greater than 10')
else:
    print('i is a number between 2 and 10')

i is greater than 10

For Loops Statements

The for loops are control flow statements used to iterate over a sequence (e.g. list, tuple, string) and execute code for each item.

for item in sequence:
    # Do Something

Example of a for loop in Python.

# Simple for loop
for i in [1,2,3]:
    print(i)

1
2
3

While Loops Statements

The while loops are control flow statements are used to execute code as long as a given condition is True.

Similar to a if else block that would be repeated over and over.

while condition:
    # do something

Example of a while loop in Python.

# Simple while loop
while True:
    print('hello')
    break

hello

Above the break statement stops the loop to prevent an infinite loop caused by the while True statement.

break and continue Statements

The break and continue keywords define control flow statements to either stop a loop early or skip to the next iteration:

• The break statement is used to stop a loop early.
• The continue statement is used to skip to the next iteration of the loop.

# Break and continue example
letters = ['a', 'b', 'c', 'd', 'e', 'f']

for letter in letters:
    if letter == 'e':
        # if the letter is "e", stop the loop
        break
    elif letter == 'c':
        # if the letter is "c", skip to the next iteration
        continue
    print(letter)

a
b
d

pass statement

The pass control flow statement is a null operation that is used as a placeholder when the code should do nothing.

The pass keyword is often used to list the functions to be created in the code, but not yet ready to be worked on.

# Example pass
def a_func():
    pass

try/except Statements

The try/except control flow statements are used to handle errors (exceptions).

• The try block contains the code that may return an exception.
• The except block contains the code to execute if there is an exception.

It has the following structure:

try:
    # code that may break
except:
    # code to execute if it breaks

Handling exceptions with Try and Except

# Example Try and Except
try:
    1 / 0
except:
    print("You can't divide by zero") 

You can't divide by zero

match Statement

The match keyword define the control flow statement used perform pattern matching for code execution.

If a pattern match the expression, code is executed.

# Example match statement
def http_error(status):
    match status:
        case 400:
            return "Bad request"
        case 404:
            return "Not found"
        case 418:
            return "I'm a teapot"
        case _:
            return "This code was not matched"
        
print('404:', http_error(404))
print('1000:', http_error(1000))  

404: Not found
1000: This code was not matched

Conclusion

We have now learned about the various control flow statements in Python.

Jean-Christophe Chouinard

SEO Strategist at Tripadvisor, ex- Seek (Melbourne, Australia). Specialized in technical SEO. Writer in Python, Information Retrieval, SEO and machine learning. Guest author at SearchEngineJournal, SearchEngineLand and OnCrawl.