Python Flow Control

Learn Programming for Data Science
Author

Juma Shafara

Published

November 1, 2023

Modified

November 7, 2024

Keywords

function, conditional statements, if, if else, if elif, lambda functions, loops, for loop, while loop

Photo by DATAIDEA

Python control flow tools change the flow of how code is executed by the Python interpreter.

Since the Python interpreter executes code in a line-by-line manner, Python control flow tools help dictate what line(s) of code should run in a Python program. There are different types of control flow tools available to us in Python and we will go through them in detail in this lesson.

Functions

A function in python is a group statements that perform a particular task

The function below calculates and returns one’s body mass index rounded to 2 decimal places

# This function calculates Body Mass Index
def calculateBodyMassIndex(weight_kg, height_m):

    body_mass_index = weight_kg / pow(height_m, 2)
    rounded_bmi = round(body_mass_index, 2)

    return rounded_bmi
# lets try
calculateBodyMassIndex(67, 1.6)
26.17

Creating a function

To create a function, we need the following:
  • The def keyword
  • A function name
  • Round brackets () and a colon :
  • A function body- a group of statements 
def greeter():
    message = 'Hello'
    print(message)

Note!

A group of statements must have the same indentation level, in the example above, 4 whitespaces were used to indent the function body

Calling a Function

  • To execute a function, it needs to be called
  • To call a function, use its function name with parentheses () 
greeter()
Hello

Function Parameters/Arguments

When calling a function, we can pass data using parameters/arguments

A parameter is a variable declared in the function. In the example below, number1 and number2 are parameter

The argument is the value passed to the function when its called. In the example below 3 and 27 are the arguments

# define the function
def addNumbers(number1, number2):
    sum = number1 + number2
    print(sum)

# Call the function
addNumbers(3, 27)
30
# setting a default argument
def greet(name='you'):
    message = 'Hello ' + name
    print(message)

greet('Tinye')
greet()
Hello Tinye
Hello you

Default Arguments:

A function can have default arguments.

It can be done using the assignment operator (=).

If you don’t pass the argument, the default argument will be used instead.

def hello(name = 'Agaba'):
    print('Hello ' + name)

hello('John') # calling with John
hello() # calling with no name
Hello John
Hello Agaba

Return Statement

The return statement is used to return a value to a function caller

def addNumbers(number1, number2):
    sum = number1 + number2
    return sum

summation = addNumbers(56, 4)
print(summation)
60

Note!

The return statement stops the execution of a function.

### lambda functions
  • Lambda functions (also called anonymous functions) are functions that donot have names
  • The body of a lambda function can only have one expression, but can have multiple arguments
  • The result of the expression is automatically returned

Syntax: python lambda parameters: expression

# Example of lambda function
calculateBMI = lambda weight_kg, height_m: round((weight_kg/(height_m ** 2)), 2)

# Calling a labda function
calculateBMI(67, 1.7)
23.18

Note!

In the example above, the body mass index is automatically return, even without using the return statement

Practice functions

Calculate CGPA

# Assume 4 course units
# 1. Math - A
# 2. Science - B
# 3. SST - B
# 4. English - C


def calculate_CGPA(GPs_list, CUs_list):
    length = len(GPs_list)
    product_sum = 0

    for item in range(length):
        product_sum += GPs_list[item] * CUs_list[item]

    CUs_sum = sum(CUs_list)

    CGPA = product_sum / CUs_sum

    return CGPA

# calculate_CGPA(4, 5)

Get someones age given birth month and year

def getAge(month, year):
    month_diff = 12 - month
    year_diff = 2023 - year

    return str(year_diff) + ' years ' + str(month_diff) + ' months'  
    
age = getAge(year=2000, month=10) # keyword argument
age2 = getAge(10, 2000) # positional argument

print(age)
23 years 2 months

Loops

  • Loops are used to repetitively execute a group of statements
  • we have 2 types, for and while loop

For Loop

A for loop is used to loop through or iterate over a sequence or iterable objects

Syntax:

for variable in sequence:
    statements

Looping through a list

The for loop is commonly used with lists.

pets = ['cat', 'dog', 'rabbit']
# iterate through pets
for pet in pets:
    print(pet)
cat
dog
rabbit

Here’s another example to convert many weights from kilograms(kgs) to pounds(pds)

# convert all weights in list from kg to pounds
weights_kg = [145, 100, 76, 80]
weights_pds = []

for weight in weights_kg:
    pounds = weight * 2.2
    rounded_pds = round(pounds, 2)
    weights_pds.append(rounded_pds)

print(weights_pds)

This example displays all letters in my name.

# Display all letters in a name
name = 'Shafara'

for letter in name:
    print(letter)
S
h
a
f
a
r
a

While loop

The while loop executes a given group of statements as long as the given expression is True

Syntax:

while expression:
    statements

In the example below, Hello you will be printed 5 times, that is for each time counter is still less than 5.

counter = 0

while counter < 5:
    print('Hello you')
    counter += 1
Hello you
Hello you
Hello you
Hello you
Hello you
# Convert the weights in the list from kgs to pounds
weights_kg = [145, 100, 76, 80]
weights_pds = []

counter = 0
end = len(weights_kg)

while counter < end:

    pound = weights_kg[counter] * 2.2
    rounded_pds = round(pound, 3)
    weights_pds.append(rounded_pds)

    counter += 1

print(weights_pds)
[319.0, 220.0, 167.2, 176.0]

Conditional Statements

Conditional statements in Python are fundamental building blocks for controlling the flow of a program based on certain conditions. They enable the execution of specific blocks of code when certain conditions are met. The primary conditional statements in Python include if, elif, and else.

Basic Syntax

If Statement

The if statement is used to test a condition. If the condition evaluates to True, the block of code inside the if statement is executed.

if condition:
    # block of code

Example:

x = 10
if x > 5:
    print("x is greater than 5")

Else Statement

The else statement is used to execute a block of code if the condition in the if statement evaluates to False.

if condition:
    # block of code if condition is True
else:
    # block of code if condition is False

Example:

x = 3
if x > 5:
    print("x is greater than 5")
else:
    print("x is not greater than 5")

Elif Statement

The elif (short for else if) statement allows you to check multiple conditions. If the first condition is False, it checks the next elif condition, and so on. If all conditions are False, the else block is executed.

if condition1:
    # block of code if condition1 is True
elif condition2:
    # block of code if condition2 is True
else:
    # block of code if none of the above conditions are True

Example:

x = 7
if x > 10:
    print("x is greater than 10")
elif x > 5:
    print("x is greater than 5 but less than or equal to 10")
else:
    print("x is 5 or less")

Nested Conditional Statements

Conditional statements can be nested within each other to handle more complex decision-making processes.

Example:

x = 15
if x > 10:
    if x > 20:
        print("x is greater than 20")
    else:
        print("x is greater than 10 but not greater than 20")
else:
    print("x is 10 or less")

Conditional Expressions (Ternary Operator)

Python also supports conditional expressions, which allow for a more concise way to write simple if-else statements.

variable = value_if_true if condition else value_if_false

Example:

x = 10
result = "greater than 5" if x > 5 else "5 or less"
print(result)  # Output: greater than 5

Combining Conditions

Multiple conditions can be combined using logical operators (and, or, not).

Example:

x = 8
if x > 5 and x < 10:
    print("x is between 5 and 10")

Practical Usage

Conditional statements are used in a wide variety of scenarios, such as:
  • Validating user input.
  • Controlling the flow of loops.
  • Implementing different behaviors in functions or methods.
  • Handling exceptions or special cases in data processing.

Understanding and effectively using conditional statements are crucial for writing efficient and readable code in Python. They enable developers to build programs that can make decisions and respond dynamically to different inputs and situations.

What’s on your mind? Put it in the comments!

Back to top