Understanding the difference between expressions and statements is crucial for writing effective Python code. In this guide, you’ll learn how operators work, how expressions produce values, and how statements control program flow. Practice with exercises and workshops to solidify your skills.

Expressions vs. Statements

What is an Expression?

An expression is a piece of code that evaluates to a value. Examples include arithmetic operations, function calls, or comparisons.

2 + 3          # Evaluates to 5
len("Hello")   # Evaluates to 5
3 > 2          # Evaluates to True

What is a Statement?

A statement is an instruction that performs an action. Examples include loops, conditionals, and variable assignments.

if x > 0:       # if statement
    print(x)

for i in range(5):  # for loop statement
    print(i)

Python Operators

1. Arithmetic Operators

print(10 + 3)   # 13 (Addition)
print(10 - 3)   # 7 (Subtraction)
print(10 * 3)   # 30 (Multiplication)
print(10 ** 3)  # 1000 (Exponentiation)
print(10 / 3)   # 3.333... (Division)
print(10 // 3)  # 3 (Floor Division)
print(10 % 3)   # 1 (Modulus)

2. Comparison Operators

print(5 == 5)   # True
print(5 != 3)   # True
print(5 > 3)    # True
print(5 <= 5)   # True

3. Logical Operators

print(True and False)  # False
print(True or False)   # True
print(not True)        # False

4. Assignment Operators

x = 5
x += 3   # Equivalent to x = x + 3 (x becomes 8)

Practice Work

Exercise 1: Identify Expressions vs. Statements

Label the following as expression or statement:

5 * (10 - 3)
x = 25
print("Hello")
sum = 2 + 3

Exercise 2: Fix Invalid Expressions

Correct the invalid code below:

result = 10 + * 3
if 5 > 3 and < 10:
    print("Valid")

result = 10 * 3
if 5 > 3 and 5 < 10:
    print("Valid")

Exercise 3: Evaluate Expressions

What do these expressions evaluate to?

15 % 4
(5 > 3) or (2 == 3)
not (10 <= 5)

Workshop: Real-World Applications

Workshop 1: Temperature Converter

Write a script that converts Celsius to Fahrenheit using the formula: F = (C * 9/5) + 32.

celsius = float(input("Enter temperature in Celsius: "))
fahrenheit = (celsius * 9/5) + 32
print(f"{celsius}°C = {fahrenheit}°F")

Workshop 2: User Login Validation

Check if a user’s input meets these conditions:

• Username is at least 5 characters long.
• Password contains at least 8 characters.

username = input("Enter username: ")
password = input("Enter password: ")

is_valid = (len(username) >= 5) and (len(password) >= 8)
print("Valid credentials?" , is_valid)

Best Practices

• Use Parentheses for Clarity: (5 + 3) * 2 is clearer than 5 + 3 * 2.
• Avoid Complex One-Liners: Break complicated expressions into multiple lines.
• Prefer Descriptive Variables: discount_rate = 0.1 instead of dr = 0.1.

Conclusion

Mastering operators and understanding the difference between expressions and statements will help you write efficient and readable Python code. Practice the exercises and workshops to reinforce these concepts.

Next Steps: Explore our posts on Python Control Flow and Python Functions.

Variables and identifiers are fundamental to programming in Python. In this guide, you’ll learn how to name variables correctly, avoid syntax errors, and follow Python’s naming conventions. We’ll also explore reserved keywords and practice with hands-on exercises.

Variables and Identifiers in Python

What Are Variables?

Variables are containers for storing data. They are created using an identifier (name) and the assignment operator =.

age = 25
name = "Mourad"
user_name = 'admin'
server_ip = '192.168.1.1'

Rules for Naming Identifiers

• Can include letters, numbers, and underscores.
• Cannot start with a number.
• Cannot use reserved keywords (e.g., def, class).
• Case-sensitive (Age ≠ age).

Invalid Identifiers Example

#def = 5  # SyntaxError: "def" is a reserved keyword

Reserved Keywords

Python has predefined keywords that cannot be used as identifiers. View them using the keyword module:

import keyword
print(keyword.kwlist)

Practice Work

Exercise 1: Fix Invalid Identifiers

Correct the invalid identifiers in the code below:

2nd_name = "Ali"
class = "Python101"
user-email = "[email protected]"

second_name = "Ali"
course_class = "Python101"
user_email = "[email protected]"

Exercise 2: Create Valid Identifiers

Write variables for the following data:

1. A constant for maximum login attempts (value: 3).
2. A string storing a server’s domain name (“api.example.com”).
3. A boolean indicating whether a user is active (True/False).

MAX_LOGIN_ATTEMPTS = 3
server_domain = "api.example.com"
is_user_active = True

Exercise 3: Reserved Keywords Check

Write a script to check if the word "async" is a reserved keyword in Python.

import keyword
print("async" in keyword.kwlist)  # Output: True (in Python 3.7+)

Workshop: Real-World Practice

Workshop 1: User Registration Script

Create a script that asks for a user’s first name, last name, and age. Store the data in variables and print a summary. Follow these rules:

• Use snake_case for variable names.
• Avoid reserved keywords.

first_name = input("Enter your first name: ")
last_name = input("Enter your last name: ")
user_age = int(input("Enter your age: "))

print(f"User: {first_name} {last_name}, Age: {user_age}")

Workshop 2: Data Processing

Calculate the total price of items in a shopping cart using variables:

item1_price = 20.5
item2_price = 15.75
item3_price = 5.0
total = item1_price + item2_price + item3_price
print(f"Total: ${total:.2f}")

Best Practices

• Use Descriptive Names: user_age instead of a.
• Follow Case Conventions:
  • snake_case for variables (e.g., server_ip).
  • UPPER_CASE for constants (e.g., MAX_USERS).
• Avoid Abbreviations: first_name is clearer than fn.

Conclusion

Mastering variables and identifiers is the first step to writing clean, maintainable Python code. Practice the exercises and workshops to solidify your understanding, and always follow Python’s naming conventions.

Want more? Check out our post on Python Data Types or Python Functions.

Understanding Python’s code structure is essential for writing reusable, maintainable, and executable programs. In this guide, we’ll explore the differences between scripts and modules, how to use the shebang line (#!/usr/bin/env python3), and best practices for organizing and executing Python code.

Scripts vs. Modules in Python

1. Scripts: Executable Files

A script is a Python file (.py) designed to be executed directly. Scripts typically perform tasks like data processing, automation, or running applications.

#!/usr/bin/env python3
# This is a script named "greet.py"
print("Hello, World!")

2. Modules: Reusable Code Libraries

A module is a Python file intended to be imported and reused in other scripts or modules. Modules contain functions, classes, or variables.

# This is a module named "math_utils.py"
def add(a, b):
    return a + b

def subtract(a, b):
    return a - b

Shebang Line and File Permissions

Shebang Line: #!/usr/bin/env python3

The shebang line tells the operating system which interpreter to use to execute the script. Place it at the top of your script:

#!/usr/bin/env python3
print("This script uses Python 3!")

Setting File Permissions

To run a script directly, you must make it executable using chmod:

chmod +x script.py  # Grant execute permission
./script.py         # Run the script

Example: Script Execution Workflow

1. Create a script: greet.py
2. Add the shebang line and code:

#!/usr/bin/env python3
name = input("Enter your name: ")
print(f"Hello, {name}!")

3. Make it executable:

chmod +x greet.py

4. Run it:

./greet.py

Practice Work

Exercise 1: Create a Script

Write a script called calculator.py that asks the user for two numbers and prints their sum. Include the shebang line and make it executable.

#!/usr/bin/env python3
num1 = float(input("Enter first number: "))
num2 = float(input("Enter second number: "))
print(f"Sum: {num1 + num2}")

Exercise 2: Convert Script to Module

Convert the calculator.py script into a reusable module with functions for addition, subtraction, multiplication, and division.

# Module: calculator.py
def add(a, b):
    return a + b

def subtract(a, b):
    return a - b

def multiply(a, b):
    return a * b

def divide(a, b):
    return a / b

Exercise 3: Fix Permissions

If a script returns a “Permission denied” error, what command would you use to fix it? Write the answer below.

# Answer:
chmod +x script_name.py

Best Practices

• Use Shebang for Clarity: Always include #!/usr/bin/env python3 in scripts to specify the Python version.
• Organize Code into Modules: Reuse functions/classes across projects by separating logic into modules.
• Name Scripts and Modules Clearly: Use descriptive filenames like data_analysis.py or utils.py.

Conclusion

Mastering Python’s code structure—scripts, modules, and execution—is key to building scalable and maintainable applications. Practice creating scripts, converting them to modules, and using the shebang line to ensure your code runs smoothly across systems.

Need more Python tutorials? Check out our posts on Python Functions and Object-Oriented Programming in Python.

Python is a powerful and beginner-friendly programming language known for its simplicity and readability. One of the most distinctive features of Python is its use of indentation to define code blocks. Unlike other programming languages that use braces {}, Python relies on proper spacing and alignment to group statements. In this chapter, we’ll explore Python’s indentation rules, comments, and line continuation, along with examples, exercises, and practical work (TP) to help you master these concepts.

Why Indentation Matters in Python

In many programming languages like C, Java, or JavaScript, curly braces {} are used to define blocks of code. However, Python takes a different approach. It uses indentation (spaces or tabs) to group statements logically. This makes Python code visually clean and easy to read.

Example of Indentation

if True:
    print('This is indented properly')  # This code is indented

In the example above:

• The if statement is followed by a colon :.
• The print() statement is indented with 4 spaces (or a tab) to indicate that it belongs to the if block.
• If the indentation is incorrect, Python will throw an IndentationError.

Comments in Python

Comments are essential for explaining your code. Python supports two types of comments:

1. Single-line comments: Use the # symbol.
2. Multi-line comments: Use triple quotes ''' or """.

Example of Comments

# This is a single-line comment

"""
This is a multi-line comment.
It can span across multiple lines.
"""

Line Continuation in Python

Sometimes, a single line of code can become too long. Python allows you to break it into multiple lines using the backslash \ for line continuation.

Example of Line Continuation

total = 1 + 2 + 3 + \
        4 + 5 + 6
print(total)  # Output: 21

Here, the backslash \ tells Python that the statement continues on the next line.

Exercises to Practice

Now that you’ve learned the basics, let’s test your understanding with some exercises.

Exercise 1: Fix the Indentation

The following code has incorrect indentation. Fix it so it runs without errors.

if 5 > 2:
print('5 is greater than 2')

Exercise 2: Add Comments

Add a single-line comment and a multi-line comment to the code below.

x = 10
y = 20
sum = x + y
print(sum)

Exercise 3: Line Continuation

Rewrite the following code using line continuation to make it more readable.

result = 10 + 20 + 30 + 40 + 50 + 60 + 70 + 80 + 90 + 100
print(result)

TP (Practical Work)

To solidify your understanding, here are some practical tasks to complete. These tasks will help you apply the concepts of indentation, comments, and line continuation in real-world scenarios.

TP 1: Create a Simple Calculator

Write a Python program that performs basic arithmetic operations (addition, subtraction, multiplication, and division) using proper indentation and comments to explain each step.

# Example structure
num1 = 10
num2 = 5

# Addition
sum = num1 + num2
print("Sum:", sum)

# Subtraction
difference = num1 - num2
print("Difference:", difference)

# Add more operations here...

TP 2: Fix and Improve Code

The following code is poorly formatted and lacks comments. Fix the indentation, add comments, and use line continuation where necessary.

if 10 > 5:
print('10 is greater than 5')
else:
print('5 is greater than 10')
total = 10 + 20 + 30 + 40 + 50 + 60 + 70 + 80 + 90 + 100
print(total)

TP 3: Write a Program with Nested Conditions

Write a Python program that uses nested if statements to check multiple conditions. Ensure proper indentation and add comments to explain the logic.

# Example structure
age = 18
has_license = True

if age >= 18:
    if has_license:
        print("You are eligible to drive.")
    else:
        print("You need a license to drive.")
else:
    print("You are too young to drive.")

Conclusion

Understanding Python’s indentation, comments, and line continuation is crucial for writing clean and error-free code. By mastering these basics and completing the exercises and practical work (TP), you’ll be well on your way to becoming a proficient Python programmer. Practice the exercises provided, and don’t forget to experiment with your own code!

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