Indentation & Execution Flow in Python

Video Lesson:

Lesson Transcript:

Indentation refers to the whitespaces at the beginning of a code line. Python uses indentation to indicate which lines of code are part of a function and which are not.

Take this function for example:

def say_hello():
  print("Hello World")
  print("Welcome to AlgoCademy!")

Both print statements would run when say_hello() is called because they have the same level of indentation (2 spaces).

As for this code:

def say_hello():
  print("Hello World")
  print("Welcome to AlgoCademy!")
print("Outside the function!")

The unindented line print("Outside the function!") is not part of the function. Even more, it marks the end of the function.

The output of this program is:

Outside the function!

Execution flow

Lastly, note that the execution of a program always begins on the first line. The code is then executed one line at a time from top to bottom. This is known as execution flow and is the order a program in Python executes code.

For example, this program:

print("It's a beautiful day.")

def say_hello():
  print("Hello coders!")
  print("Welcome to AlgoCademy!")

print("Students are joining.")
say_hello()
print("We'll learn about functions.")

prints:

It's a beautiful day.
Students are joining.
Hello coders!
Welcome to AlgoCademy!
We'll learn about functions.

This is how the Python interpreter goes through this code line by line:

print("It's a beautiful day.") # <- Print "It's a beautiful day."

print("It's a beautiful day.")

def say_hello(): # <- A function say_hello() is about to be defined

print("It's a beautiful day.")

def say_hello():
  print("Hello coders!") # <- Indented line, so this is part of the function. Not executing the print tho.

print("It's a beautiful day.")

def say_hello():
  print("Hello coders!")
  print("Welcome to AlgoCademy!") # <- Indented line, so this is also part of the function. Not executing the print tho.

print("It's a beautiful day.")

def say_hello():
  print("Hello coders!")
  print("Welcome to AlgoCademy!")
  
print("Students are joining.") # <- Unindented line, so the function definition is over. 
                               #    Print "Students are joining."

print("It's a beautiful day.")

def say_hello():
  print("Hello coders!")
  print("Welcome to AlgoCademy!")
  
print("Students are joining.")
say_hello() # <- Function call of say_hello(). Executing the code inside function:
            # Print "Hello coders!"
            # Print "Welcome to AlgoCademy!"

print("It's a beautiful day.")

def say_hello():
  print("Hello coders!")
  print("Welcome to AlgoCademy!")
  
print("Students are joining.")
say_hello()
print("We'll learn about functions.") # <- Print "We'll learn about functions."

Assignment
Follow the Coding Tutorial and let's practice with indentation and execution flow!

Hint
Look at the examples above if you get stuck.

Introduction

In this lesson, we will explore the concepts of indentation and execution flow in Python. Understanding these concepts is crucial for writing clear and error-free code. Indentation helps define the structure of your code, while execution flow determines the order in which your code runs. These concepts are fundamental in programming and are particularly useful in scenarios involving functions, loops, and conditional statements.

Understanding the Basics

Indentation in Python refers to the spaces at the beginning of a code line. Unlike many other programming languages, Python uses indentation to define the scope of loops, functions, and other constructs. This means that all the lines of code within a particular block must be indented at the same level.

For example:

def greet():
    print("Hello, World!")
    print("Welcome to Python programming.")

In the above code, both print statements are part of the greet function because they are indented at the same level.

Main Concepts

Let's delve deeper into the key concepts of indentation and execution flow:

Indentation: Python uses indentation to define the scope of code blocks. Consistent indentation is crucial for the correct execution of the code.
Execution Flow: The execution flow in Python is the order in which the code is executed. It starts from the first line and proceeds line by line, following the logical structure defined by indentation.

Consider the following example:

print("Start of the program")

def greet():
    print("Hello, World!")
    print("Welcome to Python programming.")

print("Before calling the function")
greet()
print("After calling the function")

In this example, the execution flow is as follows:

Print "Start of the program".
Define the greet function (but do not execute it yet).
Print "Before calling the function".
Call the greet function, which prints "Hello, World!" and "Welcome to Python programming."
Print "After calling the function".

Examples and Use Cases

Let's look at some more examples to understand these concepts better:

# Example 1: Simple function with indentation
def add(a, b):
    result = a + b
    return result

print(add(5, 3))  # Output: 8

# Example 2: Conditional statements with indentation
def check_even_odd(number):
    if number % 2 == 0:
        print(f"{number} is even")
    else:
        print(f"{number} is odd")

check_even_odd(4)  # Output: 4 is even
check_even_odd(7)  # Output: 7 is odd

In these examples, indentation is used to define the scope of the function and the conditional statements.

Common Pitfalls and Best Practices

Here are some common mistakes to avoid and best practices to follow:

Inconsistent Indentation: Ensure that you use consistent indentation throughout your code. Mixing tabs and spaces can lead to errors.
Indentation Levels: Use the same number of spaces for each level of indentation. The Python style guide (PEP 8) recommends using 4 spaces per indentation level.
Readability: Proper indentation improves the readability of your code, making it easier to understand and maintain.

Advanced Techniques

As you become more comfortable with basic indentation and execution flow, you can explore advanced techniques such as nested functions, loops, and exception handling. These techniques often require careful attention to indentation to ensure correct execution.

# Example of nested functions
def outer_function():
    print("This is the outer function")

    def inner_function():
        print("This is the inner function")

    inner_function()

outer_function()

Code Implementation

Let's implement a more complex example that combines multiple concepts:

def process_numbers(numbers):
    for number in numbers:
        if number % 2 == 0:
            print(f"{number} is even")
        else:
            print(f"{number} is odd")

numbers_list = [1, 2, 3, 4, 5, 6]
process_numbers(numbers_list)

In this code, we define a function process_numbers that takes a list of numbers and checks if each number is even or odd. The function uses a loop and conditional statements, both of which require proper indentation.

Debugging and Testing

Debugging and testing are essential parts of writing reliable code. Here are some tips:

Print Statements: Use print statements to track the execution flow and identify where the code might be going wrong.
Unit Tests: Write unit tests to verify the correctness of your functions. Use testing frameworks like unittest or pytest.

import unittest

def add(a, b):
    return a + b

class TestMathFunctions(unittest.TestCase):
    def test_add(self):
        self.assertEqual(add(2, 3), 5)
        self.assertEqual(add(-1, 1), 0)

if __name__ == '__main__':
    unittest.main()

Thinking and Problem-Solving Tips

When approaching problems related to indentation and execution flow, consider the following strategies:

Break Down the Problem: Divide the problem into smaller parts and solve each part step by step.
Write Pseudocode: Outline the logic of your code in plain language before writing the actual code.
Practice: Regular practice with coding exercises and projects will help you become more proficient in these concepts.

Conclusion

In this lesson, we covered the importance of indentation and execution flow in Python. These concepts are fundamental for writing clear and error-free code. By mastering these basics, you will be well-equipped to tackle more complex programming challenges. Remember to practice regularly and explore further applications to deepen your understanding.

Additional Resources

For further reading and practice, consider the following resources: