Chapter 2: Python Syntax and Core Concepts

Now that your development environment is ready, it’s time to get hands-on with Python syntax and core programming concepts. In this chapter, we’ll cover:

• Variables and Data Types
• Conditional Statements
• Loops
• Functions and Modules
• A practical project: Building a simple CLI-based calculator
• Writing unit tests for your calculator

2.1 Variables and Data Types

Python Data Types

Python Variables

In Python, a variable is a name that refers to a value stored in memory. Variables allow you to store, retrieve, and manipulate data in your programs.

Key Features of Python Variables

Example Variable Declarations

# Integer variable
age = 25

# String variable
name = "Alice"

# Float variable
price = 19.99

# Boolean variable
is_active = True

Variable Naming Rules

1. Variable names can contain letters, numbers, and underscores.
2. They cannot start with a number.
3. No spaces allowed.
4. Reserved keywords like class, if, return, etc., cannot be used as variable names.

✅ Valid examples:

user_name = "Bob"
userAge = 30
_user_location = "NY"

❌ Invalid examples:

2user = "Error"      # Starts with a number
user-name = "Error"  # Contains a hyphen
class = "Error"      # Reserved keyword

Dynamic Typing Example

x = 10          # x is an integer
x = "Hello"     # x is now a string
x = 3.14        # x is now a float

Python doesn’t require you to declare the type beforehand.

Multiple Variable Assignment

a, b, c = 1, 2, 3

# Or assign the same value to multiple variables
x = y = z = 0

Swapping Variables

Python makes it easy to swap values without a temporary variable:

x, y = 5, 10
x, y = y, x
print(x, y)  # Output: 10 5

Type Checking:

You can check the type of a variable using:

name = "Alice"
print(type(name))  # Output: <class 'str'>

Notes:

• Type Checking: Use type(x) or isinstance(x, DataType)
• Type Conversion: Built-in functions like int(), str(), float()
• Further Reading: Python Standard Types - Official Docs

2.2 Conditional Statements

Python uses indentation (not curly braces) for blocks of code.

x = 10

if x > 5:
    print("x is greater than 5")
elif x == 5:
    print("x equals 5")
else:
    print("x is less than 5")

2.3 Loops

For Loop

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

While Loop:

count = 0

while count < 3:
    print(f"Count is {count}")
    count += 1

Loop Control Statements:

• break: Exit the loop early
• continue: Skip to the next iteration

2.4 Functions and Modules

The def keyword is used to define a function in Python. Functions help organize and reuse your code.

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

result = add(2, 3)
print(result)  # Output: 5

You can also import functions from modules (files).

Example: If you had a file math_utils.py:

# math_utils.py
def multiply(a, b):
    return a * b

You can import and use it like this:

from math_utils import multiply

print(multiply(3, 4))  # Output: 12

Example Project: CLI-Based Calculator

Let’s build a simple command-line calculator supporting addition, subtraction, multiplication, and division.

File: 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):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def main():
    print("Simple CLI Calculator")
    print("Operations: add, subtract, multiply, divide")

    operation = input("Enter operation: ").strip().lower()
    try:
        num1 = float(input("Enter first number: "))
        num2 = float(input("Enter second number: "))

        if operation == "add":
            result = add(num1, num2)
        elif operation == "subtract":
            result = subtract(num1, num2)
        elif operation == "multiply":
            result = multiply(num1, num2)
        elif operation == "divide":
            result = divide(num1, num2)
        else:
            print("Invalid operation")
            return

        print(f"Result: {result}")

    except ValueError as e:
        print(f"Error: {e}")

if __name__ == "__main__":
    main()

Running the Calculator

python calculator.py

Example interaction:

Simple CLI Calculator
Operations: add, subtract, multiply, divide
Enter operation: add
Enter first number: 10
Enter second number: 5
Result: 15.0

2.6 Writing Unit Tests for the Calculator

Now, let's create a unit test file.

File: test_calculator.py

import pytest
from calculator import add, sub, mul, div

def test_addition():
    assert add(1, 2) == 3

def test_subtraction():
    assert sub(5, 2) == 3

def test_multiplication():
    assert mul(2, 3) == 6

def test_division():
    assert div(6, 2) == 3

def test_division_by_zero():
    with pytest.raises(ValueError, match="Division by zero"):
        div(2, 0)

Running Your Tests:

pytest

Expected Output:

collected 5 items

test_calculator.py .....                       [83%]

5 passed in 0.03s

The reason why the test coverage isn't 100% is because the main() function in the module wasn't tested. We could achieve 100% test coverage of the business logic by moving the arithmatic functions into a separate module, calculator_operations.py for instance, and writing tests for it.

2.7 How This Ties to Web Development Later

Understanding data types, control structures, and functions lays the groundwork for writing API routes, services, and business logic layers in frameworks like FastAPI and Django.

For example:

• The main() function above mirrors a simple web controller
• Validation logic (like checking for divide by zero) will later appear in API request validation

Conclusion

You’ve learned:

• Python variables and types
• Conditionals and loops
• Functions and modules
• How to write and test a simple calculator app

What is Next?

In Chapter 3: Working with Files and Data Serialization, we’ll cover:

• Reading and writing files
• Parsing JSON and CSV
• Practical example: Log file parser that outputs JSON