Python Basics for Data Science

# CHAPTER 3

Python Basics for Data Science #

1. Chapter Introduction #

2. Python Syntax and Indentation #

Most programming languages use curly braces {} to define blocks of code and semicolons ; to end lines. Python does not.

Python relies on indentation (whitespace) to structure code. You must be consistent!

# CORRECT INDENTATION (4 spaces)
if 5 > 2:
    print("Five is greater than two!")
    print("This line is also inside the if block.")

# INCORRECT INDENTATION (Will throw an IndentationError)
if 5 > 2:
print("This will fail!")

3. Comments #

Comments are notes written in the code that the computer completely ignores. They are used to explain *why* the code is doing something to other humans (or yourself in 6 months).

# This is a single-line comment using the hashtag symbol.
print("Hello World") # This is an inline comment

"""
This is a multi-line comment (or docstring).
It uses three double-quotes.
Data scientists use this to explain complex functions.
"""

4. Variables #

A variable is a labeled container for storing data. In Python, you do not need to declare what type of data the variable will hold; Python figures it out automatically. You assign data to a variable using the = sign.

# Assigning values to variables
customer_name = "Alice"
age = 28
account_balance = 1500.50

# Variables can be updated later
age = 29 

Naming Rules for Variables:

• Must start with a letter or an underscore .

• Can only contain alphanumeric characters and underscores (A-Z, 0-9, and ).

• Are case-sensitive (Age and age are different).

• *Best Practice:* Use snakecase for Python variable names (e.g., totalsales).

5. Printing Output #

To display the contents of a variable to the screen, use the print() function.

first_name = "John"
last_name = "Doe"

# Printing a single variable
print(first_name)

# Printing multiple items (automatically separated by a space)
print("The customer is:", first_name, last_name)

# Formatted Strings (f-strings) - THE MODERN WAY
# Prefix the string with 'f' and use {} for variables
print(f"The customer's full name is {first_name} {last_name}.")

6. User Input #

For interactive scripts, you can ask the user to type something using the input() function.

# The program will pause here and wait for the user to type
user_city = input("What city are you from? ")

print(f"Wow, {user_city} is a beautiful city!")

*Note: The input() function ALWAYS returns text (a string), even if the user types a number. We will learn how to convert types in the next chapter.*

7. Mini Project: Simple Calculator Input #

Let's combine what we've learned into a simple interactive script.

print("--- Welcome to the Data Portal ---")

# Gather variables
user = input("Enter your username: ")
department = input("Enter your department (e.g., Sales, HR): ")

# Output formatted summary
print("\n--- Portal Access Granted ---")
print(f"User: {user}")
print(f"Dept: {department}")
print("Loading analytics dashboard...")

8. Common Mistakes #

• Mixing Tabs and Spaces: Python hates it when you use the Tab key on one line, and 4 Spacebar taps on the next line to indent code. Pick one (spaces are the industry standard) and stick to it. Most modern IDEs convert Tabs to spaces automatically.

• Forgetting f-string syntax: Writing print("Hello {name}") without the f in front will literally print the text "Hello {name}". It must be print(f"Hello {name}").

9. MCQs #

10. Interview Questions #

• Q: Why does Python use indentation instead of brackets for code blocks? What are the pros and cons of this design?

• Q: Explain the difference between = and == in Python syntax (Briefly prep for next chapter).

11. Summary #

12. Next Chapter Recommendation #