Chapter 2 — Python Basics: Variables, Types, Operators, and I/O

Before analyzing data we need to speak Python fluently. This chapter covers the fundamentals: variables, data types, operators, expressions, type conversion, and input/output formatting — the building blocks of every program that follows.

Learning Objectives

Declare variables and apply naming rules.
Use numeric types and arithmetic operators, including //, %, and **.
Apply operator precedence to evaluate expressions.
Convert between types and round numbers.
Manipulate strings with indexing and slicing.
Use Booleans and comparison operators.
Format output with f-strings, format(), and print options.
Read console input and convert it safely.

Prerequisites / Imports

Pure Python — no third-party libraries needed.

In [1]:

import math

1 Syntax, Comments, and Mode

Python uses indentation to define blocks. Comments start with #. You can run Python interactively (the REPL) or as a script. In Jupyter, each cell behaves like a mini-script with shared state.

In [1]:

# This is a comment
message = 'Hello, Data Science'   # variable assignment
print(message)                  # output

Hello, Data Science

2 Variables and Assignment

A variable is a name bound to a value. Identifiers must start with a letter or underscore, are case-sensitive, and cannot be a keyword. Python supports simultaneous assignment.

In [1]:

x = 10
y = 3.5
name = 'Ada'
print(x, y, name)

# Simultaneous assignment (swap without a temp)
a, b = 1, 2
a, b = b, a
print('a =', a, 'b =', b)

10 3.5 Ada
a = 2 b = 1

3 Numeric Types and Operators

Python has integers (int), floating-point (float), and complex numbers. Key operators: + - * / // % **. Augmented assignment (+=, -=, ...) updates a variable in place.

In [1]:

print('integer division 7 // 2 =', 7 // 2)
print('modulo 7 % 2 =', 7 % 2)
print('power 2 ** 10 =', 2 ** 10)
print('true division 7 / 2 =', 7 / 2)

integer division 7 // 2 = 3
modulo 7 % 2 = 1
power 2 ** 10 = 1024
true division 7 / 2 = 3.5

In [1]:

total = 0
total += 5      # augmented assignment
total += 10
print('total =', total)

total = 15

4 Operator Precedence

Python follows standard math precedence: ** binds tighter than unary minus, then * / // %, then + -. Use parentheses to make intent explicit.

In [1]:

print('2 + 3 * 4 =', 2 + 3 * 4)
print('(2 + 3) * 4 =', (2 + 3) * 4)
print('-2 ** 2 =', -2 ** 2, '(note: -(2**2))')
print('(-2) ** 2 =', (-2) ** 2)

2 + 3 * 4 = 14
(2 + 3) * 4 = 20
-2 ** 2 = -4 (note: -(2**2))
(-2) ** 2 = 4

5 Type Conversion and Rounding

Convert with int(), float(), str(), bool(). Round with round(x, n).

In [1]:

print(int(3.9), float(5), str(42), bool(0))
print(round(3.14159, 2))
print(round(2.675, 2))   # note floating-point surprise

3 5.0 42 False
3.14
2.67

6 Strings: Indexing and Slicing

Strings are immutable sequences of characters. Index from 0; slice with [start:stop:step].

In [1]:

s = 'DataScience'
print('first:', s[0], 'last:', s[-1])
print('slice [0:4]:', s[0:4])
print('every other:', s[::2])
print('reverse:', s[::-1])

first: D last: e
slice [0:4]: Data
every other: DtSine
reverse: ecneicSataD

7 Booleans and Comparisons

Comparisons return True/False. Combine with and, or, not.

In [1]:

age = 25
print(age >= 18 and age <= 65)
print(not (age < 18))
print(5 > 3, 5 == 5, 5 != 3)

True
True
True True True

8 Reading Input

input() reads a string from the console. In a notebook it blocks, so we show the pattern with a hardcoded value and demonstrate type conversion.

Code shown for illustration; not executed in this notebook.

# raw = input('Enter your age: ')
# age = int(raw)
# print('Next year you will be', age + 1)

In [1]:

# Safe conversion with error handling (runnable with a fixed value)
raw = '27'
try:
    age = int(raw)
    print('Parsed age:', age)
except ValueError:
    print('That was not an integer.')

Parsed age: 27

9 Output Formatting

f-strings are the modern, readable way to format output. print accepts sep and end.

In [1]:

price = 19.995
qty = 3
print(f'Unit price: ${price:.2f}')
print(f'Total: ${price * qty:,.2f}')
print('2026', '09', '16', sep='-')
print('no newline', end=' ')
print('continued')

Unit price: $20.00
Total: $59.98
2026-09-16
no newline continued

In [1]:

# format() method alternative
print('pi is roughly {:.4f}'.format(math.pi))

pi is roughly 3.1416

10 Named Constants and Style

Use UPPER_CASE for constants to signal they should not change. Follow PEP 8: spaces around operators, descriptive names, lines under ~79 characters.

In [1]:

TAX_RATE = 0.0825
subtotal = 100.0
tax = subtotal * TAX_RATE
print(f'Tax: ${tax:.2f}')

Tax: $8.25

Case Study: A Simple Sales Receipt

Compute a formatted receipt from item prices and quantities, applying tax.

In [1]:

items = [('Widget', 3, 4.50), ('Gadget', 2, 12.75), ('Cable', 5, 1.20)]
TAX_RATE = 0.0825

subtotal = sum(qty * price for _, qty, price in items)
tax = subtotal * TAX_RATE
total = subtotal + tax

print('=== RECEIPT ===')
for name, qty, price in items:
    print(f'{name:<8} x{qty} @ ${price:.2f} = ${qty*price:.2f}')
print('-' * 30)
print(f'Subtotal:        ${subtotal:.2f}')
print(f'Tax (8.25%):     ${tax:.2f}')
print(f'TOTAL:           ${total:.2f}')

=== RECEIPT ===
Widget   x3 @ $4.50 = $13.50
Gadget   x2 @ $12.75 = $25.50
Cable    x5 @ $1.20 = $6.00
------------------------------
Subtotal:        $45.00
Tax (8.25%):     $3.71
TOTAL:           $48.71

Exercises

Compute and print the area of a circle with radius 5 using math.pi.
Evaluate 1 + 2 * 3 ** 2 by hand, then verify in Python.
Swap two variables without a temporary variable.
Convert the string '3.14' to a float and round it to one decimal.
Slice the string 'DataScience' to get 'Science'.
Write an f-string that prints 1234567 with thousands separators.
Given celsius = 25, compute and print Fahrenheit with two decimals.
Write a receipt program for two items with a 10% discount before tax.
Explain why round(2.675, 2) does not give 2.68.
Use sep and end in print to output 2026/09/16 on one line.

Python Data Science: From Foundations to Applications — Chapter 2