2.1.1 What is a Variable?

Think of a variable as a labeled box where you store a piece of data. You give the box a name (the label), and Python remembers what's inside. Whenever you need that data, you just refer to the label.

In Python, creating a variable is as simple as picking a name and using the assignment operator =:

name = "Alice"        # String variable
age = 15              # Integer variable
height = 1.68         # Float variable
is_student = True     # Boolean variable

The = sign doesn't mean "equals" like in math. It means "assign the value on the right to the name on the left". Read it as: "name gets the value 'Alice'".

2.1.2 Python's Basic Data Types

Every value in Python has a type. Python has four fundamental data types that you'll use all the time:

🔤 Strings (str)

Strings hold text. They're created by wrapping characters in quotes — either single ' ' or double " ".

greeting = "Hello, World!"
name = 'Alice'
empty_string = ""

🔢 Integers (int)

Integers are whole numbers — positive, negative, or zero. No decimal point allowed!

score = 95
temperature = -3
population = 8_000_000_000  # Underscores make large numbers readable!

📐 Floats (float)

Floats represent decimal numbers. Use them when you need precision, like measurements or money.

pi = 3.14159
gpa = 3.75
price = 19.99
temperature = 36.6

✅ Booleans (bool)

Booleans have only two possible values: True or False. They're perfect for yes/no questions.

is_raining = True
has_passed = False
is_enrolled = True

Note: In Python, True and False must be capitalized!

2.1.3 Checking Types with type()

Python provides a built-in function called type() that tells you the data type of any value. This is extremely useful when you're debugging or unsure what type a variable holds:

name = "Alice"
age = 15
gpa = 3.8
passed = True

print(type(name))    # 
print(type(age))     # 
print(type(gpa))     # 
print(type(passed))  # 

2.1.4 Variable Naming Rules

Python has strict rules about what makes a valid variable name. Follow these to avoid errors:

Python Keywords to Avoid

These words have special meanings in Python and can't be used as variable names:

and     as      assert  break   class   continue  def
del     elif    else    except  finally  for      from
global  if      import  in      is      lambda    not
or      pass    raise   return  try     while     with
yield

Best Practice: Snake Case

Python programmers follow a naming convention called snake_case — all lowercase letters with underscores between words:

student_name       # ✅ Good: clear and readable
age_of_student     # ✅ Good: descriptive
total_score        # ✅ Good: follows convention
a                  # ❌ Too vague — what does it mean?
x1y2               # ❌ Meaningless — no context
studentName        # ⚠️ camelCase — JavaScript style, not Python!

2.1.5 Dynamic Typing — The Python Superpower

Unlike many other programming languages, Python is dynamically typed. This means:

• ✅ You don't need to declare the type — just assign a value
• ✅ A variable can change its type if you assign a new value of a different type

x = 10           # x is an integer
print(type(x))   # 

x = "Hello"      # Now x is a string!
print(type(x))   # 

x = 3.14         # Now x is a float!
print(type(x))   # 

value = 100
print(type(value))
value = "one hundred"
print(type(value))
value = 100.0
print(type(value))

2.1.6 Type Conversion (Casting)

Sometimes you need to convert a value from one type to another. Python provides built-in functions for this:

# Convert to string
age = 15
message = "You are " + str(age) + " years old"
print(message)  # You are 15 years old

# Convert to integer
price_str = "29"
price_int = int(price_str)
print(price_int + 1)  # 30

# Convert to float
whole = 10
decimal = float(whole)
print(decimal)  # 10.0

# Boolean conversion
print(bool(1))    # True (any non-zero number is True)
print(bool(0))    # False (zero is False)
print(bool(""))   # False (empty string is False)
print(bool("Hi")) # True (non-empty string is True)

📋 Section 2.1 Summary Checklist

2.2.1 Creating Strings

In Python, a string is a sequence of characters enclosed in quotes. Strings are everywhere in programming — names, messages, passwords, file paths, and even the HTML code of websites.

Single vs Double Quotes

Python treats both identically. The key is consistency — pick one style and stick with it!

# Single quotes
name = 'Alice'
greeting = 'Hello, World!'

# Double quotes
name = "Alice"
greeting = "Hello, World!"

# Choosing quotes to avoid escaping:
sentence1 = "It's a beautiful day"     # ✅ Double quotes = no escape needed for '
sentence2 = 'He said "Python is fun"'  # ✅ Single quotes = no escape needed for "

Multi-line Strings with Triple Quotes

For strings that span multiple lines, use triple quotes (''' or """):

poem = '''Roses are red,
Violets are blue,
Python is awesome,
And so are you!'''

print(poem)
# Output:
# Roses are red,
# Violets are blue,
# Python is awesome,
# And so are you!

2.2.2 String Concatenation & Repetition

🔗 Concatenation with +

The + operator joins strings together. This is called concatenation:

first = "Python"
second = "Rocks"
result = first + " " + second
print(result)  # Python Rocks

# Concatenating multiple strings
full_name = "Alice" + " " + "Smith"
print(full_name)  # Alice Smith

🔁 Repetition with *

The * operator repeats a string a given number of times:

print("Ha" * 3)       # HaHaHa
print("-" * 20)        # --------------------
print("🐍" * 5)        # 🐍🐍🐍🐍🐍

# Great for creating visual separators!
separator = "=" * 30
print(separator)
print("WELCOME TO PYTHON")
print(separator)

*
**
***
****
*****

2.2.3 Essential String Methods

Python strings come with many built-in methods that let you manipulate text. Here are the most important ones for beginners:

Seeing Them in Action

text = "  Hello, Python!  "

print(text.upper())                    # "  HELLO, PYTHON!  "
print(text.lower())                    # "  hello, python!  "
print(text.strip())                    # "Hello, Python!"
print(len(text))                       # 19 (counts ALL characters including spaces)
print(text.replace("Python", "World")) # "  Hello, World!  "
print(text.find("Python"))             # 8 (starts at index 8)
print(text.count("o"))                 # 3 ('o' appears 3 times)
print("python".capitalize())           # "Python"

Method Chaining

You can chain multiple methods together — each operates on the result of the previous one:

message = "  Hello, World!  "
print(message.strip().upper())                           # "HELLO, WORLD!"
print(message.strip().replace("World", "Python").upper()) # "HELLO, PYTHON!"

2.2.4 f-Strings — The Modern Way to Format Text

f-strings (formatted string literals) are the cleanest way to insert variables into text. Just put an f before the opening quote and use {} as placeholders:

name = "Bob"
age = 16
print(f"My name is {name} and I am {age} years old.")
# Output: My name is Bob and I am 16 years old.

Expressions Inside {}

You can put any Python expression inside the curly braces, not just variable names:

a = 10
b = 20
print(f"{a} + {b} = {a + b}")
# Output: 10 + 20 = 30

print(f"Temperature: {36.6}°C")
# Output: Temperature: 36.6°C

score = 85
print(f"You scored {score}% — {'Pass' if score >= 50 else 'Fail'}")
# Output: You scored 85% — Pass

Formatting Numbers

You can control how numbers are displayed:

pi = 3.1415926535
print(f"Pi to 2 decimals: {pi:.2f}")    # Pi to 2 decimals: 3.14
print(f"Pi to 4 decimals: {pi:.4f}")    # Pi to 4 decimals: 3.1416

big_number = 1000000
print(f"Formatted: {big_number:,}")     # Formatted: 1,000,000

percentage = 0.875
print(f"Percentage: {percentage:.1%}")  # Percentage: 87.5%

f-Strings vs Older Methods

2.2.5 The print() Function — Beyond the Basics

The print() function is your window to the outside world — it sends text to the console. Let's explore its hidden powers:

Multiple Arguments

You can pass multiple values separated by commas. print() automatically adds a space between them:

print("Apple", "Banana", "Cherry")
# Output: Apple Banana Cherry

print(1, 2, 3, "Go!")
# Output: 1 2 3 Go!

Customizing the Separator (sep)

The sep parameter changes what goes between items:

print("apple", "banana", "cherry", sep=", ")
# Output: apple, banana, cherry

print("10", "20", "30", sep=" | ")
# Output: 10 | 20 | 30

print("hello", "world", sep="")
# Output: helloworld (no space at all!)

Customizing the End Character (end)

By default, print() ends with a newline (\n). You can change this with end:

print("Hello", end=" ")
print("World")
# Output: Hello World (both on the same line!)

print("Loading", end="")
print(".", end="")
print(".", end="")
print(".")
# Output: Loading....

Combining sep and end

print("a", "b", "c", sep=" - ", end="!!!\n")
# Output: a - b - c!!!

2.2.6 Escape Characters

Escape sequences let you include special characters inside strings. They start with a backslash \ followed by a character code:

Escape Characters in Action

# New line
print("Line 1\nLine 2\nLine 3")
# Output:
# Line 1
# Line 2
# Line 3

# Tab for alignment
print("Name\tAge\tGrade")
print("Alice\t15\tA")
print("Bob\t16\tB")
# Output:
# Name    Age    Grade
# Alice   15     A
# Bob     16     B

# Including quotes inside strings
print("She said, \"Python is amazing!\"")
# Output: She said, "Python is amazing!"

# Backslash in file paths
path = "C:\\Users\\Student\\Documents"
print(path)
# Output: C:\Users\Student\Documents

  *
 ***
*****

📋 Section 2.2 Summary Checklist

📋 Chapter 2: Python Basics — Complete Checklist