Variables and Data Types

Variables are named containers that store values in your program. In Python, you create a variable simply by assigning a value to a name — no type declaration needed.

Creating Variables

name = "Alice"
age = 30
height = 5.7
is_active = True

Python infers the type from the value on the right. Variable names are case-sensitive, so age and Age are different variables.

Core Data Types

Strings (str)

Strings hold text. You can use single or double quotes:

greeting = "Hello, World!"
language = 'Python'
multi = """This spans
multiple lines"""

Useful string operations:

name = "Alice"
print(len(name))        # 5 — length
print(name.upper())     # ALICE
print(name.lower())     # alice
print(name[0])          # A — indexing
print(name[1:3])        # li — slicing

Integers (int) and Floats (float)

count = 42          # integer
price = 9.99        # float
big = 1_000_000     # underscores for readability

Booleans (bool)

is_valid = True
has_error = False

Booleans are the result of comparisons and are used in conditions.

None

None is Python's way of representing "no value" or "nothing":

result = None
print(result)   # None

Type Checking and Conversion

Use type() to inspect a variable's type:

x = 42
print(type(x))   # <class 'int'>

Convert between types with built-in functions:

age_str = "25"
age_int = int(age_str)    # string to integer: 25
price = 9.99
price_int = int(price)    # float to integer: 9 (truncates)
count = 7
count_float = float(count) # integer to float: 7.0
number = 42
number_str = str(number)   # integer to string: "42"

Naming Rules

Valid variable names:

• Start with a letter or underscore
• Contain letters, digits, and underscores
• Are case-sensitive
• Cannot be Python keywords (like if, for, while)

By convention, Python uses snake_case for variable names:

first_name = "Alice"
total_price = 99.99
max_retries = 3

Multiple Assignment

Python lets you assign multiple variables at once:

x, y, z = 1, 2, 3
a = b = c = 0       # all three set to 0

Understanding data types is fundamental — every value in Python has a type, and knowing the type tells you what operations are valid.