Variables & Data Types

Variables & Data Types

Python is dynamically typed — you don’t declare a type, the interpreter infers it at runtime.

Declaring Variables

name = "Alice"
age = 30
price = 9.99
is_active = True
nothing = None

Built-in Data Types

Category	Types
Text	str
Numeric	int, float, complex
Boolean	bool
Sequence	list, tuple, range
Mapping	dict
Set	set, frozenset
Binary	bytes, bytearray, memoryview
None	NoneType

Type Checking

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

Type Conversion (Casting)

# Implicit
result = 5 + 2.0      # float: 7.0

# Explicit
n = int("42")         # 42
f = float("3.14")     # 3.14
s = str(100)          # "100"
b = bool(0)           # False
lst = list("abc")     # ['a', 'b', 'c']

Multiple Assignment

a = b = c = 0
x, y, z = 1, 2, 3
first, *rest = [1, 2, 3, 4]  # first=1, rest=[2,3,4]

Constants (Convention)

Python has no const keyword. Use ALL_CAPS by convention:

MAX_SIZE = 100
PI = 3.14159

Variable Scope Preview

• Variables defined inside a function are local.
• Variables defined at module level are global.
• Use global keyword to modify a global inside a function.

count = 0

def increment():
    global count
    count += 1

Common Pitfalls

# Mutable default — avoid this
def append_item(item, lst=[]):   # BAD: lst shared across calls
    lst.append(item)
    return lst

# Safe version
def append_item(item, lst=None):
    if lst is None:
        lst = []
    lst.append(item)
    return lst