Python variables and data types explained
int, float, str, bool, type function, dynamic typing, variable naming rules, type conversion
Variables and Types
Python is dynamically typed — you do not declare a type. The interpreter infers it from the value assigned.
age = 25 # int
price = 9.99 # float
name = "Alice" # str
is_active = True # bool
print(type(age)) # <class 'int'>
Naming Rules
Variable names must start with a letter or underscore, contain only letters, digits, and underscores, and cannot be a reserved keyword (if, for, class, etc.). Use snake_case by convention.
Type Conversion
Convert explicitly using built-in functions. Implicit conversion does not happen between incompatible types — Python raises a TypeError.
x = "42"
n = int(x) # 42
f = float(x) # 42.0
s = str(3.14) # "3.14"
b = bool(0) # False — 0, "", [], None are all falsy
# This raises TypeError:
# result = "5" + 5
result = int("5") + 5 # correct: 10
Use type() during debugging to verify what a variable holds. Mismatched types are the most common beginner error.
Python infers the type from the value, but that does not mean types do not matter — they absolutely do. Passing a string where an integer is expected still raises a TypeError, just at runtime rather than compile time. Calling type() mid-debug is a fast way to confirm what you are actually working with. For user-facing applications, always validate and convert input before using it in calculations. The most common beginner trap is reading input(), which always returns a string, and then trying arithmetic on it without converting first.