Variables & Types

Variables and dynamic typing

In Python you assign a value to a name — no type declaration needed. The variable’s type is determined by the object it currently references, not by a declaration.

x = 10          # x references an int
x = "hello"     # x now references a str — perfectly valid
x = [1, 2, 3]   # x now references a list

This is called dynamic typing. The type travels with the object, not the variable name.

Built-in scalar types

Python ships with four scalar types you will use in every program:

Type	Literal example	Notes
`int`	`42`, `-7`, `0`	Arbitrary precision — never overflows
`float`	`3.14`, `-0.5`	IEEE 754 double precision
`str`	`"hello"`, `'world'`	Immutable Unicode sequence
`bool`	`True`, `False`	Subclass of `int`; `True == 1`, `False == 0`

There is also None — a singleton that represents the absence of a value. None has its own type, NoneType, and there is exactly one None object in the entire Python process. Always test for it with is None, never == None.

Checking types at runtime

type() returns the exact type of an object. isinstance() tests whether an object is an instance of a type or any of its subclasses.

age = 30
print(type(age))           # <class 'int'>
print(isinstance(age, int))  # True
print(isinstance(True, int)) # True — bool is a subclass of int

Type conversions

Python does not silently coerce types. You convert explicitly using the type name as a function:

n = int("42")        # str -> int
f = float("3.14")    # str -> float
s = str(100)         # int -> str
b = bool(0)          # int -> bool  ->  False

If the conversion is impossible, Python raises a ValueError:

int("hello")   # ValueError: invalid literal for int()

Truthiness

Every Python object has a boolean interpretation used in if and while conditions. The following values are falsy — everything else is truthy:

False, None
Zero: 0, 0.0, 0j
Empty containers: "", [], {}, (), set()

name = ""
if name:
    print("has name")
else:
    print("name is empty")   # this branch runs

Full runnable demo

Open in an online REPL

age: int = 30
price: float = 9.99
greeting: str = "Hello"
active: bool = True
nothing = None

print(type(age))
print(type(price))
print(type(greeting))
print(type(active))
print(type(nothing))

x = int("42")
y = float("3.14")
z = str(100)
print(x, y, z)

falsy_values = [0, "", None, False, [], 0.0]
for v in falsy_values:
    print(f"bool({v!r}) = {bool(v)}")