Data Types: Numbers, Text, Booleans and None

Every value in Python has a type. The type decides what you can do with the value: you can divide numbers, make text uppercase, and use a boolean in an if. This lesson covers the five basic types you will use in every program.

The map of built-in types

Checking a type

print(type(42))
print(type(3.14))
print(type("42"))
print(type(True))
print(type(None))

<class 'int'>
<class 'float'>
<class 'str'>
<class 'bool'>
<class 'NoneType'>

Notice that "42" in quotes is text, not a number. That difference causes many beginner bugs. In real code, prefer isinstance(value, int) to compare types – it also accepts subclasses.

Integers (int)

Whole numbers, positive or negative. Python integers have no size limit – they grow as big as your memory allows, which is rare among languages.

followers = 1_250_000
print(followers * 3)
print(2 ** 100)        # 2 to the power 100
print(0b1010, 0o17, 0xff)   # binary, octal, hex

3750000
1267650600228229401496703205376
10 15 255

Floats (float)

Numbers with a decimal point. Any division with / gives a float, even when the result is whole:

print(10 / 2)
print(7 / 2)
print(1.5e3)       # scientific notation: 1.5 × 10³

5.0
3.5
1500.0

The famous 0.1 + 0.2 puzzle

print(0.1 + 0.2)
print(0.1 + 0.2 == 0.3)
print(round(0.1 + 0.2, 2))

0.30000000000000004
False
0.3

Computers store floats in binary, and 0.1 cannot be written exactly in binary – just as 1/3 cannot be written exactly in decimal (0.3333…). The tiny error is normal in every language.

import math
from decimal import Decimal

print(math.isclose(0.1 + 0.2, 0.3))
print(Decimal("0.10") + Decimal("0.20"))

True
0.30

Strings (str)

Text in single or double quotes. Strings are a big topic with their own chapter; here are the basics:

first = "Ada"
last = "Lovelace"
full = first + " " + last      # join with +
print(full)
print(len(full))               # number of characters
print("=" * 12)                # repeat with *

Ada Lovelace
12
============

Booleans (bool)

A boolean is either True or False (capital first letter). Comparisons produce booleans, and if statements consume them:

age = 19
is_adult = age >= 18
print(is_adult)
print(type(is_adult))
print(True + True)    # bool is a kind of int: True is 1

True
<class 'bool'>
2

Truthy and falsy

Any value can be tested in an if. These count as false; everything else is true:

for value in [0, 0.0, "", [], {}, None, "0", " ", [0]]:
    print(repr(value), "→", bool(value))

0 → False
0.0 → False
'' → False
[] → False
{} → False
None → False
'0' → True
' ' → True
[0] → True

Empty things and zero are falsy. The string "0" and a list containing 0 are not empty, so they are truthy – a classic interview trick.

None: the absence of a value

None means "nothing here yet" or "no result". Functions that do not return anything give back None. Always test for it with is:

discount = None
if discount is None:
    print("No discount applied")

result = print("hi")   # print() returns None
print(result)

No discount applied
hi
None

Mutable or immutable?

All the types in this lesson are immutable: you cannot change a value in place, you can only create a new one. Lists, dictionaries and sets are mutable. This idea comes back again and again, especially with functions.

Practice

Summary