CODESARRAY

Lesson 2, by Codesarray, , 5 min read

Python variables and data types: int, float, str, bool, None

The five basic types, how to ask Python what type a value is, and how to turn one type into another.

Every value in Python has a data type. By the end of this page you will know the five basic Python data types, how to ask Python what type a value is with the type() function, and how to turn one type into another.

What a variable is

A variable is a name for a value. Write the name, an equals sign, and the value:

score = 10

Python stores 10 and calls it score. There is no keyword and no type to declare. Use the name and you get the value back. Assign again and the name points to the new value:

print(score)
score = score + 5
print(score)
10
15

The five basic types

Every value has a type.

  • 42 is an int: a whole number.
  • 3.14 is a float: a number with a decimal point.
  • "hello" is a str, short for string: text in quotes.
  • True and False are bool.
  • None means no value at all.

Ask Python with the type function and it tells you:

>>> type(42)
<class 'int'>

Dynamic typing

The type belongs to the value, not to the name. So the same name can hold a number now and text later:

x = 5
x = "five"

Python checks types while the program runs. That is called dynamic typing.

Arithmetic

Plus, minus and times work as you expect. Division has two forms, and the difference matters:

>>> 7 / 2
3.5
>>> 7 // 2
3
>>> 7 % 2
1
>>> 2 ** 10
1024

A single slash always gives a float. A double slash drops the fraction. Percent gives the remainder, and two stars is a power.

Comparisons and logic

A comparison asks a question and answers with a bool. A double equals tests whether two values are equal; a single equals assigns. Combine answers with and, or and not:

>>> age = 36
>>> is_admin = True
>>> age >= 18
True
>>> age == 36
True
>>> age >= 18 and not is_admin
False

Converting between types

Types do not mix by themselves. Adding text to a number is an error:

>>> "3" + 4
TypeError: can only concatenate str (not "int") to str

Convert first. int turns text into a whole number, str turns a number into text, and float handles decimals:

>>> int("3") + 4
7
>>> "3" + str(4)
'34'
>>> float("3.5")
3.5

The whole program

name = "Ada"
age = 36
height = 1.68
is_admin = True
nickname = None

print(type(name), type(age))
next_year = age + 1
half = age / 2
print(next_year, half)

print(age >= 18 and not is_admin)
print("Age: " + str(age))
print(int("42") + 8)

Lines 1 to 5 store five values, one of each type. Line 7 prints two of the types. Line 8 adds one to age and line 9 halves it; the slash makes a float. Line 12 asks a question and prints the answer. Line 13 converts age to text so it can join the label, and line 14 converts text to a number before adding.

<class 'str'> <class 'int'>
37 18.0
False
Age: 36
50

What you learned

  • int, float, str, bool and None are the basic types.
  • type(x) tells you which one you have.
  • 7 / 2 gives a float; 7 // 2 gives a whole number.
  • Comparisons give a bool.
  • int(), float() and str() convert between types.

Common questions

**Why does 7 / 2 give 3.5 and not 3?** In Python a single slash is true division and always gives a float, even when the numbers divide exactly: 8 / 2 is 4.0. When you want the whole-number part, use //, which gives 3 for 7 // 2.

**Why does my number print as 18.0 instead of 18?** Because it came from a division. age / 2 is a float, so it prints with a decimal point. Use age // 2 if you want an int, or int(age / 2) to convert the result.

**What is the difference between = and ==?** A single equals assigns: age = 36 stores 36 under the name age. A double equals compares: age == 36 asks whether age is 36 and answers True or False. Writing = where you meant == inside an if is a syntax error, so Python will tell you.

**Is None the same as 0 or an empty string?** No. None is its own value, meaning "nothing here". None == 0 is False, and so is None == "". Use None when a variable has no value yet, and a real 0 or "" when the value is genuinely zero or empty.

Do I have to declare a type like in other languages? No. The type belongs to the value, and Python reads it from the value you assign. The same name can hold an int now and a str later. That freedom is why conversions matter: Python will not guess what you meant by "3" + 4.

This lesson follows on from Your first Python program. Next in the series: control flow.

This article goes with the video Variables and types. Watch it on YouTube.