Data types

What you'll learn

The four basic types of value in Python, how to check a type with type(), and how to convert between them.

Why it matters

Many beginner bugs are type bugs, like trying to add a number to text. Knowing the types explains the error messages.

Explanation

The everyday types:

• int: whole numbers, 7, -3.
• float: numbers with a decimal point, 3.14.
• str: text, "hello".
• bool: True or False (capital first letter).

type(value) tells you the type.

You can convert with int(), float(), str(). This is called casting.

Useful arithmetic: + - * / and also // (whole-number division), % (remainder) and ** (power). Note that / always gives a float.

Example

Step-by-step walkthrough

1. 7 / 2 is 3.5, because / always returns a float.
2. 7 // 2 drops the remainder and gives 3; 7 % 2 is that remainder, 1.
3. 7 ** 2 is seven squared, 49.
4. type() shows int, float, str and bool.
5. int("5") turns the text into the number 5 so adding 3 gives 8. "5" + "3" joins two strings into 53.

Try it yourself

Convert the string "42" to an integer and double it. Then try "42" + 1 and read the error.

Types and castingPython

Editable codetext = "42" print(type(text)) number = int(text) print(number * 2)

Run ResetCopyCtrl+Enter to run

Common mistakes

• Adding text and numbers: "5" + 3 raises a TypeError. Convert first.
• Lower-case booleans: true is not defined. Use True.
• Expecting / to give a whole number. Use // for that.
• Casting text that is not a number: int("abc") raises a ValueError.

Quick quiz

Summary

• Basic types: int, float, str, bool.
• type() checks a type; int(), float(), str() convert.
• / returns a float, // returns a whole number.

Next: making decisions.

a = 7
b = 2

print(a / b)       # 3.5
print(a // b)      # 3
print(a % b)       # 1
print(a ** b)      # 49

print(type(a))     # <class 'int'>
print(type(3.5))   # <class 'float'>
print(type("hi"))  # <class 'str'>
print(type(True))  # <class 'bool'>

text = "5"
print(int(text) + 3)    # 8
print(text + "3")       # 53