Everything the user types is collected by input as a string, which is the data type Python uses for text. That is true even when what they typed looks like a number. Computers cannot do maths with text, so before you can calculate with an answer the user typed, you have to convert it into a number.

Converting a piece of data from one type into another is called casting.

Input is always text

Here is what goes wrong without casting. The user types 12, and the program tries to add 1 to it:

print("How old are you?")
age = input()

print(age + 1)

How old are you?
12
TypeError: can only concatenate str (not "int") to str

Python is telling you that age holds a string, and that a string and a number cannot be added together. The + on that line means “join two pieces of text”, because that is what + does to a string, and there is no sensible way to join the number 1 onto the end of a piece of text.

Worse than an error is a program that runs and gives a nonsense answer. If both sides are strings, Python happily joins them:

print("How old are you?")
age = input()

print(age + age)

How old are you?
12
1212

That is not 24. It is the text 12 written down twice.

int() and float()

int() converts a piece of data into a whole number, and float() converts it into a decimal number. Wrap the input in whichever one you need, and the variable holds a number from the very start:

print("How old are you?")
age = int(input())

print(age + 1)

How old are you?
12
13

Read that line from the inside out: input collects the text, int converts that text into a whole number, and = assigns the number to age. The two closing brackets at the end are both needed, one for input and one for int.

Use float() whenever the number can have a decimal point in it, such as a price or a measurement. Here the user types 23.50:

print("What did it cost?")
price = float(input())

print(price * 2)

What did it cost?
23.50
47.0

str() converts back

Casting works in the other direction too. To join a number onto a string with +, you first need a string version of that number, and str() makes one:

num1 = 12

print("You picked number " + str(num1))

You picked number 12

str(num1) does not change what is stored in num1. It produces a string copy for use on that one line, and the variable itself still holds the number.

Try it yourself

Run the code below and type a number when it asks. Then remove the int() from the second line and run it again, to see the error that casting was preventing.

print("Enter a number and I will double it.")
num1 = int(input())

result = num1 * 2

print("Twice " + str(num1) + " is " + str(result))