Input and Type Conversion

Definition

Why does adding the input "5" to "5" give "55" instead of 10?

In simple words

Everything typed on a keyboard arrives as text, even digits. If someone types 15 for their age, Python gets "15". To do maths with it, convert it: int("15") is the number 15. To join a number into a sentence, convert the other way: str(15).

Why it matters

Data from users, files and websites usually arrives as text. Converting it correctly, and handling text that isn't a valid number, is a daily task in every AI project.

How it works

In code

The age arrives as text, is converted to a number, used in maths, then converted back to text for the message.

# On your computer: age_text = input("Your age: ")
age_text = "15"
print("Typed:", repr(age_text), "of type", type(age_text).__name__)

age = int(age_text)
print("In 10 years you will be", age + 10)
print("Days lived, roughly:", age * 365)

message = "Age " + str(age) + " confirmed"
print(message)

print(int(3.9), float("2.5"), bool(0), bool("hi"))

Typed: '15' of type str
In 10 years you will be 25
Days lived, roughly: 5475
Age 15 confirmed
3 2.5 False True

Try it yourself

Common mistakes

• Forgetting that input() always returns a string.
• Expecting int() to round. int(3.9) is 3.
• Converting text that isn't a number, such as int("twelve"): ValueError.

Check yourself

Summary

• input() returns text, always.
• int(), float(), str() and bool() convert between types.
• Converting invalid text raises ValueError.