Type Conversion

A currency exchange booth turns pesos into dollars. You get the same value, just in a form the next shop will accept.

In Numbers, Strings, and Booleans, Python refused to run "Age: " + 17. It won't join text and a number, because it can't be sure what you mean. And in the next lesson, you'll read what a user types, which always arrives as text, even when they type a number.

Both problems have the same fix: converting a value from one type to another, on purpose.

A currency exchange

When you travel, a shop in another country won't take your pesos. So you go to an exchange booth, hand over pesos, and get dollars back. The value is (roughly) the same; it's just in a form the shop accepts.

Sometimes the exchange isn't perfect. Coins often can't be exchanged, so the booth rounds your money down. And if you hand over something that isn't money at all, like a bus ticket, the booth refuses.

Python's conversion functions are exchange booths:

Function	Converts to	Example	Result
int(x)	a whole number	int("42")	42
float(x)	a decimal	float("3.5")	3.5
str(x)	text	str(17)	"17"
bool(x)	True or False	bool(0)	False

Each one is named after the type it produces, and each gives back a new value. The original stays exactly as it was.

Numbers to text: str()

To join a number to text with +, turn the number into text first:

age = 17
print("Age: " + str(age))

Age: 17

str(age) gives back the string "17", and two strings join happily. (Separating with commas in print, or using an f-string, often reads more nicely. You'll see f-strings in Formatting Output with f-strings.)

Text to numbers: int() and float()

Going the other way is how you'll turn typed input into numbers you can do math with:

typed_age = "17"
typed_height = "1.62"

age = int(typed_age)
height = float(typed_height)

print(age + 1)
print(height * 100)

18
162.0

Without the conversion, typed_age + 1 would be an error, and typed_age * 2 would give "1717", the string repeated twice.

int() and float() ignore spaces at the start and end, so int(" 42 ") is 42. But the text has to look like a number of the right kind. If it doesn't, the booth refuses:

int("seventeen")  # error: ValueError: invalid literal for int() with base 10: 'seventeen'

int("3.5")  # error: ValueError: invalid literal for int() with base 10: '3.5'

The second one surprises people. "3.5" is a perfectly good number, but it isn't a whole number, so int() won't take it. Use float("3.5") instead. (A ValueError means "right kind of value, but this particular one doesn't work." In Exceptions, you'll learn to catch it and ask the user again instead of crashing.)

Decimals to whole numbers: what gets lost

int() also converts a float to a whole number. But like the exchange booth with your coins, it doesn't round. It just chops off everything after the decimal point:

print(int(9.99))
print(int(-9.99))

9
-9

If you want the nearest whole number, use round() instead:

print(round(9.99))
print(round(2.4))

10
2

round() can also keep some decimal places. round(3.14159, 2) gives 3.14.

One oddity to know about: when a number is exactly halfway, round() goes to the nearest even number. So round(2.5) is 2, but round(3.5) is 4. This is called "banker's rounding," and it avoids always rounding halves up, which would slowly push totals too high. For everyday display, you'll rarely notice.

Anything to True or False: bool()

bool() turns any value into True or False. The rule is short: empty or zero values become False, and everything else becomes True.

print(bool(0), bool(0.0), bool(""), bool(None))
print(bool(5), bool(-1), bool("hi"), bool("False"))

False False False False
True True True True

Look at that last one: bool("False") is True. The string isn't empty, so it counts as true. Python doesn't read the word inside it.

Values that act like False are called falsy, and the rest are truthy. This matters in Making Decisions, where you can write if name: to mean "if the name isn't empty."

Checking before converting

Sometimes you want to know whether text can become a number before you try. Strings have a method called isdigit() that answers True when every character is a digit:

print("2026".isdigit())
print("20x6".isdigit())
print("".isdigit())

True
False
False

It doesn't handle minus signs or decimal points, though: "-5".isdigit() is False. For anything beyond simple whole numbers, the try/except approach you'll learn in Exceptions is the reliable way.

Try it

A class of 7 students collected 250 bottles for recycling, and the goal arrived as text. This program converts in every direction. Predict each line.

Running Python here

Running Python right in the browser is coming to Syntaxia soon. For now, you can read and edit the code here. If you have Python installed on your computer (see Setting Up), save it as main.py and run it with python main.py (or python3 main.py on macOS and Linux).

Check your prediction

Exact share: 35.714285714285715
Chopped: 35
Rounded: 36
Two decimals: 35.71
Still needed: 50
Goal as text times 2: 300300
Goal as number times 2: 600
Any bottles left to find? True

int(share) chops 35.71 down to 35, while round goes up to 36. The text "300" times 2 repeats the text, but the number 300 times 2 is 600. And goal - bottles is 50, which isn't zero, so bool gives True.

Try it yourself

1. Change typed_goal to "three hundred". What would happen when the program runs, and which line would fail? Change it back.
2. Predict bool(" "), a string containing just one space. Then check it in the interactive shell. Why is it that value?
3. Write a line that converts the text "19.5" into a number, adds 0.5, and prints the result as a whole number: 20.

Check your understanding

What does int(7.9) give you?

8

7.9

7

An error

What happens with int("3.5")?

A ValueError, because the text is not a whole number

It gives 3

It gives 4

It gives 3.5

What is bool("False")?

False

An error

None

True

Up next

So far, every value in your programs was typed straight into the code. In Reading User Input with input(), your programs start asking the user for values instead, and int() and float() will come in handy right away.