Python Type Conversion (Casting)

Data doesn't always arrive in the type you need. A price typed into a web form comes in as text, but you need a number to do math with it. Python type conversion is how you change a value from one type to another, for example turning the text "19" into the number 19. In this lesson, you'll learn which conversions Python does by itself, how to convert values yourself, and how to avoid the errors that trip up nearly every beginner.

What is type conversion in Python?

Type conversion means taking a value of one type (like str) and producing a value of another type (like int). It's also called casting. There are two kinds:

• Implicit conversion: Python converts automatically, behind the scenes.
• Explicit conversion: you ask for it, using a function like int() or str().

If you need a refresher on the types themselves, see Python data types.

Implicit type conversion: what Python does for you

When you mix an int and a float in a calculation, Python quietly converts the int to a float so no precision is lost. The result is always a float.

items = 3
price = 2.5

total = items * price
print(total)
print(type(total))

7.5
<class 'float'>

Booleans join in too, since True counts as 1 and False as 0 (you saw this in Python booleans):

print(True + 2)
print(4 + 0.5)

3
4.5

Python only does this for types where the meaning is obvious. It will never silently turn text into a number or a number into text. You have to do that yourself.

Explicit type conversion: int(), float(), str() and bool()

Python gives each basic type a function with the same name. Pass in a value, and you get back a new value of that type. The original value doesn't change.

int(): convert to a whole number

print(int("42"))    # text to int
print(int(9.99))    # float to int
print(int(True))    # bool to int

42
9
1

Notice that int(9.99) gives 9, not 10. int() doesn't round; it simply chops off everything after the decimal point.

float(): convert to a decimal number

print(float(5))
print(float("3.14"))
print(float("  2.5  "))

5.0
3.14
2.5

int() and float() both ignore spaces at the start and end of a string, which is handy for messy input.

str(): convert to text

bottles = 99
label = str(bottles) + " bottles"
print(label)

99 bottles

bool(): convert to True or False

print(bool(1))
print(bool(0))
print(bool("no"))

True
False
True

Remember: bool() gives False only for zero, empty values and None. Even the text "no" is True.

list(): turn something into a list

Other types have conversion functions too. For example, list() splits a string into its individual characters:

print(list("cab"))

['c', 'a', 'b']

Python type conversion table

Why does int("3.7") give a ValueError?

This one surprises almost everyone. int(3.7) works fine, but int("3.7") fails:

print(int("3.7"))

ValueError: invalid literal for int() with base 10: '3.7'

When int() receives a string, the string must look exactly like a whole number. A decimal point isn't allowed. The fix is to convert in two steps: first to a float, then to an int.

print(int(float("3.7")))
print(round(float("3.7")))

3
4

Use int() if you want to drop the decimals, or round() if you want the nearest whole number.

Does int() round or truncate?

int() truncates toward zero, which means it removes the decimal part and moves closer to zero. For positive numbers that looks like rounding down, but for negative numbers it rounds up:

print(int(3.7))
print(int(-3.7))

3
-3

If you need true "round down" behavior for negative numbers, use math.floor(-3.7), which gives -4.

How do you fix "can only concatenate str (not int) to str"?

This is probably the most common error in beginner Python. It happens when you try to join text and a number with +:

age = 25
print("Age: " + age)

TypeError: can only concatenate str (not "int") to str

With strings, + means "glue together", but Python can't glue a number to text without knowing what you want. Here are three fixes:

age = 25

print("Age: " + str(age))   # 1. convert the number to text
print("Age:", age)          # 2. let print() separate the values
print(f"Age: {age}")        # 3. use an f-string

Age: 25
Age: 25
Age: 25

An f-string is a string with an f before the quotes; anything inside {} is replaced by its value. You'll learn more about them in the print() lesson.

Real-world example: values from a web form

Online forms send every field as text. Here, an order form has sent a price and a quantity, and we want the total plus a converted amount in another currency.

price_text = "12.50"     # from the form
quantity_text = "3"      # from the form
exchange_rate = 0.92     # 1 dollar in euros

price = float(price_text)
quantity = int(quantity_text)

total = price * quantity
in_euros = round(total * exchange_rate, 2)

print("Total in dollars:", total)
print("Total in euros:", in_euros)

Total in dollars: 37.5
Total in euros: 34.5

Without the conversions, price_text * quantity_text would fail, because you can't multiply two strings together.

Common mistakes with type conversion

1. Expecting int() to round

int(4.9) is 4. Use round(4.9) if you want 5.

2. Converting text that isn't a number

print(float("ten"))

ValueError: could not convert string to float: 'ten'

The text must contain digits Python can read. Words like "ten" or symbols like "$10" won't work.

3. Forgetting to store the converted value

score = "80"
int(score)
print(score + 5)

TypeError: can only concatenate str (not "int") to str

int(score) creates a new value but doesn't change score. Save it: score = int(score).

4. Adding numbers that are still strings

print("5" + "3")

53

No error, but the wrong answer: the two strings were glued together. Convert first: int("5") + int("3") gives 8.

Practice exercise

monday = "5.2"
wednesday = "3.8"
saturday = "6"

total = float(monday) + float(wednesday) + float(saturday)
print("Total: " + str(int(total)) + " km")

Total: 15 km

What's next?

Now you can convert between types, which is exactly what you need when your program asks the user a question. Next, learn how to read what people type with Python user input.