Python Casting (Type Conversion)

Casting means converting a value from one type to another. You need it all the time, because data often arrives as text: everything a user types with input, every line read from a file and every value in a CSV table is a string, even when it looks like a number. Before you can calculate with it, you convert it.

The conversion functions

Each built-in type has a function with the same name that converts a value to that type:

print(int("42") + 1)       # text to a whole number
print(float("3.5") * 2)    # text to a decimal number
print(str(100) + " points")  # a number to text
print(int(9.99))           # float to int: the decimals are cut off
print(float(7))
print(int("  15  "))       # spaces around the number are allowed

43
7.0
100 points
9
7.0
15

Note that int(9.99) gives 9: converting a float to an int cuts off the decimals, it does not round. Use round(9.99) when you want 10.

Text and numbers do not mix

Python never converts text to a number, or a number to text, on its own. Adding them together is an error:

age = 21
print("Age: " + age)

Traceback (most recent call last):
  File "main.py", line 2, in <module>
    print("Age: " + age)
          ~~~~~~~~^~~~~
TypeError: can only concatenate str (not "int") to str

The fix is to convert the number with str(age), or to let print join the values with a comma, or to use an f-string, which you will learn in String Formatting.

When a conversion fails

int and float raise a ValueError when the text is not a number of the right form. int("3.5") fails because 3.5 is not a whole number, and int("ten") fails because it is not a number at all. The Try ... Except topic shows how a program can handle this gracefully, for example by asking again.

Booleans

bool converts a value to True or False. Zero, empty text, empty lists and None become False; everything else becomes True:

print(bool(0), bool(5), bool(""), bool("no"), bool([]), bool(None))

False True False True False False

Practice exercises

1. Convert the strings "12" and "30" to numbers and print their sum and their product.
2. Print int(7.9), round(7.9) and float(int(7.9)) and explain each result.
3. Fix the error example above in three different ways.
4. Try int("3.5") and then int(float("3.5")). Why does the second one work?