Type casting

Lesson 8 of 62, about 3 min

Casting means converting a value from one type to another. Python never converts between text and numbers silently, so you will do this often, especially with user input and file data.

The constructor functions

Function	Converts to	Notes
`int(x)`	integer	Truncates floats, parses whole number strings
`float(x)`	float	Parses decimal strings
`str(x)`	string	Works on anything
`bool(x)`	boolean	See the truthiness rules in the Booleans lesson
`list(x)`, `tuple(x)`, `set(x)`	collection	Works on any iterable

Numbers to numbers

python

print(int(3.99))      # 3, decimal part dropped
print(int(-3.99))     # -3, truncates towards zero
print(float(7))       # 7.0
print(round(3.99))    # 4, if you want rounding instead

Strings to numbers

python

age = int("21")
price = float("49.99")
print(age + 1, price * 2)

Whitespace around the number is fine, but anything else raises a ValueError:

python

print(int("  42  "))
try:
    int("42 rupees")
except ValueError as e:
    print("Error:", e)

int("3.7") also fails because the string is not a whole number. Convert to float first if you need to: int(float("3.7")).

Numbers to strings

The most common need is building a message. str() works, but f-strings are cleaner:

python

count = 3
print("You have " + str(count) + " new messages")
print(f"You have {count} new messages")

Strings and lists

python

print(list("hello"))
print("".join(["h", "e", "l", "l", "o"]))
print("a,b,c".split(","))

Between collections

python

numbers = [3, 1, 3, 2, 1]
print(set(numbers))          # removes duplicates
print(tuple(numbers))
print(sorted(set(numbers)))  # unique and ordered

Reading input

input() always returns a string, even if the user types digits. Cast it before doing maths.

python

raw = input("How many tickets? ")
tickets = int(raw)
print("Total:", tickets * 250, "rupees")

Practice

Convert the string "3.14159" to a float, multiply by 2, and print the result rounded to 3 decimal places.
Take the list ["10", "20", "30"] and produce the integer sum 60.
What does bool("False") return? Run it and explain why.