Type conversion in Python

Python rarely converts types for you. "5" + 1 is an error, not 6. When you need a number from text, or text from a number, you call a conversion function yourself. The functions have the same names as the types, so int("5") builds an int from the string “5”.

Every example here was run with Python 3.13. The behavior was checked against the official int() and related docs.

Text to number

int() and float() read a string and return a number:

print(int("42"), int("-7"), int("  8  "))
print(float("3.5"), float("1e3"))

42 -7 8
3.5 1000.0

int ignores spaces at both ends of the text. Only digits (and an optional sign) are allowed in the middle.

Pass a second argument to int to read another base. The docs say the allowed bases are 0 and 2 to 36:

print(int("ff", 16), int("1010", 2))

255 10

Number to text

str() turns any value into text. Use it when you want to join text and a number with +:

print(str(42) + "!")
print(str(3.0), str(True))

42!
3.0 True

You can also write f"{age}", which is usually clearer. The f-strings post covers it.

Float to int: truncation

int() on a float cuts off the decimal part, toward zero:

print(int(3.9), int(-3.9))

3 -3

This is not rounding. To round, use round() first, and then convert if you need an int. round() is covered in the number operations post.

True and False as numbers

bool is a subclass of int, so True counts as 1 and False as 0:

print(int(True) + 1)

Truthiness: bool()

bool() returns True or False based on the value’s truth test. The docs say: “If the argument is false or omitted, this returns False; otherwise, it returns True.” These values are false: 0, 0.0, "", [], and None. Everything else is true:

print(bool(0), bool(""), bool([]), bool(None), bool(0.0))
print(bool("0"))

False False False False False
True

"0" is a non-empty string, so it’s true, and so is "False". The check is about the text being empty, not about its value.

Between sequence types

list(), tuple() and set() turn one collection into another:

print(list("abc"))
print(tuple([1, 2]), set([1, 1, 2]))

['a', 'b', 'c']
(1, 2) {1, 2}

A set drops duplicates.

When conversion fails

Some text can’t become a number. Python raises a ValueError:

int("3.5")

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

"3.5" is a float, so int refuses it. Convert with float if you want decimals. An empty string fails the same way:

int("")

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

When the argument has the wrong type, you get a TypeError. For example, None is not text or a number:

int(None)

TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'

Check user input first, or catch the error with try and except (exceptions post).

What’s next

Next: input(), which reads text from the user and needs type conversion right away.