Python Type Casting

Type casting means changing a value from one data type to another. For example, the text "25" is a string, and with type casting you can turn it into the number 25 so that you can do maths with it.

Python does this with built-in functions like int(), float() and str(). You should already know the basic types, so read Python Data Types first if needed.

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Implicit and explicit type conversion

There are two ways a conversion happens in Python.

• Implicit conversion: Python changes the type by itself, when it is safe to do so.
• Explicit conversion: You change the type yourself by calling a function. This is what we call type casting.

Implicit conversion

When you add an integer and a float, Python converts the integer to a float automatically. So no data is lost.

a = 5          # int
b = 2.5        # float
c = a + b

print(c)
print(type(c))

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Type casting functions

Function	What it does
int()	Converts to an integer
float()	Converts to a floating point number
str()	Converts to a string
bool()	Converts to True or False
list()	Converts to a list
tuple()	Converts to a tuple
set()	Converts to a set
dict()	Converts to a dictionary
complex()	Converts to a complex number
ord() and chr()	Character to number and number to character
hex(), oct(), bin()	Integer to hexadecimal, octal and binary string

int() function

The int() function converts a float, or a string that has a whole number, into an integer. It does not round the number, it just cuts the decimal part. You can also give a base as the second argument.

print(int(7.9))
print(int(-7.9))
print(int("321"))
print(int("101", 2))   # binary 101 to decimal

float() function

The float() function converts an integer or a numeric string into a float.

print(float(5))
print(float("3.14"))
print(float("2"))

str() function

The str() function converts any value into a string. It is very useful when you want to join a number with text, because Python does not do that for you.

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

Convert the number with str() and the error goes away.

age = 25
print("Age: " + str(age))
print(str(3.5))
print(str([1, 2]))

bool() function

The bool() function gives False for zero, empty strings and empty collections, and True for almost everything else.

print(bool(0))
print(bool(""))
print(bool([]))
print(bool("False"))
print(bool(5))

Note that bool("False") is True, because the string is not empty.

list(), tuple(), set() and dict()

These functions change one collection into another. A string is also a collection of characters.

print(list("hi"))
print(tuple([1, 2, 3]))
print(set([1, 2, 2, 3]))
print(dict([("a", 1), ("b", 2)]))

When you convert a list to a set, the duplicate values are removed, and the order is not guaranteed. Also you cannot convert a single number like list(5), because a number is not iterable.

ord() and chr() functions

ord() gives the Unicode (ASCII) number of a character, and chr() does the opposite.

print(ord("a"))
print(ord("A"))
print(chr(97))

hex(), oct() and bin() functions

These functions take an integer and return a string that shows the number in another number system.

print(hex(31))
print(oct(31))
print(bin(5))

complex() function

print(complex(2))
print(complex(21.3))

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Type casting with user input

The input() function always returns a string. So when you ask for a number, you must convert it.

age = int(input("Enter your age: "))
print("Next year you will be", age + 1)

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Summary

• Type casting means converting a value from one type to another.
• Python converts types implicitly when it is safe, like int to float in addition.
• Use int(), float(), str() and bool() for simple values.
• Use list(), tuple(), set() and dict() for collections.
• A wrong conversion, like int("abc"), raises a ValueError. Handle it with try and except.

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