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LESSON 14 OF 344 · VARIABLES

Type Conversion in Python: int(), float() and str()

Lesson Read the full explanation

In Python, every value has a type: int, float, str, and so on. Sometimes you need to convert a value from one type to another. This is called type conversion.

Use these built-in functions to convert between types:

int(x)    # converts x to an integer
float(x)  # converts x to a float
str(x)    # converts x to a string

Here are some examples:

score = 42
score_str = str(score)        # "42"  (integer → string)

weight = 70.8
weight_int = int(weight)      # 70    (float → integer, decimal dropped)

count = "100"
count_int = int(count)        # 100   (string → integer)

Note that converting a float to int simply drops the decimal - it does not round.

int(9.99)  # → 9
int(3.1)   # → 3

Assignment
Follow the Coding Tutorial and convert each variable to a different type!

Hint
Use str(), int(), and float().

STEP 1 OF 3
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    PYTHON · VARIABLES

    Type Conversion in Python, explained

    What type conversion is

    Every value in Python has a type, such as int for whole numbers, float for numbers with a decimal part and str for text. Python does not guess when you mix them up. The number 25 and the text "25" look alike when printed, but they behave differently: you can add two numbers, and you can join two strings, but Python refuses to add a number to a string.

    Type conversion means creating a value of one type from a value of another. In Python you do this by calling the type's name as a function. This is useful whenever data arrives in the wrong form, for example when you read a number from text, or when you need to put a number inside a message.

    The three conversion functions

    The three you need first are int(), float() and str():

    print(int("100"))    # 100
print(float("3.5"))  # 3.5
print(float(7))      # 7.0
print(str(3.0))      # 3.0
print(str(True))     # True

    None of these functions changes the original value. Each one builds and returns a new value, which you normally store in a variable. The original variable keeps its old value and its old type.

    Converting to a string

    str() accepts almost anything and gives back its text form. It is the standard way to put a number into a message:

    print("Age: " + str(25))  # Age: 25

    Without the conversion, "Age: " + 25 raises TypeError: can only concatenate str (not "int") to str.

    Converting a float to an int

    int() applied to a float throws away the decimal part. It does not round. It simply cuts the number off at the decimal point, which means it moves toward zero:

    print(int(9.99))   # 9
print(int(3.1))    # 3
print(int(-9.99))  # -9
print(round(9.99)) # 10

    This surprises many beginners: 9.99 becomes 9, not 10. If you want rounding, use the built-in round() instead.

    Converting a string to a number

    int() and float() read text and turn it into a number. The text must look like a number of the right kind. Spaces and a newline around the digits are allowed, and a leading minus sign is fine:

    print(int("-5"))     # -5
print(int(" 42 "))   # 42
print(float("2.50")) # 2.5

    Common mistakes

    • Converting text that is not a whole number. int("3.7") raises ValueError: invalid literal for int() with base 10: '3.7'. The same error appears for int("abc") and for the empty string. When the text holds a decimal, go through a float first: int(float("3.7")) gives 3.
    • Expecting int() to round. It truncates toward zero, so int(-0.5) is 0, and int(9.99) is 9.
    • Forgetting to save the result. Writing int(price) on a line by itself computes a value and throws it away. You must assign it, as in price_int = int(price).
    • Mixing up text and numbers. "5" + "5" is "55", while int("5") + int("5") is 10. If a calculation gives a strangely long result, check whether you are working with strings.
    • Thinking the variable changed type. After age_str = str(age), the variable age is still an integer. Only age_str is a string.

    The exercise, step by step

    The lesson starts with three variables of three different types: an integer age, a float price and a string year. You add one new variable for each, converting it to a different type.

    1. Convert the integer to a string. Call str() on age and store the result in age_str. The new variable holds the text "25".
    2. Convert the float to an integer. Call int() on price and store it in price_int. Because int() drops the decimal part, 9.99 becomes 9.
    3. Convert the string to an integer. Call int() on year and store it in year_int. The text "2024" is a valid whole number, so this works and gives the number 2024.
    age = 25
price = 9.99
year = "2024"

age_str = str(age)
price_int = int(price)
year_int = int(year)

    To see what you got, print the values and their types:

    print(repr(age_str), type(age_str))
print(price_int, type(price_int))
print(year_int + 1, type(year_int))

    '25' <class 'str'>
9 <class 'int'>
2025 <class 'int'>

    The last line shows why the conversion is worth doing: year could not take part in arithmetic, but year_int can. The original price is still 9.99 and year is still the string "2024".

    Key takeaways

    • Convert with int(), float() and str(). Each returns a new value and leaves the original alone.
    • int() on a float drops the decimal part toward zero. Use round() when you want rounding.
    • int() on text accepts only text that looks like a whole number, otherwise it raises ValueError.
    • Use str() to place numbers inside text.
    • Always assign the result of a conversion to a variable if you want to keep it.
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