Converting Between Numbers and Strings

Why type conversion matters

In Python, you can't just add or compare values of different types as-is.

The classic case is a mix of numbers (int / float) and strings (str). For example, "25" looks like the number 25 at a glance, but to Python it's just two characters lined up — it can't be used in arithmetic.

This is where type conversion (casting) comes in to line up the types. Once you're comfortable with str() / int() / float(), you can freely convert between numbers and strings.

First, let's see what happens if you don't match the types — check the error message. Run the code as-is.

Number → String (str())

When you pass a number to str(), you get back a string that represents that number.

Whether it's an int, float, or bool, str() turns almost anything into a human-readable string form.

age = 25
temp = -20.5

# Number -> String
print(str(age))        # '25'
print(type(str(age)))  # <class 'str'>

# Once it's a string, + can join it
print("Age: " + str(age) + " yrs")         # Age: 25 yrs
print("Current temp is " + str(temp) + " deg") # Current temp is -20.5 deg

When to use f-strings instead

Inside an f-string (f"Age: {age} yrs"), values are automatically converted to str, so you don't need to call str() yourself. str() shines when you're building strings with + or .join(), or when you want to lock something in as a string — filenames, log lines, JSON keys, and the like.

Using user = "Alice" and score = 95, build a one-line report.

① Using only str() and +, print "Alice's score is 95".

② Write the same thing with an f-string, without calling str().

String → Number (int() / float())

For the other direction, use int() and float(). Even if a value looks like a string, as long as its contents are a number, you can convert it and use it in calculations.

Things like user input, CSV columns, and API responses often arrive as str, so converting them to numbers is something you'll do a lot.

quantity = "12"
price    = "198.5"

# String -> int
q = int(quantity)
print(q + 3)            # 15 (works as a number)

# String -> float
p = float(price)
print(p * q)            # 2382.0 (float, so it has .0)

print(type(q), type(p)) # <class 'int'> <class 'float'>

Non-numeric strings raise ValueError

Passing something that isn't readable as a number, like int("abc") or float("hello"), raises ValueError and stops your program. When converting user input directly, the usual patterns are to check it first with isdigit() or to catch it with try / except (covered in a later chapter).

Let's compute the total for a shopping cart. All the values come in as strings.

You're given unit_price_str = "198", count_str = "5", and a tax rate TAX_RATE = 0.1.

① Convert the count and unit price to numbers with int().

② Multiply the total (unit price × count) by the tax rate and print() the price with tax.

Common conversion patterns

Finally, here are three conversion patterns that trip people up in practice.

int("3.14") raises ValueError. That's because int() only accepts strings that look like integers, so a string with a decimal point needs to be run through float() first and then passed to int() — two steps.

Also note that int() truncates (drops the decimal part). If you want rounding, use round().

You want to use the price string price_str = "1980.6" as an integer.

① Run int(price_str) as-is and confirm the error, then comment it out.

② Convert it to an integer through float() and print() the result.

In this article, we walked through type conversion with str() / int() / float(). An easy way to remember it: number → string is for building display output, and string → number is for processing input and data.