How to Parse a String to int or float in Python

By the end of this page, you will know how to convert numeric strings into int and float values in Python using the built-in int() and float() functions. You will also learn how these conversions behave, how to handle invalid input, and when to choose one numeric type over the other.

In Python, values read from places like user input, files, APIs, and CSV data often arrive as strings. Even if a string looks like a number, such as "31" or "545.2222", Python still treats it as text until you explicitly convert it.

The main tools for this are:

• int() for whole numbers
• float() for decimal numbers

Examples:

age = int("31")
price = float("545.2222")

This matters because numbers and strings behave differently:

"3" + "4"    # "34"  (string concatenation)
3 + 4         # 7     (numeric addition)

If you do not convert a numeric string first, your program may produce the wrong result or raise an error.

A few important rules:

• int("31") works because the string represents a valid integer.
• float("545.2222") works because the string represents a valid floating-point number.
• int("545.2222") fails because that string is not a valid integer literal.

Think of a string as a label on a box, and a number as the actual item inside the box.

• "31" is a text label containing the characters 3 and 1
• 31 is an actual integer value that Python can do math with

The functions int() and float() are like unpacking tools:

• int() opens the box and turns the text into a whole number
• float() opens the box and turns the text into a decimal number

If the label does not describe a valid number, Python cannot unpack it and raises an error.

Basic syntax

int(string_value)
float(string_value)

Convert a string to an integer

count = int("31")
print(count)
print(type(count))

Output:

31
<class 'int'>

Convert a string to a float

price = float("545.2222")
print(price)
print(type(price))

Output:

545.2222
<class 'float'>

Converting user input

user_age = int(input("Enter your age: "))
print(user_age + 1)

Consider this example:

text = "42"
number = int(text)
result = number + 8
print(result)

Step-by-step

1. text = "42"

   • The variable text stores a string, not a number.

2. number = int(text)

   • Python reads the string "42".
   • It recognizes that this is a valid integer representation.
   • It converts it to the integer 42.

3. result = number + 8

   • Now number is an integer.
   • Python performs numeric addition: 42 + 8.
   • The result is 50.

4. print(result)

   • The program prints:

String-to-number conversion appears everywhere in Python programs.

User input

quantity = int(input("How many items? "))

Used in command-line tools, scripts, and beginner programs.

Reading CSV or text files

row = ["Alice", "29", "88.5"]
age = int(row[1])
score = float(row[2])

CSV data is often read as strings first.

API responses

Some APIs send numbers as strings:

data = {"price": "19.99"}
price = float(data["price"])

Configuration values

Environment variables are strings by default:

import os
port = int(os.getenv("PORT", "8000"))

Data cleaning

When processing imported data, numeric fields may need conversion before sorting, filtering, or calculating totals.

In real projects, developers rarely convert values blindly. They usually combine conversion with validation and error handling.

Validation before use

value = "42"
number = int(value)
if number > 0:
    print("Valid positive number")

Guarding against bad data

def parse_age(text):
    try:
        return int(text)
    except ValueError:
        return None

This pattern is common when reading forms, query parameters, files, or API payloads.

Early return pattern

def process_discount(text):
    try:
        discount = float(text)
    except ValueError:
        return "Invalid discount value"

    if discount < 0:
        return "Discount cannot be negative"

    return discount

1. Forgetting that input() returns a string

Broken code:

age = input("Enter age: ")
print(age + 1)

This fails because age is a string.

Correct version:

age = int(input("Enter age: "))
print(age + 1)

2. Using int() on a decimal string

Broken code:

value = int("12.5")

This raises ValueError.

Correct version:

value = float("12.5")

Or, if you intentionally want an integer:

value = int(float())

int() vs float()

Quick reference

int("31")        # 31
float("31")      # 31.0
float("545.2222")# 545.2222
str(31)           # "31"

Rules

• int() converts valid integer strings to integers.
• float() converts valid numeric strings to floating-point numbers.
• input() always returns a string.
• int("12.5") raises ValueError.
• int(12.5) returns 12 by truncation.
• round(12.5) is for rounding, not parsing.

Safe parsing pattern

try:
    num = int(text)
except ValueError:
    num = None

Common conversions

How do I convert a string to an integer in Python?

Use int():

num = int("31")

How do I convert a string to a float in Python?

Use float():

num = float("545.2222")

Why does int("12.5") fail?

Because "12.5" is not a valid integer string. Parse it with float() first if needed.

Does input() return a number in Python?

No. input() always returns a string.

What error happens if the string is not numeric?

Python raises ValueError.

Does int() round numbers?

No. It truncates the decimal part.

Can float() parse whole numbers too?

• str() conversion — the reverse operation, turning numbers into strings.
• input() — important because user input is always returned as text.
• ValueError — the exception raised when a string cannot be parsed as a number.
• try/except — used to safely handle invalid conversions.
• Numeric types in Python — helps explain the difference between int and float.
• round() — related because beginners often confuse rounding with integer conversion.
• Type checking with type() — useful for confirming what kind of value you have.