How to convert Python strings to floats

Con­vert­ing a Python string into a float can help you avoid errors that result from including non-numeric strings in arith­metic op­er­a­tions.

What kind of Python strings can be converted to floats?

If you want to convert a Python string to a float, the string needs to have a valid numerical rep­re­sen­ta­tion as a floating-point value. This means that it can contain numbers in decimal format, which includes decimal points and sci­en­tif­ic notation (such as "3.14" or "2.7e-3"). However, it can’t contain ad­di­tion­al char­ac­ters, spaces or char­ac­ters that don’t represent numerical values. If it does, the con­ver­sion won’t work.

String–float con­ver­sion is a basic function and is included in every version of Python. However, there might be subtle dif­fer­ences in how some methods and functions work in different versions. Be sure to keep an eye out for errors that can arise during con­ver­sions. For example, entering a string that doesn’t represent a valid number will give rise to an error. And special values like NaN (Not a Number) and Infinity can’t be converted into floats. That means de­vel­op­ers working with con­ver­sions need to be able to handle errors ap­pro­pri­ate­ly. Using try/except blocks to catch and react to errors is one way to do this.

How to convert Python strings with float()

The float() function is a built-in Python method that converts strings to floats. It’s often used to change user input from strings to numeric formats so that the input can be used in cal­cu­la­tions or numerical op­er­a­tions. In programs that work with various kinds of data, float() is used to ensure that data remains con­sis­tent by con­vert­ing values into a numerical format.

To convert Python strings to floats, use float() and enter a valid string as the argument:

str1 = "3.1416"
float_value = float(str1)
print(float_value) # Output: 3.1416

You can also convert other numerical types like int and complex into a floating number by running them through float().

Try/except blocks can be used to catch invalid user input when con­vert­ing strings to floats:

while True:
    user_input = input("Enter a number: ")
    try:
        float_value = float(user_input)
        print("Entered number as float:", float_value)
        break # Exit the loop if the conversion was successful
    except ValueError:
        print("Invalid input! Please enter a numeric value.")
        # The loop continues to prompt for valid input

The above code is a loop that prompts users to enter a number. It then attempts to convert that value into a floating number. In the try block, float(user_input) is meant to convert the string entered by the user into a float. If the con­ver­sion was suc­cess­ful, the number is stored under float_value and returned. The break statement then ter­mi­nates the loop and moves on with the program.

If a Val­ueEr­ror occurs when con­vert­ing the string to a float, the code in the except block is executed. This can happen if the user enters a value that can’t be converted into a float, like a letter. In this case, a message says that the input is invalid and the loop requests valid input again.

How to convert Python strings to floats with NumPy

If you want to convert a Python string to a float with NumPy, you can use the function numpy.float64(). It will give you a 64-bit floating number.

import numpy as np
string_value = "3.1416"
float_value = np.float64(string_value)
print(float_value) # Output: 3.1416

First you’ll need to import NumPy in your script if it’s not already installed. You can then use NumPy to apply the function numpy.float64() to the string string_value to convert it into a 64-bit floating number.

How to use str()

If, on the other hand, you want to convert a float into a string, you can use the str() function. This function converts other values into strings.

float_number = 3.1416
string_number = str(float_number)
print(string_number) # Output: "3.1416"

In the above example, the float value 3.1416 is converted into the string "3.1416" using str().

str() is also useful for con­cate­nat­ing several float values with f-strings (a Python string format).

float_value1 = 3.1416
float_value2 = 2.7182
float_value3 = 1.6180
string_concatenation = f"Concatenation of: {str(float_value1)}, {str(float_value2)}, and {str(float_value3)}"
print(string_concatenation) # Concatenation of: 3.1416, 2.7182, and 1.6180

Note that when con­vert­ing floats to strings with str(), the resulting string will then be rep­re­sent­ed as text and will no longer be available for numerical cal­cu­la­tions or math­e­mat­i­cal op­er­a­tions. To use it in cal­cu­la­tions, you’ll have to turn it back into a numerical form (like a float).