How to Convert a String to a Number in Python

John on October 01, 2020

Sometimes we will need to convert a string with a numerical value into a number in Python. Fortunately, Python provides built-in functions for converting strings into integers and floats.

In this tutorial, we will learn how to convert strings to numbers in Python.

The int() Function

To convert a numerical string to an integer use the int() function, passing the string to convert inside the () (parenthesis) of the function.

string = '15'

num = int(string)

print(type(num))

<class 'int'>

In the above example, we are evaluating the type of data the string has been converted to using the type() function.

note - if the input string has a floating-point numerical value or any other non-numerical values, int() won't be able to convert it and a ValueError will be thrown:

string = '15.1'

num = int(string)

print(type(num))

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-20-70832f88df42> in <module>
      1 string = '15foo'
      2 
----> 3 num = int(string)
      4 
      5 print(type(num))

ValueError: invalid literal for int() with base 10: '15.1'

The float() Function

To convert a string that contains a floating-point number we can use the float() function, passing the string inside the () (parenthesis).

note - if the number is not a float or an integer, float() will throw a ValueError exception:

string = '15bar'

num = float(string)

print(type(num))

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-25-75435004e715> in <module>
      1 string = '15bar'
      2 
----> 3 num = float(string)
      4 
      5 print(type(num))

ValueError: could not convert string to float: '15bar'

Checking a String is a Number in Python

The above two examples only work if we assume the input string will always contain only an integer or a float. If this is not the case a ValueError will be thrown. To check if all the characters in a string are numerical and therefore an integer use the isnumeric() method.

string = '15bar'

if string.isnumeric():
    num = int(string)
    
    print(num)
else:
    print('Number contains non numerical characters')

Number contains non numerical characters

Checking a String is a Float in Python

The only non-numerical character that a floating-point number contains is a . (dot). Therefore we can replace all instances of . (dot) with nothing using the replace() method before checking it with the isnumeric() method.

string = '15.1'
no_dot = string.replace('.', '')

if no_dot.isnumeric():
    num = float(string)
    
    print(num)
else:
    print('Number is not a float')

15.1

Converting a Number to a String

If the number needs to be converted back into a string, use the str() function, supplying the integer or float inside the () parenthesis of the function.

num = 15

string = str(num)

print(type(string))

<class 'str'>

Printing a Number in a String

A number can't be concatenated inside a string like this:

num = 0

print('The number is: ' + num + '')

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-11-a2b6c9d13ae3> in <module>
      1 num = 0
      2 
----> 3 print('The number is: ' + num + '')

TypeError: can only concatenate str (not "int") to str

The cleanest way to get around this to use an f string, which can accept numbers inside {} (curly brackets).

num = 0

print(f'The number is: {num}')

The number is: 0

Read more about formatting strings in Python.

Conclusion

You now know how to convert strings containing numerical values into floating-point numbers and integers in Python.