Convert User Input to a Number

In Python 3.x, the input() function parse user input as a string even if it contains only digits.

>>> import sys
>>> data=input("Enter a Value: ")
Enter a Value: 100
>>> data
'100'
>>> type(data)
<class 'str'>
>>> data=input("Enter a Value: ")
Enter a Value: Hello
>>> data
'Hello'
>>> type(data)
<class 'str'>

How do we ensure a numeric input from the user? Most common alternative is to parse return value of the input() function to integer with int() function

>>> data=int(input("Enter a Number: "))
Enter a Number: 100
>>> data
100
>>> type(data)
<class 'int'>

However, this is prone to error. If the user inputs non-numeric data, ValueError is raised.

>>> data=int(input("Enter a Number: "))
Enter a Number: hello
Traceback (most recent call last):
  File "<pyshell#34>", line 1, in <module>
    data=int(input("Enter a Number: "))
ValueError: invalid literal for int() with base 10: 'hello'

This can be taken care of by Python's exception handling technique. The following code keeps on asking for user input till an integer number is given.

while True:
    try:
        data=int(input("Enter a Number: "))
        print ("You entered: ", data)
        break;
    except ValueError:
        print ("Invalid input")

Enter a Number: hello
Invalid input
Enter a Number: abcd
Invalid input
Enter a Number: 100
You entered: 100

You can use the built-in float() function if a floating-point number is expected to be input.

Another method is to use the eval() function. Apart from other applications of this built-in function, it is a convenient tool to check if the input is a valid number. In case it is not, the Python interpreter raises NameError

while True:
    try:
        data=eval(input("Enter a Number: "))
        print ("You entered: ",data)
        break;
    except NameError:
        print ("Invalid input")

Enter a Number: hello
Invalid input
Enter a Number: abcd
Invalid input
Enter a Number: 12.34
You entered: 12.34

Convert Input to Number in Python 2.x

Python 2.x has two built-in functions for accepting user input. the raw_input() and input(). The input() function is intelligent as it judges the data type of data read, whereas the raw_input() always treats the input as a string. So, always use the input() function in Python 2.x.

>>> data=input("enter something : ")
enter something : 100
>>> data
100
>>> type(data)
<type 'int'>
>>> data=input("enter something : ")
enter something : Hello'
>>> data
'Hello'
>>> type(data)
<type 'str'>

Malhar Lathkar

Author

Malhar Lathkar is an independent software professional having 30+ years of experience in various technologies such as Python, Java, Android, PHP, and Databases. He is an author of Python Data Persistence: With SQL and NOSQL Databases