String to Number Type Conversion in Python

Today we’ll be talking about numeric data type conversion, and in particular about converting string representations of numbers to numbers.

From user input we get strings into the program:

 >>> a = input("What's your name?")
 What's your name? Bill 

But what if you need a number? For example:

 >>> a = input("Enter a number: ")
 Enter a number: 42

 >>> a   # we can print the number
 '42' 

But you can’t use the number in calculations because technically it’s a string, not a number:

 >>> a * 10   # what we get is repetition
 '42424242424242424242' 

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If you want the input to be treated as a number you must cast it to an int or float. You can use the int or float function to achieve that:

 >>> x = input("Enter a number: ")
 Enter a number: 5

 >>> int(x) * 10
 50

 >>> float(x) + 3.48
 8.48 

You can cast strings to numbers directly in user’s input:

 >>> x = int(input("Enter x: "))
 Enter x:
 12

 >>> y = int(input("Enter y: "))
 Enter y:
 4

 >>> x * y - x
 36 

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Using the eval Function for Type Conversion

We also use the eval function if we need the input to not be treated as a string, but rather as a different data type. For example, we can use it for numbers:

 >>> number = eval(input("What's the number?"))
 What's the number?
 5 

Now the input is no longer treated as a string, but as an integer:

 >>> type(number)
 <class 'int'> 

And it can be used in calculations just like any integer:

 >>> number * 2
 10 

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