2.5. Input

So far, every program we’ve seen and run will do the exact same thing every time it runs. Most useful programs don’t do the same thing every time because they operate with some kind of input. That is, when the program runs, it takes in information from outside of itself and uses that information to change how it runs. By using input, a program can solve a variety of problems without having to change the code itself.

For example, look at the example program from the previous section. It would be more useful if it asked the user for the number of days to convert each time it runs.

Python has a built-in function called input() that provides a way to get user input. The input() function allows the programmer to provide a prompt string in the code. When the program is run, the prompt is display. Whatever the user then types (followed by pressing Enter) is returned by the function to and can be used by the program.

Syntax Pattern

input() is most frequently used in input statements with the following form:

<variable> = input("<prompt>")

See how this is an example of variable assignment? This will take the string of characters typed in by the user after the prompt and assign it to the given variable.

Try running the following example a number of times, typing different things in the input box that appears each time.

One critical detail of the input() function is that it returns a string value. Even if you asked the user to enter their age, you would get back a string like "19". If you want to use that as a number in your program (e.g., if you want to do arithmetic with it), you have to convert it into the appropriate type using the int() or float() functions we saw earlier in Type Conversion Functions. The following code runs, but it doesn’t do what you might expect (another semantic error). Try fixing it so it runs correctly:

Often we put the conversion function in the input statement itself. Here, in a corrected version of the above example, the first line of the program both asks for user input and converts that input to an int:


But watch out! Try running the above code and type in something that isn’t an integer.

Code that needs to work correctly in all cases without crashing has to do more work to check for such errors and handle them gracefully. Starting out, your code will probably not include much error checking, so you should be careful to provide appropriate inputs when you run it.

If we want to modify the example from the previous section to be more useful, then we need to add an input statement. Here, we’ve separated out the type conversion from the input statement to make the addition more explicit:

In the program above, the variable str_days refers to the string that is entered by the user. Even though this string may be 100, it is still a string of characters (‘1’, ‘0’, and ‘0’) and not a number when it is entered by the user.

To convert it to an integer, we use the int() function. The result is stored in the days variable. Now, each time you run the program, you can enter a new value for the number of days to be converted.