The first way to get interactive input is the raw_input( [prompt] ) function. This function accepts a string parameter, which is the user's prompt, written to standard output. The next line available on standard input is returned as the value of the function.

The raw_input( [prompt] ) function reads from a file often called stdin. When running from the command-line, this will be the keyboard, and what you type will be echoed in the command window or Terminal window. If you try, however, to run these examples from Textpad, you'll see that Textpad doesn't have any place for you to type any input. In BBEdit, you'll need to use the Run In Terminal item in the #! menu.

Here's an example script that uses raw_input( [prompt] ).

#!/usr/bin/env python
# show how raw_input works
a= raw_input( "yes?" )
print a

When we run this script from the shell prompt, it looks like the following.

MacBook-3:Examples slott$ 

python rawdemo.py

yes?

why not?

why not?
$ 

This program begins by evaluating the raw_input function. When raw_input is applied to the parameter of "yes?", it writes the prompt on standard output, and waits for a line of input. We entered why not?. Once that line was complete, the input string is returned as the value of the function.

The raw_input function's value was assigned to the variable a. The second statement printed that variable.

If we want numeric input, we must convert the resulting string to a number.

#!/usr/bin/env python
# Compute the value of a block of stock
shares = int( raw_input("shares: ") )
price = float( raw_input("dollars: ") )
price += float( raw_input("eights: ") )/8.0
print "value", shares * price

We'll chmod +x stock.py this program; then we can run it as many times as we like to get results.

MacBook-3:Examples slott$ 

./stock.py

shares: 

150

dollars: 

24

eights: 

3

value 3656.25
$ 

The raw_input mechanism is very limited. If the string returned by raw_input is not suitable for use by int, an exception is raised and the program stops running. We'll cover exception handling in detail in Chapter 17, Exceptions .

Here's what it looks like.

MacBook-3:Examples slott$ 

./stock.py

shares: 

a bunch



Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "stock.py", line 3, in ?
    shares = int( raw_input("shares: ") )
ValueError: invalid literal for int(): a bunch


MacBook-3:Examples slott$ 

In addition to the raw_input function, which returns the exact string of characters, there is the input( [prompt] ) function. This also accepts an optional parameter, which is the prompt. It writes the prompt, then reads a line of input. It then applies the Python eval( string ) function to evaluate the input. Typically, we expect to decode a string of digits as an integer or floating point number. Actually, it will evaluate any legal Python expression! Of course, illegal expressions will raise an exception and the program will stop running.

The value of the input ( p ) function is eval( raw_input( p ) ). Here's an example:

#!/usr/bin/env python
shares = input('shares: ')
print shares

When we chmod +x inputdemo.py and run this program, we see the following.

$ 

./inputdemo.py

shares: 

2+1/2.0

2.5
$ 

In this case, the input function evaluated the expression 2+1/2.0.

The input function is an unreliable tool. For yourself, the author of the program, problems with input are easily solved. For someone else, the vagaries of what is legal and how Python responds to things that are not legal can be frustrating. In Chapter 17, Exceptions , we'll add some sophistication that will solve some of these problems.