Input

There are two basic ways to get input from the user: through the raw_input function, and through command-line arguments.

The `raw_input` function

If you call the raw_input function, it will wait until the user to types some text, and presses the Enter key. The function returns the user's text, as a string.

      user_text = raw_input()
      user_text2 = raw_input()
      print "You typed:", user_text, 'then you typed', user_text2

In the example above, the user won't get any guidance: he or she will just see a blinking cursor. So, you should pass a prompt to the raw_input function:

      name      = raw_input('What is your first name?')
      last_name = raw_input('What is your last name?')
      full_name = name + ' ' + last_name
      print 'Hello,', full_name, 'it\'s nice to meet you'

You should remember that raw_input returns strings. If you are expecting another kind of variable, you'll have to convert the string value:

      principal    = float(raw_input('Loan principal: '))
      pct_interest = float(raw_input('Annual % interest: '))
      months       = int(raw_input('Loan length (months): '))
      rate = pct_interest / 1200
      payment = principal * (rate + rate / ((1 + rate)**months - 1))
      print 'Monthly payment:', payment

Notice how the principal and interest are floats, while the length of the loan is an int.

Command-line arguments

In many of your exercises, you will simply create your program in a text file, using IDLE or another text editor, and then you will run the program inside IDLE. But it's possible to run the program from the command line.

To do that, you will open a command shell window, and run the program within the python interpreter. For example, if your program is in a file called loan.py, you would type:

        python loan.py

This is called "using the command line", and the text above is called a "command".

This will work if you've set things up correctly: you should have navigated within the command shell, to place yourself in the folder where the program loan.py is. Also, the PATH variable has to be set up correctly, so the shell knows where the python program is. Here, I won't tell you how do that.

Suppose the text file loan.py contains the program I showed you above:

          principal    = float(raw_input('Loan principal: '))
          pct_interest = float(raw_input('Annual % interest: '))
          months       = int(raw_input('Loan length (months): '))
          rate = pct_interest / 1200
          payment = principal * (rate + rate / ((1 + rate)**months - 1))
          print 'Monthly payment:', payment

then typing the command python loan.py would prompt the user for principal, interest, and length, and would print the payment amount. However, sometimes we want to include that information in the command itself:

          python loan.py 12345.00 8.75 60

Those three numbers 12345.00 8.75 60 are called "command-line arguments". In order for your program to access them, it must use the argv list, which is part of the sys library. Here is how:

          import sys

          principal    = float(sys.argv[1])
          pct_interest = float(sys.argv[2])
          months       = int(  sys.argv[3])
          rate = pct_interest / 1200
          payment = principal * (rate + rate / ((1 + rate)**months - 1))
          print 'Monthly payment:', payment

Notice several things:

The first argument is sys.arg[1], the second is sys.arg[2], etc.
You must tell python that you plan to use the sys library, using import.
The arguments are strings, so they may have to be converted into float or int, just as with raw_input.

What if the user forgets one the three numbers? If this was typed:

          python loan.py 12345.00 8.75

the program would crash, complaining that, in sys.argv[3] the index 3 is out of range. You could prevent this embarassing error, by telling the user to include 3 arguments. You can actually figure out how many were typed, by checking the length of the list sys.argv, as follows:

          import sys

          if len(sys.argv) != 4:
              print 'Please enter three command-line arguments:'
              print 'python loan.py principal interest loan_length'
              sys.exit(1)

          principal    = float(sys.argv[1])
          pct_interest = float(sys.argv[2])
          months       = int(  sys.argv[3])
          rate = pct_interest / 1200
          payment = principal * (rate + rate / ((1 + rate)**months - 1))
          print 'Monthly payment:', payment

Here, if the user hasn't typed three arguments, we print out some instructions, and stop the program by calling sys.exit.

Did you notice if len(sys.argv) != 4:? It turns out that the list sys.argv has an extra entry, sys.argv[0], which is the name of your program: in this case sys.argv[0] is 'loan.py'. If you expect a certain number of arguments, the length of sys.argv should be one more than that number.

Input

The raw_input function

Command-line arguments

The `raw_input` function