Formatting using a variable number of .format( ) arguments in Python

Question

I can't seem to figure out a straightforward way to make code that finds the number of items to format, asks the user for the arguments, and formats them into the original form.

A basic example of what I'm trying to do is as follows (user input starts after ">>> "):

>>> test.py
What is the form? >>> "{0} Zero {1} One"
What is the value for parameter 0? >>> "Hello"
What is the value for parameter 1? >>> "Goodbye"

The program would then use print(form.format()) to display the formatted inputs:

Hello Zero Goodbye One

However if the form had 3 arguments, it would ask for parameters 0, 1 and 2:

>>> test.py (same file)
What is the form? >>> "{0} Zero {1} One {2} Two"
What is the value for parameter 0? >>> "Hello"
What is the value for parameter 1? >>> "Goodbye"
What is the value for parameter 2? >>> "Hello_Again"
Hello Zero Goodbye One Hello_Again Two

This is the most basic application that I can think of that would use a variable amount of things to format. I have figured out how to make variables as I need them using vars() but as string.format() can't take in lists, tuples, or strings, I can't seem to make ".format()" adjust to the number of things there are to format.

Answer 1

fmt=raw_input("what is the form? >>>")
nargs=fmt.count('{') #Very simple counting to figure out how many parameters to ask about
args=[]
for i in xrange(nargs):
    args.append(raw_input("What is the value for parameter {0} >>>".format(i)))

fmt.format(*args)
          #^ unpacking operator (sometimes called star operator or splat operator)

Answer 2

The easiest way is to simply try to format using whatever data you have, and if you get an IndexError you don't have enough items yet, so ask for another one. Keep the items in a list and unpack it using the * notation when calling the format() method.

format = raw_input("What is the format? >>> ")
prompt = "What is the value for parameter {0}? >>> "
parms  = []
result = ""
if format:
    while not result:
        try:
            result = format.format(*parms)
        except IndexError:
             parms.append(raw_input(prompt.format(len(parms))))
print result