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.

myargs = ['Hello', 'Goodbye', 'Hello_Again']; myString.format(*myargs) — Joel Cornett
– Joel Cornett, Commented Jul 10, 2012 at 21:38

mgilson · Accepted Answer · 2012-07-10 21:43:09Z

6

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)

answered Jul 10, 2012 at 21:43

mgilson

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1 Comment

Razz Abuiss Over a year ago

Not only did you give a great solution, but using format on the input makes the program way simpler than the way I had it. Thanks!

kindall · Accepted Answer · 2012-07-10 21:56:03Z

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

edited Jul 10, 2012 at 21:56

answered Jul 10, 2012 at 21:50

kindall

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6 Comments

mgilson Over a year ago

Just to be complete, you should probably check that format isn't ''

mgilson Over a year ago

Yeah, that's where I would put it too. Good answer now. I like it slightly better than mine (+1) -- edit why did you move it? while format and not result seems cleaner to me. Performance isn't an issue here since you're waiting for user input anyway...

kindall Over a year ago

Yeah, I just thought it was clearer as a separate statement. Tomato, tomato.

mgilson Over a year ago

Funny, when I read that, Tomato and tomato sound the same in my head ;).

jfs Over a year ago

you don't need to check if format if you use while True: instead of while not result (add break if there is no exception)

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jfs · Accepted Answer · 2012-07-11 15:22:42Z

2

Here's a modified kindall's answer that allows empty result for non-empty format string:

format = raw_input("What is the format? >>> ")
prompt = "What is the value for parameter {0}? >>> "
params  = []

while True:
    try:
        result = format.format(*params)
    except IndexError:
        params.append(raw_input(prompt.format(len(params))))
    else:
        break
print result

answered Jul 11, 2012 at 15:22

jfs

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1 Comment

kindall Over a year ago

Yeah, this is probably the best solution. I hate manually breaking out of loops if it can be avoided (far more elegant to write a proper condition) but it doesn't seem possible in this case.

Community · Accepted Answer · 2017-05-23 12:07:50Z

1

You just need to use * in front of the last parameter name and all subsequent ones will be grouped into that one. You can then iterate over that list as you desire.

How to make variable argument lists in Python

edited May 23, 2017 at 12:07

CommunityBot

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answered Jul 10, 2012 at 21:46

chrisjleaf

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