Prompt the user to input something else if the first input is invalid

Question

I'm very new to Python, so forgive my newbish question. I have the following code:

[a while loop starts]

print 'Input the first data as 10 characters from a-f'

input1 = raw_input()
if not re.match("^[a-f]*$", input1):
    print "The only valid inputs are 10-character strings containing letters a-f"
    break
else:
[the rest of the script]

If I wanted to, instead of breaking the loop and quitting the program, send the user back to the original prompt until they input valid data, what would I write instead of break?

Just don't use `break`? (depending on the rest of the script). — Felix Kling, Mar 07 '12 at 21:20
@Felix: He'd still need to wrap his actual code into an `else` branch, though, which could be prevented by using `continue`. — Niklas B., Mar 07 '12 at 21:21

score 6 · Answer 1 · answered Mar 07 '12 at 21:19

6

To go on with the next loop iteration, you can use the continue statement.

I'd usually factor out the input to a dedicated function:

def get_input(prompt):
    while True:
        s = raw_input(prompt)
        if len(s) == 10 and set(s).issubset("abcdef"):
            return s
        print("The only valid inputs are 10-character "
              "strings containing letters a-f.")

answered Mar 07 '12 at 21:19

Sven Marnach

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As Niklas pointed out, it's worth noting that if `continue` is used, the `else` condition may be able to be removed as well. – Mr. Shickadance Mar 07 '12 at 21:23
@Sven Marnach what is the use of "PROMPT" in raw_input?? Please clearify it and me also new in python – Varun Chhangani Apr 15 '14 at 05:25
1

@VarunChhangani: It's the prompt printed before waiting for user input; see the [documentation](https://docs.python.org/2.7/library/functions.html#raw_input). – Sven Marnach Apr 15 '14 at 10:47

Silas Ray · Answer 2 · 2012-03-07T22:35:01.100

print "Input initial data.  Must be 10 characters, each being a-f."
input = raw_input()
while len(input) != 10 or not set(input).issubset('abcdef'):
    print("Must enter 10 characters, each being a-f."
    input = raw_input()

Slight alternative:

input = ''
while len(input) != 10 or not set(input).issubset('abcdef'):
    print("Input initial data.  Must enter 10 characters, each being a-f."
    input = raw_input()

Or, if you wanted to break it out in to a function (this function is overkill for this use, but an entire function for a special case is suboptimal imo):

def prompt_for_input(prompt, validate_input=None, reprompt_on_fail=False, max_reprompts=0):
    passed = False
    reprompt_count = 0
    while not (passed):
        print prompt
        input = raw_input()
        if reprompt_on_fail:
            if max_reprompts == 0 or max_reprompts <= reprompt_count:
                passed = validate_input(input)
            else:
                passed = True
        else:
            passed = True
        reprompt_count += 1
   return input

This method lets you define your validator. You would call it thusly:

def validator(input):
    return len(input) == 10 and set(input).subset('abcdef')

input_data = prompt_for_input('Please input initial data.  Must enter 10 characters, each being a-f.', validator, True)

Prompt the user to input something else if the first input is invalid

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