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I'm asking the user if they want to go first or not and use input validation so that only "y" "Y" "n" or "N" are accepted as a valid response. my code seems to crash when i run it.

choice = raw_input("Would you like to go first or not? (y/Y or n/N): ")
print ""

try:
    valid_choice = False
    while not valid_choice:
        if choice == "y" or choice == "Y":
            users_turn = True
            valid_choice = True
        elif choice == "n" or choice == "N":
            users_turn = False
            valid_choice = True
            break
    else:
            print "Invalid Choice."

except NameError:
    print "You can only enter y/Y or n/N"
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3
  • "my code seems to crash" -- In what way, more precisely? What's the error? Commented Feb 8, 2014 at 3:21
  • you are missing break in if clause Commented Feb 8, 2014 at 3:22
  • If any of these answers helped you, you should select one as a solution. Commented Feb 10, 2014 at 4:49

3 Answers 3

Reset to default
2

Ahm, the flow in your code has a bit too many possible paths. You could simplify it with something like:

question = "Would you like to go first or not? (y/Y or n/N): "
choice = raw_input(question)
while choice not in ['y', 'Y', 'n', 'N']:
    print 'Invalid choice'
    choice = raw_input(question)
users_turn = choice in ['y', 'Y']

Having said that, I should say that it is a common pattern for a console application to use (Y/n) to indicate that it accepts the letters y or n as responses (in both lower and upper cases), but the upper case indicates the default option.

So, if you can trust the user to make a decision the first time you ask for an input to the question, you can set a default response and code it like this:

choice = raw_input('Would you like to go first? (Y/n)')
users_turn = choice.lower() not in ['n', 'no']
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3 Comments

@zhangxaochen I saw your edit to use a string, but I think I still prefer using a list -- that way you can add other possible answers likes 'yes', 'no' and initials to support other languages.
yes and I removed that edit also because 'Yn' in 'yYnN' gives True. sorry ;P
@zhangxaochen oh yeah, there's also that! =)
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Just as an alternative to elias' answer, I'd like to present the following:

while True:
    choice = raw_input("Would you like to go first or not? (y/Y or n/N): ")
    if choice in ["y", "Y", "n", "N"]:
        break
    print "Invalid choice"
users_turn = choice in ["y", "Y"]

It's better in that it doesn't duplicate the ask line, but worse in that the while True/break syntax is a bit ugly (it would be nice if Python had do...while like C, but alas). Choose for yourself which you like the most. :)

Also, just to point out some of the several errors you have in your own code:

  • The try...except NameError block is completely redundant, since nothing in your code will throw NameError, and especially not to signal an actual user error.
  • You have your else line at the wrong indentation. As it stands, it matches the while block rather than if/elif block, causing behavior you most likely do not intend.
  • The break in the elif block is redundant, since the while loop will exit anyway when you've set valid_choice to True.
  • Most important of all, you don't reask the user when he types an invalid answer, causing your code to loop forever. Perhaps this is the unspecified crash you speak of?
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0 
def get_one_of(prompt, options, default=None):
    options = set(options)
    while True:
        val = raw_input(prompt)
        if val == '' and default is not None:
            return default
        elif val in options:
            return val

response = get_one_of('Do you want to go first? [Yn] ', 'yYnN', 'y')
users_turn = response in 'yY'
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