def query_yes_no(question, default="yes"):
"""Ask a yes/no question via raw_input() and return their answer.
"question" is a string that is presented to the user.
"default" is the presumed answer if the user just hits <Enter>.
It must be "yes" (the default), "no" or None (meaning
an answer is required of the user).
The "answer" return value is one of "yes" or "no".
"""
valid = {"yes":"yes", "y":"yes", "ye":"yes",
"no":"no", "n":"no"}
if default == None:
prompt = " [y/n] "
elif default == "yes":
prompt = " [Y/n] "
elif default == "no":
prompt = " [y/N] "
else:
raise ValueError("invalid default answer: '%s'" % default)
while 1:
sys.stdout.write(question + prompt)
choice = raw_input().lower()
if default is not None and choice == '':
return default
elif choice in valid.keys():
return valid[choice]
else:
sys.stdout.write("Please respond with 'yes' or 'no' "\
"(or 'y' or 'n').\n")
