Code doesn't behave like conditions are being met when they are? [Python 3]

Question

So I have the following function:

def ask_question():
    """Asking a question which requires a yes or no response"""
    response=input()
    while response.lower()!="yes" or response.lower()!="no":
        print("Invalid response! Answer must be 'Yes' or 'No'.")
        response=input()
    return response

Yet when I execute the function ask_question() and type "yes" or "no", it comes up with the response "Invalid response! Answer must be 'Yes' or 'No'."

I can't figure out for the life of me why and i've been staring at it for a while now. Can someone help me out?

Answer 1

You need to use and in your loop. But why?

According to De Morgan's law , your condition

response.lower()!="yes" or response.lower()!="no":

Is equivalent to: (not A) or (not B) which is the same as not (A and B) - which is not what you want (ie not (Yes and No) does not give you what you want).

Therefore changing your query to use the and would change to this:

response.lower()!="yes" and response.lower()!="no":

which is equivalent to (not A) and (not B) which is the same as not (A or B) which is what you want. In other words:

if input is "not (Yes or No)", print invalid reponse msg

Answer 2

Your code checks prints the message if the input in lower case is not "yes" , or the input in lower case is not "no" , which is True pretty much for all possible inputs the user can give. Of course you could do if response.lower() != "yes" and response.lower() != "no": but it would not be very pythonic.

Instead you'd probably want to do the following with the in operator:

def ask_question():
    """Asking a question which requires a yes or no response"""

    while True:
        response = input("Please answer 'yes' or 'no'> ").lower()
        if response not in ('yes', 'no'):
            print("Invalid response! Answer must be 'yes' or 'no'.")
        else:
            return response

This code also ensures that the the user has answered correctly the second time when prompted.