Else-statement after while-statement, and string comparison with a list of strings?

问题

I'm attempting to create a small test script that appends something to a note. Contained below is the main function that I will execute in the script. The problem seems to be that I can't get the else block to run when the while block evaluates to false (that is to say, when it evaluates to anything that isn't one of those four options), the while block just continues in an infinite loop. I've also attempted to insert a break into the while loop but this terminates the script after the while loop has executed.

How do I move from the while to the else block when it evaluates to false? And why doesn't the current way I do things work as I would like it to? Thanks.

def start():
    q01 = input("What is the subject of your note?\n")
    q02 = input("Are you certain that the subject of your note is " + q01 + "?\n")
    while q02 == 'No' or 'no' or 'NO' or 'n':
       q01 = input("So, what is the subject of your note?\n")
       q02 = input("Are you certain now that the subject of your note is " + q01 + "?\n")
    else:
       q03 = Enter("Enter the content of your note")

回答1:

Your culprit is the while loop condition:

while q02 == 'No' or 'no' or 'NO' or 'n':

This is equivalent to:

while (q02 == 'No') or 'no' or 'NO' or 'n':

As 'no', 'NO' and 'n' are all non-empty strings they evaluate to True and so your condition evaluates to:

while (q02 == 'No') or True or True or True:

which is clearly always True.

To fix this you need to adjust the condition to:

while q02 == 'No' or q02 == 'no' or q02 == 'NO' or q02 == 'n':

Although to be more pythonic you could instead make this:

while q02 in ['No','no','NO','n']:

回答2:

The problem is that the guard condition on the while block is always True!

>>> q02 = 'y'
>>> q02 == 'No' or 'no'
'no'

The or operator is interesting. It evaluates it's left operand and if it is "truthy" then the left operand is the result of the operation; otherwise, it evaluates it's right operand. In your case, the left operand is a boolean value (the result of q02 == 'No') and the right operand is a non-empty string. The non-empty string is "truthy" so that is the result.

IOW, q02 == 'No' or 'no' or 'NO' or 'n' evaluates to True if and only if q02 is 'No'. Otherwise it evaluates to the string 'no' which is "truthy" as far as the while loop is concerned.

>>> q02 = 'y'
>>> q02 == 'No' or 'no' or 'NO' or 'n'
'no'
>>> bool(q02 == 'No' or 'no' or 'NO' or 'n')
True

回答3:

Change this statement

while q02 == 'No' or 'no' or 'NO' or 'n':

while q02 == 'No' or q02 == 'no' or q02 == 'NO' or q02 == 'n':

One more elegant way of doing it:

def startMe():
    q01 = input("What is the subject of your note?\n")
    q02 = input("Are you certain that the subject of your note is " + q01 + "?\n")
    negList = ['No', 'no', 'NO', 'nO', 'n', 'N']  # <<< Easily modifiable list.

    while any(q02 in s for s in negList):  
       q01 = input("So, what is the subject of your note?\n")
       q02 = input("Are you certain that the subject of your note is " + q01 + "?\n")
    break:
       q03 = input("Enter the content of your note")

回答4:

Your logic and code is correct except 1 syntax. Use while q02 == 'No' or q02 == 'no' or q02 == 'NO' or q02 == 'n': insted of while q02 == 'No' or 'no' or 'NO' or 'n':

You can try:

def start():
    q01 = input("What is the subject of your note?\n")
    q02 = input("Are you certain that the subject of your note is " + q01 + "?\n")
    while q02 == 'No' or q02 == 'no' or q02 == 'NO' or q02 == 'n':
       q01 = input("So, what is the subject of your note?\n")
       q02 = input("Are you certain now that the subject of your note is " + q01 + "?\n")
    else:
       q03 = input("Enter the content of your note")
start()

来源：https://stackoverflow.com/questions/41612095/else-statement-after-while-statement-and-string-comparison-with-a-list-of-strin

标签

python

if-statement

while-loop

conditional