Verifying keyboard input

Question

I have been writing a Python 3 program that includes a keyboard input and I wanted to test what comes in, to make sure it meets certain criteria, namely it should be a single character and only a, b or c. Being new to Python, I have cobbled together some code from other pages here, and it seems to work, but I'm not sure that it follows best practice (or even is correct programming). So any comments/improvements or alternatives most welcome.

chars = set('abc')
trigger = 0
while (trigger == 0):
    answer = input(':')
    if len(answer) != 1:
        print ('Please enter only one character')
    if any ((c in chars) for c in answer):
        trigger = 1
    else:
        print ('please answer a,b or c')

If this is **working code** that you think could be improved, see [codereview.se]. If you're unsure, delete the question and go and **test it**. If you are aware of a specific problem, [edit] to provide a [mcve]. In general, the guidance on validating user input is at https://stackoverflow.com/questions/23294658/asking-the-user-for-input-until-they-give-a-valid-response. — jonrsharpe, Dec 31 '17 at 14:40
Many thanks, the link you provided took me to what I was looking for, although I hadn't come across that page in my searches. — N. Blain, Dec 31 '17 at 22:05

score 2 · Answer 1 · answered Dec 31 '17 at 17:08

2

Create a list of valid inputs, and use in to test. Example:

>>> rv = input()
a
>>> rv in ['a', 'b', 'c']
True
>>> rv = input()
test
>>> rv in ['a', 'b', 'c']
False

answered Dec 31 '17 at 17:08

Roland Smith

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You would also have to add `len(rv) == 1' for multi-char input – Joe Iddon Dec 31 '17 at 17:32
@JoeIddon That is not necessary. As you can see in the second try, a string with a length > 1 fails. If you tried `'aa' in ['a', 'b', 'c']` it would also fail. – Roland Smith Dec 31 '17 at 18:01
Sorry! My mistake, I assumed you were using the `in` operator on a `string` in which case you would have to check it was length `1`, my bad! – Joe Iddon Dec 31 '17 at 18:08
Of course `len(rv) == 1 and rv in 'abc'` is also a valid answer. – Roland Smith Dec 31 '17 at 18:26
Many thanks for your help folks, I had a feeling there was a better way – N. Blain Dec 31 '17 at 22:03

Verifying keyboard input

1 Answers1