Is there a way to do a while loop on a variable that's undefined? (How can I rewrite this, Python)

Question

I want to do a while loop that runs until a user puts nothing in the input.

This is what I have that currently works, but I want to remove the answer = None instantiation.

def answer_as_ul(question, input_prefix='• '):
    print(question)
    answer_list = list()
    answer = None
    while answer != '':
        answer = input(input_prefix)
        answer_list.append(answer) if answer else None
    return answer_list

Is there a way to remove answer = None here and keep the functionality?

Using a conditional expression like that is pretty unidiomatic. Which version of Python, can you use `while answer := input(input_prefix):` (available from 3.8, see [PEP 572](https://www.python.org/dev/peps/pep-0572/))? — jonrsharpe, Jan 06 '21 at 14:40
@jonrsharpe yep! I'm on Pyhton 3.8+. That worked like a charm! Beautiful solution. — Zack Plauché, Jan 06 '21 at 14:43

Timur Shtatland · Answer 1 · 2021-01-06T14:48:03.010

1

Use the code below. I also added a few minor tweaks for more Pythonic code:

def answer_as_ul(question, input_prefix='• '):
    print(question)
    answer_list = []
    while True:
        answer = input(input_prefix)
        if not answer: break
        answer_list.append(answer)
    return answer_list

edited Jan 06 '21 at 14:48

answered Jan 06 '21 at 14:42

Timur Shtatland

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Is it more pythonic to use `[]` instead of `list()`? – Zack Plauché Jan 06 '21 at 15:02
@ZackPlauché Yes, I think so. I have seen it more often in otherwise good quality code. Maybe because it is shorter... – Timur Shtatland Jan 06 '21 at 15:08
1

You were right! [and they're faster.](https://stackoverflow.com/questions/5790860/and-vs-list-and-dict-which-is-better) – Zack Plauché Jan 06 '21 at 15:11

score 1 · Answer 2 · answered Jan 06 '21 at 14:42

1

Maybe like this but it doesn't make a lot of difference :

def answer_as_ul(question, input_prefix='• '):
    print(question)
    answer_list = list()
    while True:
        answer = input(input_prefix)
        if answer: break  
        else: answer_list.append(answer)
    return answer_list

answered Jan 06 '21 at 14:42

dspr

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Other way around, surely - append the `answer` if it's truthy, break otherwise. – jonrsharpe Jan 06 '21 at 14:52

Zack Plauché · Accepted Answer · 2021-01-06T14:54:41.533

1

I found the answer thanks to @jonsharpe's beautiful solution using Python 3.8's walrus operator:

def answer_as_ul(question, input_prefix='• '):
    print(question)
    answer_list = list()
    while answer := input(input_prefix):
        answer_list.append(answer)
    return answer_list

edited Jan 06 '21 at 14:54

answered Jan 06 '21 at 14:49

Zack Plauché

1,964
9
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This is very Pythonic! – Timur Shtatland Jan 06 '21 at 14:54

Is there a way to do a while loop on a variable that's undefined? (How can I rewrite this, Python)

3 Answers3