[Python-ideas] A suggestion for a do...while loop

[Joao S. O. Bueno]

and "while not except"  :-/   maybe we just stick with "while True' and put
forward a documenting
PEP advising linter packages to look for ways of getting out of the loop.

On 26 June 2017 at 08:08, Terry Reedy <tjreedy at udel.edu> wrote:

> On 6/26/2017 12:41 AM, Nick Coghlan wrote:
>
>> On 26 June 2017 at 10:25, Rob Cliffe <rob.cliffe at btinternet.com> wrote:
>>
>>>
>>>
>>> On 25/06/2017 12:58, Markus Meskanen wrote:
>>>
>>>>
>>>> I'm a huge fan of the do...while loop in other languages, and it would
>>>> often be useful in Python too, when doing stuff like:
>>>>
>>>> while True:
>>>>      password = input()
>>>>      if password == ...:
>>>>          break
>>>>
>>>> [...]I suggest [...]
>>>>
>>>> do:
>>>>      password = input('Password: ')
>>>>      until password == secret_password
>>>>
>>>>      # This line only gets printed if until failed
>>>>      print('Invalid password, try again!')
>>>>
>>>>
>>>> I don't see any significant advantage in providing an extra Way To Do
>>> It.
>>> Granted, the "while True" idiom is an idiosyncrasy, but it is frequently
>>> used and IMHO intuitive and easy to get used to.  Your suggestion doesn't
>>> even save a line of code, given that you can write:
>>>
>>>      while True:
>>>          password = input('Password:')
>>>          if password == secret_password: break
>>>          print('Invalid password, try again!')
>>>
>>
>> Right, this is the key challenge for do-while loops in Python: can you
>> come up with something that's significantly clearer than the current
>> "while True/if/break" pattern?
>>
>> The biggest weakness of that idiom is that it isn't really explicit in
>> the header line - there's nothing about "while True:" that directly
>> tells the reader "This loop is expected to exit via a break
>> statement".
>>
>> If we wanted to allow that to be expressed literally, we could
>> probably special case the "while not break" keyword sequence as a do
>> loop:
>>
>>      while not break:
>>          # Setup
>>          if condition: break
>>          # Loop continuation
>>
>
> We would then also need 'while not return:'
>
> That more explicit declaration of intent ("The code in the loop body
>> will conditionally break out of this loop") would allow a couple of
>> things:
>>
>> - the compiler could warn that an else clause attached to such a loop
>> will never execute (technically it could do that for any expression
>> that resolves to `True` as a constant)
>> - code linters could check the loop body for a break statement and
>> complain if they didn't see one
>>
>> Logically, it's exactly the same as writing "while True:", but whereas
>> that spelling suggests "infinite loop", the "while not break:"
>> spelling would more directly suggest "terminated inside the loop body
>> via a break statement"
>>
>
>
>
>
> --
> Terry Jan Reedy
>
>
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