[Python-ideas] IMAP4.__exit__ counterintuitive for with blocks

[Koos Zevenhoven]

On Mon, Oct 30, 2017 at 8:30 PM, Drew <dwarwick96 at gmail.com> wrote:

> Yeah, a flag for IMAP4 objects sounds like it'd solve
> backwards-compatibility problems. E.g.
>
> imaplib.IMAP4_SSL(..., commit=manual/auto)
>
> As for handling errors with automatic commits, we should probably just
> defer to whatever Python IO does to remain consistent.
>
>
​Yeah, at least if it makes sense here too.

BTW, it so happens that an old contextlib feature strikes again and allows
you to write this in plain English, with punctuation and all:

with imaplib.IMAP4_SSL(..) as i, closing(i):
    ...


This one does call .close() regardless of errors, though, and possibly the
kind of error may affect whether the commit succeeds or not. Not sure if
that matters.

-- Koos





> On Oct 30, 2017, at 1:33 PM, Guido van Rossum <guido at python.org> wrote:
>>
>> But maybe if __exit__ is called with an exception it should roll back.
>>
>> In any case it looks like your proposal could break existing code (e.g.
>> code that uses `with` depending on its current behavior, using some other
>> logic to decide whether to commit or not) and that feels difficult to
>> overcome. Perhaps a new method or flag argument can be added to request
>> that the transactions be automatically committed?
>>
>> On Mon, Oct 30, 2017 at 10:20 AM, Drew <dwarwick96 at gmail.com> wrote:
>>
>>> IMAP4.close closes the selected inbox and commits changes such as
>>> deletions. It is not called on IMAP4.__exit__ (only logout is, which
>>> doesn't call close in its call stack) however, so:
>>>
>>> with imaplib.IMAP4_SSL(...) as i:
>>>     ...
>>>
>>> would fail to commit those changes. close must be explicitly invoked
>>> i.e.
>>>
>>> with imaplib.IMAP4_SSL(...) as i:
>>>     ...
>>>     i.close()
>>>
>>> This is counterintuitive however, as the with statement is meant to
>>> automatically clean up. Another programmer might come along and delete
>>> i.close() because it seems unnecessary. Now changes aren't being committed
>>> and the programmer doesn't realize it's because of this weird Python
>>> idiosyncracy.
>>>
>>> Python IO such as open commits changes automatically, so I'm not sure
>>> why IMAP4 doesn't and only logs out.
>>>
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>>
>>
>> --
>> --Guido van Rossum ( python.org/~guido)
>>
>
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-- 
+ Koos Zevenhoven + http://twitter.com/k7hoven +
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