> On Nov 20, 2019, at 7:50 AM, Rhodri James <rho...@kynesim.co.uk> wrote:
>
> On 20/11/2019 01:57, Oscar Benjamin wrote:
>>> On Tue, 19 Nov 2019 at 12:03, Paul Moore <p.f.mo...@gmail.com> wrote:
>>>
>>>> On Tue, 19 Nov 2019 at 11:34, Oscar Benjamin <oscar.j.benja...@gmail.com>
>>>> wrote:
>>>
>>>> If I was to propose anything here it would not be to disallow anything
>>>> that you can currently do with context managers. Rather the suggestion
>>>> would be to:
>>>> 1. Clearly define what a well-behaved context manager is.
>>>> 2. Add convenient utilities for working with well behaved context managers.
>>>> 3. Add well-behaved alternatives for open and maybe others.
>>>> 4. Add Random832's utility for adapting misbehaving context managers.
>>>
>>> That sounds reasonable, with one proviso. I would *strongly* object to
>>> calling context managers that conform to the new expectations "well
>>> behaved", and by contrast implying that those that don't are somehow
>>> "misbehaving". File objects have been considered as perfectly
>>> acceptable context managers since the first introduction of context
>>> managers (so have locks, and zipfile objects, which might also fall
>>> foul of the new requirements). Suddenly deeming them as "misbehaving"
>>> is unreasonable.
>> Perhaps a less emotive way of distinguishing these classes of context
>> managers would be as "eager" vs "lazy". An eager context manager jumps
>> the gun and does whatever needs undoing or following up before its
>> __enter__ method is called. A lazy context manager waits until
>> __enter__ is called before committing itself.
>> I don't really want to give a sense of equality between eager and lazy
>> though. To me it is clear that lazy context managers are preferable.
>
> As context managers, yes, lazy managers make chaining them easier because
> there's no mess to clean up if the chain breaks while you are creating it.
> On the other hand, eager managers like open() can be used outside a "with"
> statement and still manage resources perfectly well for a lot of cases. It a
> matter of fitness for different purposes, so even "preferable" is a relative
> term here.
>
> --
> Rhodri James *-* Kynesim Ltd
> _______________________________________________
>
To my mind, eager context managers nest just fine, if you put each into their
own context. What it seems the lazy managers let you do is squish multiple
context managers into a single context, but then you get the question of which
of them actually is providing the context that you are in?
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