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
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