[Python-Dev] Coding practice for context managers (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Mon Oct 21 14:08:39 CEST 2013

Le Mon, 21 Oct 2013 20:46:39 +1000, Nick Coghlan <ncoghlan at gmail.com> a écrit :

On 21 Oct 2013 12:44, "Raymond Hettinger" <raymond.hettinger at gmail.com> wrote: > > Two of the new context managers in contextlib are now wrapped in pass-through factory functions. The intent is to make the help() look cleaner. This practice does have downsides however. > > The usual way to detect whether something is usable with a > with-statement is to check the presence of the enter and exit methods. Wrapping the CM in a pass-through function defeats this and other forms of introspection. > > Also, the help() output itself is worse-off. When you run help on a CM(), you're trying to find-out what happens on entry and what happens on exit. If those methods had docstrings, the question would be answered directly. The wrapper (intentionally) hides how it works. > > Since I teach intermediate and advanced python classes to > experienced Python users, I've become more sensitive to problems this practice will create. Defeating introspection can make the help look nicer, but it isn't a clean coding practice and is something I hope doesn't catch on. > > To the extent there is a problem with the output of help(), I think efforts should be directed at making help() better. A lot of work needs to be done on that end -- for example abstract base classes also don't look great in help(). > > There are a couple of other minor issues as well. One is that the wrapper function hides the class, making it harder to do type checks such as "isinstance(x, suppress)". The other issue is that wrappers cause extra jumping around for people who are tracing code through a debugger or using a visualization tool such as pythontutor. These aren't terribly important issues, but it does support the notion that usually the cleanest code is the best code. > > In short, I recommend that efforts be directed at improving help() > rather than limiting introspection by way of less clean coding practices.

That's a fair point, but I really dislike exposing implementations that don't match their documentation, and both of these are currently documented as factory functions.

Let's call them callables instead?

Exposing the full object oriented API is certainly a reasonable option, but the docs should be updated accordingly, with suitable public attributes being defined (providing access to the exception tuple for suppress and the target stream for redirectstdio).

I don't get why adding public attributes should be related to the proposed change.

Regards

Antoine.

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