[Python-Dev] Context management patterns (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sat Oct 19 14:12:59 CEST 2013

On 19 October 2013 20:47, Glenn Linderman <v+python at g.nevcal.com> wrote:

Sadly, all the documentation that references "suppressing" an exception, is misleading. Misleading enough that you convinced yourself to rename from ignore to suppress after reading it, even though the two are nearly synonymous.

You misunderstand the two components that formed my rationale for renaming it.

The first part of the rationale is that re-reading all of the current docs reminded me that this pattern already had a consistent name in the language and library reference (i.e. suppress), and this name was even mentioned all the way back in PEP 343 (although it wasn't the primary term there - that honour went to "swallow", which understandably didn't make it into the reference docs). To avoid introducing an inconsistency into the documentation for Python 3.4, I had to choose between rewording all the existing context management and with statement docs or just using the established name for the new construct (and I chose the latter path).

The second part of my rationale stems from the fact that ignore and suppress aren't quite synonyms, and that suppressing something is far more active than ignoring it. If you completely ignore something, it should be as if it never happened - you certainly don't take any kind of action in response to it. When you suppress something, by contrast, there's a clear implication that it still happens, and then you do something later to make it go away. That means the potential for confusion changes:

Plausible interpretations of ignore(SomeException):

Plausible interpretations of suppress(SomeException):

With "suppress", the implication that the "exception is raised" part still happens is much stronger, suggesting that the name is not only more consistent with the existing docs, but also more accurate in an absolute sense.

Independently of that, one of the things that has become clear to me in these threads is that many experienced Python developers don't realise that with statements actually are intended to be usable as control flow constructs (albeit in a quite restrained way, with their influence on the flow of control being limited solely to the ability to suppress exceptions). If coming to grips with the behaviour of contextlib.suppress expands people's horizons beyond merely using them for resource, state and transaction management, then I see that as a good thing.

Now, some of the people in that last group are actually more likely to be confused than beginners, since they already have the mistaken impression that with statements don't affect control flow. And if you have that impression, then you're more likely to guess that suppress() has to be a state management context manager that is giving the interpreter the instruction to not raise the named exception types. Discovering that it doesn't work that way may then help to enlighten them as to the true nature of the underlying construct.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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