[Python-Dev] cpython: Rename contextlib.ignored() to contextlib.ignore(). (original) (raw)

Zero Piraeus z at etiol.net
Tue Oct 15 03:04:45 CEST 2013

: On Tue, Oct 15, 2013 at 10🔞56AM +1000, Nick Coghlan wrote:

I didn't articulate the point very well. The reason I originally approved the change (and the reason I have kept it despite the objections raised) is because it allows correct-but-ugly code like:

try: os.unlink(fname) except FileNotFoundError: pass To be rewritten as the significantly more elegant: with ignore(FileNotFoundError): os.unlink(fname)

I thought that was already clear, to be honest. As an aside, calling the try/except construct above "ugly" seems a stretch to me - it's a little verbose if you're going to be using it a lot, but presumably in that case you'd just wrap it in a safe_unlink() convenience function if it became annoying.

This benefit when used correctly then needs to be weighed against the risk of it being used incorrectly, especially by people that expect it to work like VB's "on error resume next" rather than by suppressing the exception and resuming execution after the with statement.

The problem of overbroad exception handling isn't a new one, though, and I don't believe this feature makes that problem worse, and, by making it simpler and cleaner to suppress exceptions from a single statement, may make it better. For example, this code is wrong: try: os.unlink(fname) os.unlink(fname2) except FileNotFoundError: pass

Yes, it's bad code, but it's more intuitively obvious what it does (and therefore that it's badly written) than the equivalent

with ignore(FileNotFoundError):
    os.unlink(fname)
    os.unlink(fname2)

... as others have pointed out. The above suffers from the same (famously solved by Python) problem as C code with inconsistent braces and indentation: there's a disconnect between what it looks like and what it does.

I think one problem is that ignore() doesn't actually do what its name suggests - the flow of execution is changed if an exception occurs, which by definition means the exception isn't being ignored.

Maybe it would be better to call it silence()? That's still not perfect IMO, but it's closer to an accurate description of what's going on.

-[]z.

-- Zero Piraeus: obiter dictum http://etiol.net/pubkey.asc

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