[Python-ideas] One-line "try with" statement
ncoghlan at gmail.com
Mon Mar 4 09:22:26 CET 2013
On Mon, Mar 4, 2013 at 5:50 AM, Stefan Behnel <stefan_ml at behnel.de> wrote:
> Andrew Barnert, 03.03.2013 14:24:
>> For example, if you do "with open(path) as f:" the context manager
>> doesn't (and shouldn't) do anything to protect you from a
>> FileNotFoundError in the open, or an IOError reading inside the block.
>> If you want to, say, log, or try a backup file, how else would you
>> handle that but a with inside a try?
> If you really care about errors when opening the file (i.e. when creating
> the context manager), then the correct way to do this is to only wrap the
> creation of the context manager in a try-except clause, i.e.
> f = open("somefile.txt")
> except FileNotFoundError:
> raise # or return, or whatever
> with f:
> Otherwise, you risk accidentally catching (and potentially shadowing)
> exceptions that originated from the body of the with statement instead of
> just the context manager creation.
> This may look a bit overly complicated for a file, but it quickly becomes
> more obvious for more complex context managers, e.g. those that may raise
> more common errors like ValueError or AttributeError.
Indeed - complex try blocks in general are a red flag when paired with
exception handlers for common exceptions.
Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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