Python proper way to catch exceptions on file close

Question

I'm old at Perl and new to Python. I know in Perl that fd.close() isn't irrelevant. Writing to a full file system, close() will report the error. Also for socket errors, they appear in close(). So how to do with in Python? Some examples show putting the open() and close() in the same try block which would catch IOError on either. But other examples show close() in the finally block to close the file upon exception. However, what if the exception first occurs in close()?

Does this cover both requirements? (1) Always close the file (2) Catch all IO exceptions?

try: with open(FILE, 'w') as fd: ..... except IOError as err: .....

Thanks, Chris

With your example code, there is no file to close... you never opened it it in the first place and your exception handler it outside of the context manager. — roganjosh
– roganjosh, Commented Nov 8, 2018 at 20:39
@roganjosh: It's not clear what you're trying to say. open(FILE, 'w') doesn't require the file to already exist, and the question is about failures when closing the opened file, not failure to open the file. — user2357112
– user2357112, Commented Nov 8, 2018 at 20:42
@user2357112 actually I defer on this because of the outer try/except :) my understanding is that the file is still closed despite an extra exception handler but you can presumably confirm either way — roganjosh
– roganjosh, Commented Nov 8, 2018 at 20:42
@roganjosh: The file would be closed as the exception bubbled out of the with, and (unless the file object's __exit__ suppressed the exception intentionally, which it doesn't) would continue to bubble up until caught by the except IOError block. Exceptions raised in __exit__ would similarly be within the try, and therefore caught by the except. — ShadowRanger
– ShadowRanger, Commented Nov 8, 2018 at 21:09

ShadowRanger · Accepted Answer · 2018-11-08 21:14:21Z

Your code is correct; it can't differentiate errors on open from errors on the (implicit) close when the with block exits (nor differentiate errors from any other file operations in the block), but it will catch all such errors. By the time you reach the except block, you're guaranteed that the file tried to close (the with auto-close will occur before you reach it, whether by fallthrough or an exception being raised), though if the exception occurred during close your options are limited (because recovering meaningfully from close failing is usually not possible).

Note that IOError is not exactly correct; on Py3 it will work as expected (it's an alias of OSError, which is the actual base exception you want to catch), while on Python 2 it's separate from OSError, which means you won't catch OSError or its subclasses (which are commonly seen on Windows systems in particular).

If you want to definitely catch all such errors portably, you want to catch EnvironmentError, which is a superclass of both IOError and OSError on Python 2, and an alias for OSError on Python 3; if portability is not a concern, then OSError is the name that's actually used on Py3, so you may as well use the documented name.

hochl · Accepted Answer · 2018-11-08 21:00:14Z

Check this answer, one comment says you can open the file separately to find the error on open, then use it as a context manager with try around it again like that:

try: f = open( ... ) except IOError: ... try: with f: ... except IOError: ...

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