Message 233903 - Python tracker

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Author	sjdrake
Recipients	docs@python, sjdrake
Date	2015-01-13.07:35:44
SpamBayes Score	-1.0
Marked as misclassified	Yes
Message-id	<1421134546.13.0.154248882029.issue23227@psf.upfronthosting.co.za>
In-reply-to

Content
If a generator has its close() method called before any items are requested from it, a finally block in the generator function will not be executed. I encountered this when wrapping an open file to alter the result of iterating over it. Using a generator function with a try/finally block seemed like a simple way of acheiving this. Here's an example that logs each line as it's read: def logged_lines(f): try: for line in f: logging.warning(line.strip()) yield line finally: logging.warning('closing') f.close() If the generator is created and closed immediately, the underlying file-like object is left open: >>> f = urlopen('https://docs.python.org/') >>> lines = logged_lines(f) >>> lines.close() >>> f.closed False But once the first item is requested from the generator, close() will trigger cleanup: >>> lines = logged_lines(f) >>> next(lines) WARNING:root:<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"\n' >>> lines.close() WARNING:root:closing >>> f.closed True Having read the documentation for yield expressions, I don't believe this behaviour to be non-conformant - but it still seems like a bit of a gotcha to me. Should this usage be warned against?

If a generator has its close() method called before any items are requested from it, a finally block in the generator function will not be executed.

I encountered this when wrapping an open file to alter the result of iterating over it.  Using a generator function with a try/finally block seemed like a simple way of acheiving this.  Here's an example that logs each line as it's read:

def logged_lines(f):
    try:
        for line in f:
            logging.warning(line.strip())
            yield line
    finally:
        logging.warning('closing')
        f.close()

If the generator is created and closed immediately, the underlying file-like object is left open:
>>> f = urlopen('https://docs.python.org/')
>>> lines = logged_lines(f)
>>> lines.close()
>>> f.closed
False

But once the first item is requested from the generator, close() will trigger cleanup:
>>> lines = logged_lines(f)
>>> next(lines)
WARNING:root:<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
'<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"\n'
>>> lines.close()
WARNING:root:closing
>>> f.closed
True

Having read the documentation for yield expressions, I don't believe this behaviour to be non-conformant - but it still seems like a bit of a gotcha to me.  Should this usage be warned against?

History
Date	User	Action	Args
2015-01-13 07:35:46	sjdrake	set	recipients: + sjdrake, docs@python
2015-01-13 07:35:46	sjdrake	set	messageid: <1421134546.13.0.154248882029.issue23227@psf.upfronthosting.co.za>
2015-01-13 07:35:46	sjdrake	link	issue23227 messages
2015-01-13 07:35:44	sjdrake	create