GzipFile and BZ2File should support context manager protocol

[hagen]

BPO	3860
Nosy	@pitrou, @vstinner, @benjaminp
Files	withgzip.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2009-01-10.16:23:27.442>
created_at = <Date 2008-09-13.19:29:02.127>
labels = ['type-feature', 'library']
title = 'GzipFile and BZ2File should support context manager protocol'
updated_at = <Date 2011-02-12.19:32:25.462>
user = 'https://bugs.python.org/hagen'

bugs.python.org fields:

activity = <Date 2011-02-12.19:32:25.462>
actor = 'teamnoir'
assignee = 'none'
closed = True
closed_date = <Date 2009-01-10.16:23:27.442>
closer = 'pitrou'
components = ['Library (Lib)']
creation = <Date 2008-09-13.19:29:02.127>
creator = 'hagen'
dependencies = []
files = ['12398']
hgrepos = []
issue_num = 3860
keywords = ['patch']
message_count = 8.0
messages = ['73194', '78035', '78037', '78046', '78319', '78324', '79551', '128458']
nosy_count = 5.0
nosy_names = ['pitrou', 'vstinner', 'benjamin.peterson', 'hagen', 'teamnoir']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'patch review'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue3860'
versions = ['Python 3.1', 'Python 2.7']

[hagen]

When you've become used to writing

with open("xzy", "w") as f: 

it's strange when it doesn't work for gzip.open or bz2.BZ2File.
Or is there a reason for them not being context managers?

[pitrou]

Here is a patch for the trunk. I'm waiting for the opinion other
developers before deciding to commit or no.

[benjaminp]

Here's the StringIO discussion: bpo-1286

[pitrou]

Thanks. The outcome of the StringIO discussion isn't clearly negative,
although Guido does not see a big benefit in it. If some people are
opposed to this, please stand up :) Otherwise I'll commit the patch to
trunk.

[vstinner]

I love "with ... as f:" and I want it for gzip/bz2 files!

Comment about the patch:

(1) the IOError message is different for the two modules:
"I/O operation on closed GzipFile object"
vs
"I/O operation on closed file"

Specify the object name in both messages or remove it from both
messages. I prefer verbose error message ("I/O operation on closed
BZ2File object").

(2) For bz2module, Why don't you call BZ2File_close() directly instead
of the generic version PyObject_CallMethod(... "close" ...)?

[pitrou]

(1) the IOError message is different for the two modules:
"I/O operation on closed GzipFile object"
vs
"I/O operation on closed file"

I've reused the same error message as used in other parts of each
module.

(2) For bz2module, Why don't you call BZ2File_close() directly instead
of the generic version PyObject_CallMethod(... "close" ...)?

Because someone may subclass BZ2File and override the close() method. I
don't know if it's likely to happen, though.

[pitrou]

Committed in trunk and py3k. Thanks!

[teamnoir]

Documentation needs to be updated to state that these are now context managers. This is important since they aren't in python-2.x.

I'm not sure whether this should be added to the "new in python" blurbs.