Add devnull file-like object

[rhettinger]

BPO	28864
Nosy	@rhettinger, @ncoghlan, @pitrou, @vstinner, @tiran, @bitdancer, @vadmium, @serhiy-storchaka, @MojoVampire

^{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 2018-01-29.20:51:20.705>
created_at = <Date 2016-12-04.03:36:35.686>
labels = ['type-feature', '3.7']
title = 'Add devnull file-like object'
updated_at = <Date 2018-01-29.20:51:20.704>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2018-01-29.20:51:20.704>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2018-01-29.20:51:20.705>
closer = 'rhettinger'
components = []
creation = <Date 2016-12-04.03:36:35.686>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 28864
keywords = []
message_count = 10.0
messages = ['282314', '282316', '282358', '282382', '282408', '282422', '282423', '282424', '284535', '311170']
nosy_count = 9.0
nosy_names = ['rhettinger', 'ncoghlan', 'pitrou', 'vstinner', 'christian.heimes', 'r.david.murray', 'martin.panter', 'serhiy.storchaka', 'josh.r']
pr_nums = []
priority = 'low'
resolution = 'later'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue28864'
versions = ['Python 3.7']

[rhettinger]

Uses cases are the same as /dev/null:

with redirect_stderr(io.DevNull()):
    some_func_using_stdout_and_stderr()

[serhiy-storchaka]

If you need true file object, with name, fileno() etc, you can use open(os.devnull, 'w'). If the simple file-like object is enough, it is just few lines:

    class NullWriter:
        def write(self, s): pass

io.StringIO() works in most cases well too.

[bitdancer]

Yes, in my mind open(os.devnull) is the "obvious way" to do this.

[vadmium]

If you only need the readable interface, use BytesIO or StringIO.

I once had an implementation like Serhiy’s, called dummywriter: <https://github.com/vadmium/python-lib/blob/99ec887/streams.py#L12\>. To fully implement the writable file API it should also implement writable(), and write() should return the size of its argument.

Implementing this without opening a real file descriptor is slightly easier to manage (no need to worry about closing it, and no problem sharing instances or creating many copies). I don’t have a strong opinion about including it in the builtin library though, since it is pretty easy to implement the basics yourself.

I doubt anyone would need a readable and writable object, i.e. open(devnull, "r+"). On the other hand, it is occasionally useful to know how many bytes were written in total, so implementing tell() etc could be useful. (Linux’s /dev/null doesn’t work that way though; I find it always returns zero.)

[rhettinger]

I had in mind a pre-opened file like sys.stderr or sys.stdout.

[MojoVampire]

Didn't pre-opened (or lazily opened) file descriptors cause headaches with os.urandom? I'm not sure I'd want my programming environment eating file descriptors "just in case", even if it might make certain tasks trivially faster after startup.

[tiran]

It can be done with some extra effort. We have to get the st_dev and st_inode from os.fstat(fd), cache them together with the fd, and verify them every time we create a new DevNull object. That way we catch both closed fd and modified fd. We might have to add a lock around the check, too.

[pitrou]

Which begs the question: what's the point? As David said, open(os.devnull) is a reasonably obvious way to do it. Is there a use case that it doesn't satisfy?

[vadmium]

Example where an implementation like Serhiy’s was not good enough: <https://bugs.python.org/issue29130#msg284437\>. In that case, the lack of flush() method causes a subtle problem.

[rhettinger]

Closed due to lack of interest (I still think Python would be better-off with this feature).