Add attributes to os.stat results

[nickm]

BPO	462296
Nosy	@mwhudson, @gvanrossum, @loewis, @freddrake
Files	stat_return.diff: Patch for new os.stat behavior. patch2.diff: EXAMPLE ONLY patch for extended behavior on posix for st_{blksize

^{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 = 'https://github.com/freddrake'
closed_at = <Date 2002-03-05.16:46:33.000>
created_at = <Date 2001-09-17.17:57:02.000>
labels = ['library']
title = 'Add attributes to os.stat results'
updated_at = <Date 2002-03-05.16:46:33.000>
user = 'https://bugs.python.org/nickm'

bugs.python.org fields:

activity = <Date 2002-03-05.16:46:33.000>
actor = 'loewis'
assignee = 'fdrake'
closed = True
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2001-09-17.17:57:02.000>
creator = 'nickm'
dependencies = []
files = ['3622', '3623', '3624', '3625', '3626', '3627', '3628', '3629', '3630']
hgrepos = []
issue_num = 462296
keywords = ['patch']
message_count = 35.0
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priority = 'normal'
resolution = 'accepted'
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superseder = None
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url = 'https://bugs.python.org/issue462296'
versions = []

[nickm]

See bug bpo-111481, and PEP-0042. Both suggest that the
return values for os.{stat,lstat,statvfs,fstatvfs}
ought to be struct-like objects rather than simple tuples.

With this patch, the os module will modify the
aformentioned functions so that their results still
obey the previous tuple protocol, but now have
read-only attributes as well. In other words,
"os.stat('filename')[0]" is now synonymous with
"os.stat('filename').st_mode.

The patch also modifies test_os.py to test the new
behavior.

In order to prevent old code from breaking, these new
return types extend tuple. They also use the new
attribute descriptor interface. (Thanks for
PEP-025[23], Guido!)

Backward compatibility: Code will only break if it
assumes that type(os.stat(...)) == TupleType, or if it
assumes that os.stat(...) has no attributes beyond
those defined in tuple.

[nickm]

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BTW, if this gets in, I have another patch that adds support
for st_blksize, st_blocks, and st_rdev on platforms that
support them. It don't expose these new fields in the
tuple, as that would break all the old code that tries to
unpack all the fields of the tuple. Instead, these fields
are only accessible as attributes.

[loewis]

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I second the request for supporting additional fields
where available. At the same time, it appears
unimplementable using pure Python.

Consequently, I'd like to see this patch redone in C. The
implementation strategy could probably remain the same,
i.e. inherit from tuple for best compatibility; add the
remaining fields as slots. It may be reasonable to
implement attribute access using a custom getattr
function, though.

I have also my doubts about the naming of the fields. The
st_ prefix originates from the time where struct fields
were living in the global namespace (i.e. across different
structures), so prefixing them for uniqueness was
essential. I'm not sure whether we should inherit this
into Python...

[nickm]

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Martin: I'm not entirely sure what you mean here; while my
patch for extra fields requires a minor chunk of C (to
access the struct fields), the rest still works in pure
python. I'm attaching this second version for reference.

I'm not sure it makes much sense to do this with pure C; it
would certainly take a lot more code, with little benefit I
can descern. But you're more experienced than I; what am I
missing?

I agree that the field naming is suboptimal; I was taking my
lead from the stat and statvfs modules. If people prefer,
we can name the fields whatever we like.

[nickm]

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On further consideration, the approach taken in the second
(example only) patch is indeed too fragile. The C code
should not lengthen the tuple arbitrarily and depend on the
Python code to decode it; instead, it should return a
dictionary of extra fields. I think that this approach uses
a minimum of C, is easily maintainable, and very extensible.

[nickm]

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Here's the revised (example only) patch that takes the
more portable approach I mention below.

[gvanrossum]

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Haven't had time to review the patch yet, but the idea of
providing a structure with fields that doubles as a tuple is
a good one. It's been tried before and can be done in pure
Python as well.

Regarding the field names: I think the field names should
keep their st_ prefix -- IMO this makes the code more
recognizable and hence readable.

[loewis]

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The problem with your second and third patch is that it
includes an incompatibility for users of posix.stat (and
friends), since it changes the siye of the tuple. If you
want to continue to return a tuple (as the top-level data
structure), you'll break compatibility for applications
using the C module directly. An example of code that would
be broken is

mode, ino, dev, nlink, uid, gid, size, a, c, m =
posix.stat(filename)

To pass the additional fields, you already need your class
_StatResult available in C.
You may find a way to define it in Python and use it in C,
but that has proven to be very fragile in the past.

[nickm]

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Ah! Now I see. I hadn't realized that anybody used the
posix module directly. (People really do this?)

I'll try to write up a patch in C tonight or tomorrow
morning. A couple of questions on which I could use advice:
(1) Where is the proper place to put this kind of
tuple-with-fields hybrid? Modules? Objects? In a new file
or an existing one?
(2) Should I try to make it general enough for non-stat use?