distutils docs: data_files always uses sys.prefix

[jdemeyer]

BPO	25592
Nosy	@pitrou, @merwok, @berkerpeksag, @jdemeyer, @dstufft
PRs	bpo-25592: improve documentation of data_files #9767 [3.7] bpo-25592: Improve documentation of distutils data_files (GH-9767) #11701 [3.7] bpo-25592: Improve documentation of distutils data_files (GH-9767) #11701 [3.7] bpo-25592: Improve documentation of distutils data_files (GH-9767) #11701 [2.7] bpo-25592: Improve documentation of distutils data_files (GH-9767) #11734 [2.7] bpo-25592: Improve documentation of distutils data_files (GH-9767) #11734 [2.7] bpo-25592: Improve documentation of distutils data_files (GH-9767) #11734

^{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 2019-02-02.10:16:29.281>
created_at = <Date 2015-11-10.06:06:12.564>
labels = ['3.7', '3.8', 'type-bug', 'library', 'docs']
title = 'distutils docs: data_files always uses sys.prefix'
updated_at = <Date 2019-02-02.10:17:12.823>
user = 'https://github.com/jdemeyer'

bugs.python.org fields:

activity = <Date 2019-02-02.10:17:12.823>
actor = 'pitrou'
assignee = 'docs@python'
closed = True
closed_date = <Date 2019-02-02.10:16:29.281>
closer = 'pitrou'
components = ['Distutils', 'Documentation']
creation = <Date 2015-11-10.06:06:12.564>
creator = 'jdemeyer'
dependencies = []
files = []
hgrepos = []
issue_num = 25592
keywords = ['patch']
message_count = 21.0
messages = ['254432', '327403', '327407', '327408', '327412', '327413', '327415', '327416', '327455', '334492', '334511', '334565', '334566', '334567', '334568', '334607', '334619', '334698', '334700', '334740', '334741']
nosy_count = 6.0
nosy_names = ['pitrou', 'eric.araujo', 'docs@python', 'berker.peksag', 'jdemeyer', 'dstufft']
pr_nums = ['9767', '11701', '11701', '11701', '11734', '11734', '11734']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25592'
versions = ['Python 2.7', 'Python 3.7', 'Python 3.8']

[jdemeyer]

The documentation for distutils claims that sys.exec_prefix is used in certain cases to install data_files, but this is simply not true (maybe it was true in the past or this sentence was copy/pasted from somewhere else?)

[merwok]

If you’re not sure about the reason for that sentence, I think you should not remove it from the docs but investigate more (look at the history, test the conditions (package with extension module), etc).

[jdemeyer]

If you’re not sure about the reason for that sentence, I think you should not remove it from the docs

If the docs are wrong, their history doesn't matter that much: the docs should be fixed regardless.

test the conditions (package with extension module)

I did that before posting this bug report. I also looked at the distutils sources. I couldn't find any evidence that data_files are ever installed in sys.exec_prefix.

[jdemeyer]

Just for fun, let's look at the history. That piece of documentation goes back to

commit 632bda3
Author: Fred Drake <fdrake@acm.org>
Date: Fri Mar 8 22:02:06 2002 +0000

Add more explanation of how data_files is used (esp. where the files end up
in the installation and how that location is determined).

I haven't tried to compile that Python version from 2002 but the distutils sources from that time look quite close to what they are today (scary, no?). So my gut feeling is that those docs were wrong even at the time that they were written.

[jdemeyer]

There is also

commit fa2f4b6
Author: Greg Ward <gward@python.net>
Date: Sat Jun 24 17:22:39 2000 +0000

Changed the default installation directory for data files (used by
the "install_data" command to the installation base, which is usually just
sys.prefix.  (Any setup scripts out there that specify data files will have
to set the installation directory, relative to the base, explicitly.)

with commit message stating that data_files are installed in sys.prefix.

[merwok]

If the docs are wrong, their history doesn't matter that much

What I was saying is that I am not sure that the docs are wrong.

Distutils is more touchy that the rest of the stdlib and I err on the side of caution.

[jdemeyer]

Well, I did try it on a minimal Python project. I also read the distutils sources and understood why it installs data_files in sys.prefix by default. So what more do you need to be convinced?

[merwok]

Did you try with a minimal project containing a C extension?

Did you install in a system where sys.prefix != sys.exec_prefix?

[jdemeyer]

Did you try with a minimal project containing a C extension?
Did you install in a system where sys.prefix != sys.exec_prefix?

Yes to both questions.