abs_paths() is the another most slow parts of site module.
We can speedup it if we have C implementation of os.path.abspath() or abs_paths().

But before trying it, is abs_paths() really needed for now?
It only rewrite __file__ and __cached__ of modules imported before site is imported.
What is difference between modules loaded befere / after site module?

Here is profile output. sysconfig dependency is removed by bpo-29585 patch, and pstat is modified
to show time in ms instead of sec.

Ordered by: cumulative time

ncalls tottime percall cumtime percall filename:lineno(function)
3/1 0.004 0.001 2.525 2.525 {built-in method builtins.exec}
1 0.003 0.003 2.524 2.524 x.py:1(<module>)
1 0.010 0.010 1.806 1.806 site.py:555(main)
4/3 0.022 0.005 1.179 0.393 <frozen importlib._bootstrap>:958(_find_and_load)
4/3 0.017 0.004 1.110 0.370 <frozen importlib._bootstrap>:931(_find_and_load_unlocked)
1 0.195 0.195 0.928 0.928 site.py:99(abs_paths)
108 0.098 0.001 0.776 0.007 posixpath.py:367(abspath)
4 0.035 0.009 0.647 0.162 <frozen importlib._bootstrap>:861(_find_spec)
4 0.005 0.001 0.589 0.147 <frozen importlib._bootstrap_external>:1150(find_spec)
4 0.043 0.011 0.584 0.146 <frozen importlib._bootstrap_external>:1118(_get_spec)
2/1 0.012 0.006 0.557 0.557 <frozen importlib._bootstrap>:641(_load_unlocked)
2/1 0.006 0.003 0.511 0.511 <frozen importlib._bootstrap_external>:673(exec_module)
108 0.461 0.004 0.493 0.005 posixpath.py:329(normpath)
16 0.150 0.009 0.453 0.028 <frozen importlib._bootstrap_external>:1234(find_spec)

path normalization was added by
38cb9f1

https://github.com/python/cpython/blob/master/Lib/importlib/_bootstrap_external.py#L1089-L1095

While '' in sys.path, it is converted to getcwd() before calling PathHook.
Is there any chance relative / not normalized path is in sys.path before site is loaded?

$ python2 -S
Python 2.7.12+ (default, Sep 17 2016, 12:08:02) 
[GCC 6.2.0 20160914] on linux2
>>> import x
>>> x.__file__
'x.py'

$ python3 -S
Python 3.6.0 (default, Dec 30 2016, 20:49:54) 
[GCC 6.2.0 20161005] on linux
>>> import x
>>> x.__file__
'/home/inada-n/x.py'

I think we can remove abs_paths() in site.py, thanks to _frozen_importlib_external.

I added all import experts to nosy list.
Please give me advice.

Aye, I agree it should be redundant now - import system should always be making __file__ and __cached__ absolutely at import time these days.

So +1 for dropping this from 3.7 and getting a bit of startup time back.

CI failure indicating it isn't redundant, but could still potentially be made faster since non-absolute paths should be relatively rare now:

======================================================================

FAIL: test_s_option (test.test_site.HelperFunctionsTests)

----------------------------------------------------------------------

Traceback (most recent call last):

File "/home/travis/build/python/cpython/Lib/test/test_site.py", line 173, in test_s_option

    self.assertIn(usersite, sys.path)

AssertionError: '/home/travis/.local/lib/python3.7/site-packages' not found in ['', '/usr/local/lib/python37.zip', '/home/travis/build/python/cpython/Lib', '/home/travis/build/python/cpython/build/lib.linux-x86_64-3.7-pydebug']

======================================================================

FAIL: test_abs_paths (test.test_site.ImportSideEffectTests)

----------------------------------------------------------------------

Traceback (most recent call last):

File "/home/travis/build/python/cpython/Lib/test/test_site.py", line 365, in test_abs_paths

    .format(os__file__.decode('ascii')))

AssertionError: False is not true : expected absolute path, got ../../Lib/os.py

----------------------------------------------------------------------

Note that "os" doesn't get imported normally, it gets injected as part of the importlib bootstrapping process (since _bootstrap_external.py needs the os module in order to work).

I got it.
removeduppaths() may change relpath in sys.path to absolute path.
abs_paths() changes __file__ and __cached__ for consistency with the changed sys.path.

I updated PR 167 to call abs_paths() only if removeduppaths() modified sys.path.
Strictly speaking, abs_paths() is required only when removeduppaths() converted relpath to absolute path.

But because duplicated paths are rare too, I think this approach is practical enough.

New changeset 2e4e011 by INADA Naoki in branch 'master':
bpo-29592: site: skip abs_paths() when it's redundant (GH-167)
2e4e011