[Python-checkins] cpython: Issue #18810: Be optimistic with stat calls when seeing if a directory

[Brett Cannon]

On Fri, Oct 18, 2013 at 9:06 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

>
> On 19 Oct 2013 03:24, "brett.cannon" <python-checkins at python.org> wrote:
> >
> > http://hg.python.org/cpython/rev/11f2f4af1979
> > changeset:   86444:11f2f4af1979
> > user:        Brett Cannon <brett at python.org>
> > date:        Fri Oct 18 13:24:13 2013 -0400
> > summary:
> >   Issue #18810: Be optimistic with stat calls when seeing if a directory
> > exists when checking for a package.
> >
> > Before there was an isdir check and then various isfile checks for
> > possible __init__ files when looking for a package.
> > This change drops the isdir check by leaning
> > on the assumption that a directory will not contain something named
> > after the module being imported which is not a directory. If the module
> > is a package then it saves a stat call. If there is nothing in the
> > directory with the potential package name it also saves a stat call.
> > Only if there is something in the directory named the same thing as
> > the potential package will the number of stat calls increase
> > (due to more wasteful __init__ checks).
>
> I don't follow this logic. There's now not even an existence check for the
> base name, so it reads to me like we will look for all the possible init
> file extensions even if there's *no* directory with an appropriate name.
>
> What am I missing?
>

``if cache_module in cache:``, the line above the _path_join() call and the
guard that blocks the entire package search.

-Brett


> Cheers,
> Nick.
>
> >
> > Semantically there is no change as the isdir check moved
> > down so that namespace packages continue to have no chance of
> > accidentally collecting non-existent directories.
> >
> > files:
> >   Lib/importlib/_bootstrap.py |    19 +-
> >   Python/importlib.h          |  1537 +++++++++++-----------
> >   2 files changed, 777 insertions(+), 779 deletions(-)
> >
> >
> > diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
> > --- a/Lib/importlib/_bootstrap.py
> > +++ b/Lib/importlib/_bootstrap.py
> > @@ -1406,16 +1406,15 @@
> >          # Check if the module is the name of a directory (and thus a
> package).
> >          if cache_module in cache:
> >              base_path = _path_join(self.path, tail_module)
> > -            if _path_isdir(base_path):
> > -                for suffix, loader in self._loaders:
> > -                    init_filename = '__init__' + suffix
> > -                    full_path = _path_join(base_path, init_filename)
> > -                    if _path_isfile(full_path):
> > -                        return (loader(fullname, full_path),
> [base_path])
> > -                else:
> > -                    # A namespace package, return the path if we don't
> also
> > -                    #  find a module in the next section.
> > -                    is_namespace = True
> > +            for suffix, loader in self._loaders:
> > +                init_filename = '__init__' + suffix
> > +                full_path = _path_join(base_path, init_filename)
> > +                if _path_isfile(full_path):
> > +                    return (loader(fullname, full_path), [base_path])
> > +            else:
> > +                # If a namespace package, return the path if we don't
> > +                #  find a module in the next section.
> > +                is_namespace = _path_isdir(base_path)
> >          # Check for a file w/ a proper suffix exists.
> >          for suffix, loader in self._loaders:
> >              full_path = _path_join(self.path, tail_module + suffix)
> > diff --git a/Python/importlib.h b/Python/importlib.h
> > --- a/Python/importlib.h
> > +++ b/Python/importlib.h
> > [stripped]
> >
> > --
> > Repository URL: http://hg.python.org/cpython
> >
> > _______________________________________________
> > Python-checkins mailing list
> > Python-checkins at python.org
> > https://mail.python.org/mailman/listinfo/python-checkins
> >
>
>
> _______________________________________________
> Python-checkins mailing list
> Python-checkins at python.org
> https://mail.python.org/mailman/listinfo/python-checkins
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-checkins/attachments/20131019/c147283d/attachment-0001.html>