python-peps/peps/pep-0235.rst

PEP: 235
Title: Import on Case-Insensitive Platforms
Version: $Revision$
Last-Modified: $Date$
Author: Tim Peters <tim.peters@gmail.com>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 21-Feb-2001
Python-Version: 2.1
Post-History: 16-Feb-2001


Note
====

This is essentially a retroactive PEP: the issue came up too late
in the 2.1 release process to solicit wide opinion before deciding
what to do, and can't be put off until 2.2 without also delaying
the Cygwin and MacOS X ports.


Motivation
==========

File systems vary across platforms in whether or not they preserve
the case of filenames, and in whether or not the platform C
library file-opening functions do or don't insist on
case-sensitive matches::

                         case-preserving     case-destroying
                     +-------------------+------------------+
    case-sensitive   | most Unix flavors | brrrrrrrrrr      |
                     +-------------------+------------------+
    case-insensitive | Windows           | some unfortunate |
                     | MacOSX HFS+       | network schemes  |
                     | Cygwin            |                  |
                     |                   | OpenVMS          |
                     +-------------------+------------------+

In the upper left box, if you create "fiLe" it's stored as "fiLe",
and only ``open("fiLe")`` will open it (``open("file")`` will not, nor
will the 14 other variations on that theme).

In the lower right box, if you create "fiLe", there's no telling
what it's stored as -- but most likely as "FILE" -- and any of the
16 obvious variations on ``open("FilE")`` will open it.

The lower left box is a mix: creating "fiLe" stores "fiLe" in the
platform directory, but you don't have to match case when opening
it; any of the 16 obvious variations on ``open("FILe")`` work.

NONE OF THAT IS CHANGING!  Python will continue to follow platform
conventions w.r.t. whether case is preserved when creating a file,
and w.r.t. whether ``open()`` requires a case-sensitive match.  In
practice, you should always code as if matches were
case-sensitive, else your program won't be portable.

What's proposed is to change the semantics of Python "import"
statements, and there *only* in the lower left box.


Current Lower-Left Semantics
============================

Support for MacOSX HFS+, and for Cygwin, is new in 2.1, so nothing
is changing there.  What's changing is Windows behavior.  Here are
the current rules for import on Windows:

1. Despite that the filesystem is case-insensitive, Python insists
   on a case-sensitive match.  But not in the way the upper left
   box works: if you have two files, ``FiLe.py`` and ``file.py`` on
   ``sys.path``, and do ::

       import file

   then if Python finds ``FiLe.py`` first, it raises a ``NameError``.
   It does *not* go on to find ``file.py``; indeed, it's impossible to
   import any but the first case-insensitive match on ``sys.path``,
   and then only if case matches exactly in the first
   case-insensitive match.

2. An ugly exception: if the first case-insensitive match on
   ``sys.path`` is for a file whose name is entirely in upper case
   (``FILE.PY`` or ``FILE.PYC`` or ``FILE.PYO``), then the import silently
   grabs that, no matter what mixture of case was used in the
   import statement.  This is apparently to cater to miserable old
   filesystems that really fit in the lower right box.  But this
   exception is unique to Windows, for reasons that may or may not
   exist.

3. And another exception: if the environment variable ``PYTHONCASEOK``
   exists, Python silently grabs the first case-insensitive match
   of any kind.

So these Windows rules are pretty complicated, and neither match
the Unix rules nor provide semantics natural for the native
filesystem.  That makes them hard to explain to Unix *or* Windows
users.  Nevertheless, they've worked fine for years, and in
isolation there's no compelling reason to change them.

However, that was before the MacOSX HFS+ and Cygwin ports arrived.
They also have case-preserving case-insensitive filesystems, but
the people doing the ports despised the Windows rules.  Indeed, a
patch to make HFS+ act like Unix for imports got past a reviewer
and into the code base, which incidentally made Cygwin also act
like Unix (but this met the unbounded approval of the Cygwin
folks, so they sure didn't complain -- they had patches of their
own pending to do this, but the reviewer for those balked).

At a higher level, we want to keep Python consistent, by following
the same rules on *all* platforms with case-preserving
case-insensitive filesystems.


Proposed Semantics
==================

The proposed new semantics for the lower left box:

A. If the ``PYTHONCASEOK`` environment variable exists, same as
   before: silently accept the first case-insensitive match of any
   kind; raise ``ImportError`` if none found.

B. Else search ``sys.path`` for the first case-sensitive match; raise
   ``ImportError`` if none found.

#B is the same rule as is used on Unix, so this will improve
cross-platform portability.  That's good.  #B is also the rule the Mac
and Cygwin folks want (and wanted enough to implement themselves,
multiple times, which is a powerful argument in PythonLand).  It
can't cause any existing non-exceptional Windows import to fail,
because any existing non-exceptional Windows import finds a
case-sensitive match first in the path -- and it still will.  An
exceptional Windows import currently blows up with a ``NameError`` or
``ImportError``, in which latter case it still will, or in which
former case will continue searching, and either succeed or blow up
with an ``ImportError``.

#A is needed to cater to case-destroying filesystems mounted on Windows,
and *may* also be used by people so enamored of "natural" Windows
behavior that they're willing to set an environment variable to
get it.  I don't intend to implement #A for Unix too, but that's
just because I'm not clear on how I *could* do so efficiently (I'm
not going to slow imports under Unix just for theoretical purity).

The potential damage is here: #2 (matching on ``ALLCAPS.PY``) is
proposed to be dropped.  Case-destroying filesystems are a
vanishing breed, and support for them is ugly.  We're already
supporting (and will continue to support) ``PYTHONCASEOK`` for their
benefit, but they don't deserve multiple hacks in 2001.



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