PEP 235 – Import on Case-Insensitive Platforms
- Tim Peters <tim.peters at gmail.com>
- Standards Track
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.
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”,
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
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:
- 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,
sys.path, and do
then if Python finds
FiLe.pyfirst, 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.
- An ugly exception: if the first case-insensitive match on
sys.pathis for a file whose name is entirely in upper case (
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.
- And another exception: if the environment variable
PYTHONCASEOKexists, 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.
The proposed new semantics for the lower left box:
- If the
PYTHONCASEOKenvironment variable exists, same as before: silently accept the first case-insensitive match of any kind; raise
ImportErrorif none found.
- Else search
sys.pathfor the first case-sensitive match; raise
ImportErrorif 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
ImportError, in which latter case it still will, or in which
former case will continue searching, and either succeed or blow up
#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
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.
Last modified: 2022-03-09 16:04:44 GMT