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Python Enhancement Proposals

PEP 235 – Import on Case-Insensitive Platforms

Tim Peters <tim.peters at>
Standards Track

Table of Contents


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”, 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, and on sys.path, and do
    import file

    then if Python finds first, it raises a NameError. It does not go on to find; 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:

  1. If the PYTHONCASEOK environment variable exists, same as before: silently accept the first case-insensitive match of any kind; raise ImportError if none found.
  2. 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.


Last modified: 2023-09-09 17:39:29 GMT