Unicode has solved a lot of problems. Anyone that remembers the mess of ISO-8859-* vs. CP437 (and of course it’s even worse for non-Western languages) can attest to that. And of course, these days they’re doing the useful work of…. codifying emojis.

Emojis aside, things aren’t all so easy. Today’s cause of pain: Python 3. So much pain.

Python decided to fully integrate Unicode into the language. Nice idea, right?

But here come the problems. And they are numerous.

gpodder, for instance, frequently exits with tracebacks due to Python errors converting podcast titles with smartquotes into ASCII. Then you have the case where the pexpect docs say to use logfile = sys.stdout to show the interaction with the virtual terminal. Only that causes an error these days.

But processing of filenames takes the cake. I was recently dealing with data from 20 years ago, before UTF-8 was a filename standard. These filenames are still valid on Unix. tar unpacks them, and they work fine. But you start getting encoding errors from Python trying to do things like store filenames in strings. For a Python program to properly support all valid Unix filenames, it must use “bytes” instead of strings, which has all sorts of annoying implications. What’s the chances that all Python programs do this correctly? Yeah. Not high, I bet.

I recently was processing data generated by mtree, which uses octal escapes for special characters in filenames. I thought this should be easy in Python, eh?

• Many wrong answers – you’ll get an encoding error for certain values. Even the regular expression solution on that page doesn’t work.
• Even more wrong answers exist.

That second link had a mention of an undocumented function, codecs.escape_decode, which does it right. I finally had to do this:

    if line.startswith(b'#'):
        continue
    fields = line.split()
    filename = codecs.escape_decode(fields[0])[0]
    filetype = getfield(b"type", fields[1:])
    if filetype == b"file":

And, whatever you do, don’t accidentally write if filetype == "file" — that will silently always evaluate to False, because "file" tests different than b"file". Not that I, uhm, wrote that and didn’t notice it at first…

So if you want to actually handle Unix filenames properly in Python, you:

• Must have a processing path that fully avoids Python strings.
• Must use sys.{stdin,stdout}.buffer instead of just sys.stdin/stdout
• Must supply filenames as bytes to various functions. See PEP 0471 for this comment: “Like the other functions in the os module, scandir() accepts either a bytes or str object for the path parameter, and returns the DirEntry.name and DirEntry.path attributes with the same type as path. However, it is strongly recommended to use the str type, as this ensures cross-platform support for Unicode filenames. (On Windows, bytes filenames have been deprecated since Python 3.3).” So if you want to be cross-platform, it’s even worse, because you can’t use str on Unix nor bytes on Windows.

Update: Would you like to receive filenames on the command line? I’ll hand you this fine mess. And the environment? it’s not even clear.