<s>[INST] Turn a string into a valid filename in Python I have a string that I want to use as a filename, so I want to remove all characters that wouldn't be allowed in filenames, using Python.
I'd rather be strict than otherwise, so let's say I want to retain only letters, digits, and a small set of other characters like "_-.() ". What's the most elegant solution?
The filename needs to be valid on multiple operating systems (Windows, Linux and Mac OS) - it's an MP3 file in my library with the song title as the filename, and is shared and backed up between 3 machines.
 [/INST] You can look at the Django framework for how they create a "slug" from arbitrary text.  A slug is URL- and filename- friendly.
Their template/defaultfilters.py (at around line 183) defines a function, slugify, that's probably the gold standard for this kind of thing.  Essentially, their code is the following.
def slugify(value):
    """
    Normalizes string, converts to lowercase, removes non-alpha characters,
    and converts spaces to hyphens.
    """
    import unicodedata
    value = unicodedata.normalize('NFKD', value).encode('ascii', 'ignore')
    value = unicode(re.sub('[^\w\s-]', '', value).strip().lower())
    value = unicode(re.sub('[-\s]+', '-', value))

There's more, but I left it out, since it doesn't address slugification, but escaping.
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