[docs] [issue13828] Further improve casefold documentation

[Jim Jewett]

New submission from Jim Jewett <jimjjewett at gmail.com>:

> http://hg.python.org/cpython/rev/0b5ce36a7a24
> changeset:   74515:0b5ce36a7a24


> +   Casefolding is similar to lowercasing but more aggressive because it is
> +   intended to remove all case distinctions in a string. For example, the German
> +   lowercase letter ``'ß'`` is equivalent to ``"ss"``. Since it is already
> +   lowercase, :meth:`lower` would do nothing to ``'ß'``; :meth:`casefold`
> +   converts it to ``"ss"``.

Perhaps add the recommendation to canonicalize as well.

A complete, but possibly too long, try is below:


Casefolding is similar to lowercasing but more aggressive because it is intended to remove all case distinctions in a string. For example, the German lowercase letter ``'ß'`` is equivalent to ``"ss"``. Since it is already lowercase, :meth:`lower` would do nothing to ``'ß'``; :meth:`casefold` converts it to ``"ss"``.  Note that most case-insensitive matches should also match compatibility equivalent characters.  

The casefolding algorithm is described in section 3.13 of the Unicode Standard.  Per D146, a compatibility caseless match can be achieved by

    from unicodedata import normalize
    def caseless_compat(string):
        nfd_string = normalize("NFD", string)
        nfkd1_string = normalize("NFKD", nfd_string.casefold())
        return normalize("NFKD", nfkd1_string.casefold())

----------
assignee: docs at python
components: Documentation
messages: 151644
nosy: Jim.Jewett, benjamin.peterson, docs at python
priority: normal
severity: normal
status: open
title: Further improve casefold documentation
versions: Python 3.3

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