Break text into letters (regular Unicode graphemes+language specific digraphs)

Question

In English, "Sh" is two letters. In other languages it's considered a single letter. I'm trying to calculate the length of a string in a Unicode aware way, with this in mind. I'm imagining a function like

def count_letters(my_string, lang="en")

But here's how I would do that for just one language, using a library that has had no new versions since 2015, uniseg:

from uniseg.graphemecluster import grapheme_clusters


def albanian_digraphs(s, breakables):
    digraphs = ["Dh", "Gj", "Ll", "Nj", "Rr", "Sh", "Th", "Xh", "Zh"]
    digraphs += [d.lower() for d in digraphs]
    for i, breakable in enumerate(breakables):
        for first, second in digraphs:
            if s.endswith(first, 0, i) and s.startswith(second, i):
                yield 0
                break
        else:
            yield breakable


# from https://sq.wiktionary.org/wiki/Speciale:PrefixIndex?prefix=dh
for text in ('dhallanik', 'dhelpëror', 'dhembshurisht', 'dhevështrues', 'dhimbshëm', 'dhjamosje', 'dhjetëballësh', 'dhjetëminutësh', 'dhogaç', 'dhogiç', 'dhomë-muze', 'dhuratë', 'dhëmbinxhi', 'dhëmbçoj', 'dhëmbëkatarosh'):
    print(list(grapheme_clusters(text, albanian_digraphs)))

#['dh', 'a', 'll', 'a', 'n', 'i', 'k']
#['dh', 'e', 'l', 'p', 'ë', 'r', 'o', 'r']
#['dh', 'e', 'm', 'b', 'sh', 'u', 'r', 'i', 'sh', 't']
#['dh', 'e', 'v', 'ë', 'sh', 't', 'r', 'u', 'e', 's']
#['dh', 'i', 'm', 'b', 'sh', 'ë', 'm']
#['dh', 'j', 'a', 'm', 'o', 's', 'j', 'e']
#['dh', 'j', 'e', 't', 'ë', 'b', 'a', 'll', 'ë', 'sh']
#['dh', 'j', 'e', 't', 'ë', 'm', 'i', 'n', 'u', 't', 'ë', 'sh']
#['dh', 'o', 'g', 'a', 'ç']
#['dh', 'o', 'g', 'i', 'ç']
#['dh', 'o', 'm', 'ë', '-', 'm', 'u', 'z', 'e']
#['dh', 'u', 'r', 'a', 't', 'ë']
#['dh', 'ë', 'm', 'b', 'i', 'n', 'xh', 'i']
#['dh', 'ë', 'm', 'b', 'ç', 'o', 'j']
#['dh', 'ë', 'm', 'b', 'ë', 'k', 'a', 't', 'a', 'r', 'o', 'sh']

Answer 1

For starter, you do not need to create you digraphs list each time the function is called: they won't change, so better create them once as a global constant. You also forgot to add capitalized versions in the list so that 'HELLO' is split into ['H', 'E', 'LL', 'O'] instead of the current ['H', 'E', 'L', 'L', 'O'].

Second, your linear research in the digraphs list can be time consuming when all you want to know is if characters at position i-1 (if any) and i forms a digraph present in your list. I’d rather write it as yield 0 if s[i-1:i+1] in digraphs else breakable. Of course, for this to work efficiently, you will need digraphs to be a set instead of a list where lookups are \$\mathcal{O}(1)\$ instead of \$\mathcal{O}(n)\$.

Lastly, I would only consider lowercase variants and turn the selected two characters lowercase before checking:

from uniseg.graphemecluster import grapheme_clusters


DIGRAPHS = {"dh", "gj", "ll", "nj", "rr", "sh", "th", "xh", "zh"}


def albanian_digraphs(s, breakables):
    for i, breakable in enumerate(breakables):
        yield 0 if s[i-1:i+1].lower() in DIGRAPHS else breakable


if __name__ == '__main__':    
    # from https://sq.wiktionary.org/wiki/Speciale:PrefixIndex?prefix=dh
    for text in ('dhallanik', 'dhelpëror', 'dhembshurisht', 'dhevështrues', 'dhimbshëm', 'dhjamosje', 'dhjetëballësh', 'dhjetëminutësh', 'dhogaç', 'dhogiç', 'dhomë-muze', 'dhuratë', 'dhëmbinxhi', 'dhëmbçoj', 'dhëmbëkatarosh'):
        print(list(grapheme_clusters(text, albanian_digraphs)))

Also note the use of if __name__ == '__main__' to separate the actual code from the tests.