meadhikari meadhikari - 4 months ago 10
Python Question

Playing around with Devanagari characters

I have something like

a = "बिक्रम मेरो नाम हो"


I want to achieve something like

a[0] = बि a[1] = क्र a[3] = म


but as म takes 4 bytes while बि takes 8 bytes I am not able to get to that straight.
So what could be done to achieve that? In Python.

Answer

The algorithm for splitting text into grapheme clusters is given in Unicode Annex 29, section 3.1. I'm not going to implement the full algorithm for you here, but I'll show you roughly how to handle the case of Devanagari, and then you can read the Annex for yourself and see what else you need to implement.

The unicodedata module contains the information you need to detect the grapheme clusters.

>>> import unicodedata
>>> a = "बिक्रम मेरो नाम हो"
>>> [unicodedata.name(c) for c in a]
['DEVANAGARI LETTER BA', 'DEVANAGARI VOWEL SIGN I', 'DEVANAGARI LETTER KA', 
 'DEVANAGARI SIGN VIRAMA', 'DEVANAGARI LETTER RA', 'DEVANAGARI LETTER MA',
 'SPACE', 'DEVANAGARI LETTER MA', 'DEVANAGARI VOWEL SIGN E',
 'DEVANAGARI LETTER RA', 'DEVANAGARI VOWEL SIGN O', 'SPACE',
 'DEVANAGARI LETTER NA', 'DEVANAGARI VOWEL SIGN AA', 'DEVANAGARI LETTER MA',
 'SPACE', 'DEVANAGARI LETTER HA', 'DEVANAGARI VOWEL SIGN O']

In Devanagari, each grapheme cluster consists of an initial letter, optional pairs of virama (vowel killer) and letter, and an optional vowel sign. In regular expression notation that would be LETTER (VIRAMA LETTER)* VOWEL?. You can tell which is which by looking up the Unicode category for each code point:

>>> [unicodedata.category(c) for c in a]
['Lo', 'Mc', 'Lo', 'Mn', 'Lo', 'Lo', 'Zs', 'Lo', 'Mn', 'Lo', 'Mc', 'Zs',
 'Lo', 'Mc', 'Lo', 'Zs', 'Lo', 'Mc']

Letters are category Lo (Letter, Other), vowel signs are category Mc (Mark, Spacing Combining), virama is category Mn (Mark, Nonspacing) and spaces are category Zs (Separator, Space).

So here's a rough approach to split out the grapheme clusters:

def splitclusters(s):
    """Generate the grapheme clusters for the string s. (Not the full
    Unicode text segmentation algorithm, but probably good enough for
    Devanagari.)

    """
    virama = u'\N{DEVANAGARI SIGN VIRAMA}'
    cluster = u''
    last = None
    for c in s:
        cat = unicodedata.category(c)[0]
        if cat == 'M' or cat == 'L' and last == virama:
            cluster += c
        else:
            if cluster:
                yield cluster
            cluster = c
        last = c
    if cluster:
        yield cluster

>>> list(splitclusters(a))
['बि', 'क्र', 'म', ' ', 'मे', 'रो', ' ', 'ना', 'म', ' ', 'हो']