Efficiently list all characters in a given Unicode category

Question

Often one wants to list all characters in a given Unicode category. For example:

• List all Unicode whitespace, How can I get all whitespaces in UTF-8 in Python?
• Characters with the property Alphabetic

It is possible to produce this list by iterating over all Unicode code-points and testing for the desired category (Python 3):

[c for c in map(chr, range(0x110000)) if unicodedata.category(c) in ('Ll',)]

or using regexes,

re.findall(r'\s', ''.join(map(chr, range(0x110000))))

But these methods are slow. Is there a way to look up a list of characters in the category without having to iterate over all of them?

Related question for Perl: How do I get a list of all Unicode characters that have a given property?

Answer 1

If you need to do this often, it's easy enough to build yourself a re-usable map:

import sys
import unicodedata
from collections import defaultdict

unicode_category = defaultdict(list)
for c in map(chr, range(sys.maxunicode + 1)):
    unicode_category[unicodedata.category(c)].append(c)

And from there on out use that map to translate back to a series of characters for a given category:

alphabetic = unicode_category['Ll']

If this is too costly for start-up time, consider dumping that structure to a file; loading this mapping from a JSON file or other quick-to-parse-to-dict format should not be too painful.

Once you have the mapping, looking up a category is done in constant time of course.