This is based on Implementing Fractional Indexing by David Greenspan .
Fractional indexing is a technique to create an ordering that can be used for Realtime Editing of Ordered Sequences.
This implementation includes variable-length integers, and the prepend/append optimization described in David's article.
$ pip install fractional-indexingfrom fractional_indexing import generate_key_between
# Insert at the beginning
first = generate_key_between(None, None)
assert first == 'a0'
# Insert after 1st
second = generate_key_between(first, None)
assert second == 'a1'
# Insert after 2nd
third = generate_key_between(second, None)
assert third == 'a2'
# Insert before 1st
zeroth = generate_key_between(None, first)
assert zeroth == 'Zz'
# Insert in between 2nd and 3rd (midpoint)
second_and_half = generate_key_between(second, third)
assert second_and_half == 'a1V'Use this when generating multiple keys at some known position, as it spaces out indexes more evenly and leads to shorter keys.
from fractional_indexing import generate_n_keys_between
# Insert 3 at the beginning
keys = generate_n_keys_between(None, None, n=3)
assert keys == ['a0', 'a1', 'a2']
# Insert 3 after 1st
keys = generate_n_keys_between('a0', None, n=3)
assert keys == ['a1', 'a2', 'a3']
# Insert 3 before 1st
keys = generate_n_keys_between(None, 'a0', n=3)
assert keys == ['Zx', 'Zy', 'Zz']
# Insert 3 in between 2nd and 3rd (midpoint)
keys = generate_n_keys_between('a1', 'a2', n=3)
assert keys == ['a1G', 'a1V', 'a1l']from fractional_indexing import validate_order_key, FIError
validate_order_key('a0')
try:
validate_order_key('foo')
except FIError as e:
print(e) # fractional_indexing.FIError: invalid order key: fooBy default, this library uses Base62 character encoding. To use a different set of digits, pass them in as the digits
argument to generate_key_between(), generate_n_keys_between(), and validate_order_key():
from fractional_indexing import generate_key_between, generate_n_keys_between, validate_order_key
BASE_95_DIGITS = ' !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~'
assert generate_key_between(None, None, digits=BASE_95_DIGITS) == 'a '
assert generate_key_between('a ', None, digits=BASE_95_DIGITS) == 'a!'
assert generate_key_between(None, 'a ', digits=BASE_95_DIGITS) == 'Z~'
assert generate_n_keys_between('a ', 'a!', n=3, digits=BASE_95_DIGITS) == ['a"', 'a#', 'a$']
validate_order_key('a ', digits=BASE_95_DIGITS)This is a Python port of the original JavaScript implementation by @rocicorp. That means that this implementation is byte-for-byte compatible with: