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FractionalIndex

Overview

A FractionalIndex takes a list of existing items and allows you to insert a new item between any two existing items, or at the start or end.

Internally, it uses the fractional_indexing pypi package, which works as follows:

first = generate_key_between(None, None) # 'a0'
second = generate_key_between(first, None) # 'a1' (after 1st)
third = generate_key_between(second, None) # 'a2' (after 2nd)
zeroth = generate_key_between(None, first) # 'Zz' (before 1st)

FractionalIndex differs with fractional_indexing in how it handles skipping the before or after parameters. If you only pass before, it will insert before the specified item, but after the largest item before that (if there is none, then it will insert at the start). If you only pass after, it will insert after the specified item, but before the smallest item after that (if there is none, then it will insert at the end).

idx = FractionalIndex()
i1 = idx.insert() # starts a new index
print(i1) # 'a0'
i2 = idx.insert(after=i1) # inserts after i1
i3 = idx.insert(before=i2) # inserts before i2
i4 = idx.insert(i3, i2) # inserts between i3 and i2
i5 = idx.insert() # adds to the end

To add to the start, you can either pass the first item to insert(before=...), or use begin():

i6 = idx.begin()

You can create an index from a list of existing items, which will be sorted:

idx = FractionalIndex([i1, i2, i3, i4, i5, i6])

Implementation

Internally, FractionalIndex by default uses IndexingList, a subclass of sortedcontainers.SortedList to store the items. The sort is needed so that insert without one of both of after or before can quickly find the next, previous or start/end item. The subclass adds the following methods (which are the only methods required by FractionalIndex):

  • after(item): returns the next item after the specified item, or None if there is none.
  • before(item): returns the previous item before the specified item, or None if there is none.
  • begin(): returns the first item.
  • end(): returns the last item.

Alternative Implementations

FractionalIndex comes with a few alternative implementations.

FileIndex

FileIndex uses FileIndexing instead of IndexingList internally. FileIndexing is a simple implementation which uses a list of files in a directory, to identify the correct locations in the index. For instance, consider the following directory contents:

a0-create-table.sql
a1-add-column.sql
a2-add-index.sql

In this case, we can create a suitable FileIndex with idx = FileIndex(directory='.', separator='-'). Since these are the default values, we can simply call FileIndex() to create it. Note that the directory will be re-scanned for the latest files on every method call.

You can then use it just like a regular FractionalIndex:

idx = FileIndex(directory='migrations', separator='-')
new_id = idx.begin()  # Creates ID before first file
new_id = idx.insert()  # Creates ID after last file
new_id = idx.insert(after='a0')  # Creates ID between a0 and next file
new_id = idx.insert(before='a1')  # Creates ID between previous file and a1
new_id = idx.insert('a0', 'a1')  # Creates ID between a0 and a1

When you get a new ID, you'll need to create the corresponding file (e.g., {new_id}-something.sql) for it to be included in future operations.

SqliteIndex

SqliteIndex indexes a sqlite DB table by using the SqliteIndexing class. Generally, the fractional index will be the primary key of the table. To use it, you need to pass a Connection, table and column (defaults to 'id') to the constructor:

import sqlite3

conn = sqlite3.connect("database.db")
conn.execute("CREATE TABLE migrations (id TEXT PRIMARY KEY)")

# Create the index
idx = SqliteIndex(conn, 'migrations', 'id')

# Use it like any other FractionalIndex
new_id = idx.begin()  # Creates ID before first row
new_id = idx.insert()  # Creates ID after last row
new_id = idx.insert(after='a0')  # Creates ID between a0 and next row
new_id = idx.insert(before='a1')  # Creates ID between previous row and a1
new_id = idx.insert('a0', 'a1')  # Creates ID between a0 and a1

# Don't forget to insert the new ID into your table
conn.execute("INSERT INTO migrations (id) VALUES (?)", (new_id,))
conn.commit()

Note that SqliteIndex will query the database on every operation to ensure it's working with the latest data, but it won't automatically insert new IDs - you need to do that yourself after getting a new ID.