aod

AttrOnDemand — a descriptor for instance attributes whose values are determined on demand on first access.

Installation

pip install aod

Requires Python 3.12 or later (uses PEP 695 generic syntax).

Usage

The name of the method that will set the attribute values can be passed to the constructor. Several attributes can share the same fetch method, so a single call can populate them all at once:

from aod import AttrOnDemand

class Person:
	firstname = AttrOnDemand[str]("fetch")
	lastname = AttrOnDemand[str]("fetch")

	def __init__(self, dbid):
		self.dbid = dbid

	def fetch(self):
		cursor = db.cursor()
		cursor.execute("select firstname, lastname from person where id=:id", id=self.dbid)
		row = cursor.fetchone()
		self.firstname = row.firstname
		self.lastname = row.lastname

p = Person(42)
print(p.firstname)   # triggers fetch, sets both firstname and lastname
print(p.lastname)    # already known, no fetch

Internally the value of an attribute foo will be stored in the instance dictionary as _foo. As long as the value hasn't been set yet, _foo will not be in the instance dictionary, and accessing the attribute will call the fetch method. The setter simply sets _foo in the instance dictionary, so the fetch method should set the attribute values via the setter.

Deleting an attribute forgets its cached value; the next access will fetch it again:

del p.firstname
print(p.firstname)   # fetches again (and refreshes lastname too)

Use AttrOnDemand.known(instance) to check whether a value has already been computed without triggering a fetch.

Note that using AttrOnDemand descriptors only works for instances that have instance dictionaries.

License

MIT — see LICENSE.