lazy imports¶

every import and from import statement in a .by file is automatically marked lazy. the transpiler prepends the lazy keyword (PEP 810, Python 3.15+) so the runtime defers module loading until first use

import os

print(os)

transpiles to:

lazy import os

print(os)

Python 3.15's runtime registers os in sys.modules immediately but defers executing its body until the first attribute access on the module object. accessing os.sep (or print(os), which calls __repr__) is what actually loads the module

supported forms¶

basedpython	Python output
`import os`	`lazy import os`
`import os as o`	`lazy import os as o`
`import os.path as p`	`lazy import os.path as p`
`from os import path`	`lazy from os import path`
`from os import path as p`	`lazy from os import path as p`
`from os import path, getcwd`	`lazy from os import path, getcwd`
`from .pkg import x`	`lazy from .pkg import x`

import a.b without an alias stays eager (write import a.b as ab to opt in). from __future__ import … and from x import * are always eager

target version¶

on python 3.15 and later, the PEP 810 lazy keyword is used directly. on older runtimes, a runtime polyfill is emitted. from __future__ and from x import * are always left eager.

set the target with --min-version 3.15 on by transpile/by build/by run (by check uses --python-version for the same concept)