On Sat, Feb 6, 2021 at 12:51 AM Matt del Valle <matthewgdv@gmail.com> wrote:
Unfortunately, [lazy importers are] fundamentally incapable of dealing with 'from x import y' syntax, because this is always loaded eagerly by the Python interpreter.
This is because it's fundamentally hard.
With the increasingly widespread adoption of type hints I think it is time to revive the proposal for lazy imports as an official language feature.
Are you writing this proposal assuming PEP 563 behaviour? If type hints are the primary justification, it may be less necessary. But lazy importing does have other value.
It also is fundamentally incapable of handling this other extremely common usage pattern:
some_library/ __init__.py
__all__ = ["SomeClass", "AnotherClass"]
from .extremely_expensive_module import SomeClass from .another_extremely_expensive_module import AnotherClass
some_module.py
from some_library import SomeClass
Hmm. Okay. Let's talk semantics, not syntax. What exactly should the interpreter do with this line? Is there an actual object in the module dictionary under the name "SomeClass", and if so, what sort of object? One possible meaning for this kind of "lazy from import" would be to maintain a table of import definitions, just like the module dictionary. (I would be inclined to say that this can ONLY be done at module level; lazy imports inside a class or function seem less useful and more hassle than they're worth. But if you disagree, feel free to expand this to other namespaces.) Whenever a module name is looked up, the interpreter looks first in the module dictionary, and then if it's not found, tries the lazy imports; upon finding the import definition, it removes it, performs the import, and stuffs the object into the module dictionary. This would be a fair amount of hassle, and it'd have a performance impact on EVERY module name lookup (and everything that goes beyond that to the builtins). But it would, I believe, give you an actual lazy import system. So the question is: how often is this going to be useful?
Where a library author wants to make several classes in their library's public API available at package level in the __init__.py, but doesn't want to eagerly load all of them when many users will only ever use one in a given program. In this situation if I try to import SomeClass from some_library/__init__.py I will always trigger AnotherClass to be loaded as well, totally unnecessarily.
I wonder if this particular use-case is better served by a module subclass that does the lazy loading. Effectively, you get to break up a module into a package of N separately-loadable modules, and on first use of anything from a particular shard, that shard gets loaded.
This would be insanely useful. I really hope this sparks some discussion :)
Not sure HOW useful it would be in general, but yes, there definitely are use-cases for it. I'm pretty dubious about having a separate lookup on every name load just in case it's been lazily imported, though. ChrisA