How about checking sys.modules? You could do that in a subprocess as part of your unit tests.
>>> lazy import email
>>> import sys
>>> sys.modules['email']
Traceback (most recent call last):
File "<console>", line 1, in <module>
KeyError: 'email'
>>> email
<module 'email' from '/lib/python315.zip/email/__init__.pyc'>
>>> sys.modules['email']
<module 'email' from '/lib/python315.zip/email/__init__.pyc'>
I don’t think there’s a good way to do this automatically on behalf of users, because it requires knowing whether or not their use intended to import.
I maintain different internal tools at work, some where we eagerly import things, including intentionally triggering lazy loaders in scipy, because it’s part of a long lived internal api, so we’d rather pay a predictable extra cost on startup to not have the first api request that hits a lazy code path be slower, but others where we want everything not used to not be imported (internal, frequently used cli tools. We don’t want our nondev users of these tools feeling like these are sluggish to respond and have to wait for someone else to fix it for them)
In terms of ensuring certain things are lazy or eager, once you define the behavior that’s desirable for your use case, you can write tests for it so long as it’s possible to introspect the lazy objects. The pep authors have made changes that address it being possible to check if you have the actual object or the deferal of the object, so as long as that remains the case, after whatever changes are required during implementation and acceptance, my answer to this would be to write a test for this if you have to care.
I’d be mildly in favor of a function in importlib that the only purpose of was to give a yes or no on if a module has been eagerly imported or not yet if it’s undesirable to have users poking at this themselves (it might be prudent to allow the implementation details to not be guaranteed on this in case issues arise later)
If the lazy module type gets exposed in types then I would expect an isinstance() check to be enough to determine if a module has been reified or not.
Thank you for your patience and civility.
I could have done a bit better on my end.
Don’t think I am stopping this thread.
In my opinion, what I said legitimately belongs here.
In any case, I have opened Alternative path for explicit lazy imports with complete lay out.
If anyone wants to discuss anything that I have said in this thread, please reply there, so not to take any more space here.
Perhaps the slow-mode (8h per post) can be removed now that there is a different threat, as to allow the natural discussion of PEP 810 (not other ideas) to continue?
Done, let’s see how it goes.
Freudian slip?
I have a couple questions about what should be suggested best practice for library authors. First, if I want to add lazy import functionality to my library but still support old versions of Python, how could I do it? There is no __future__ import and even if I put the lazy import inside a if sys.version > (X, Y): block, I would guess you still get a syntax error.
It would be nice if we could have a recommended way to do this and it’s not too clunky. Or, do we think using global lazy import control is sufficient? To me, it seems better that libraries are able to opt-in to the feature yet still be usable by old Python versions. If we had importlib.lazy_import_module() then we could use that.
A related question would be what is our advice about making a library work with -X lazy_imports=<mode> set to either all or none. My gut feeling is it would be best practice if libraries work with either mode. Ideally you would have CI tests that run tests in both modes. However, one thing that seems likely to trip that up is circular imports. In my experience, a lazy import is a convenient way to break the circularity. However, it would break if you set the global mode to none.
This might be solvable with another function, e.g. have both importlib.lazy_import_module() and importlib.maybe_lazy_import_module(). If I’m breaking circular imports, I can use the function where laziness doesn’t get disabled by the global mode.
That’s the idea behind the __lazy_modules__ dunder. If you want to have lazy imports in 3.15 and eager in older versions, you can just use the syntax of regular imports and add the modules you want to be lazy to the list. Older versions will just ignore the variable while 3.15 recognises it as enabling lazy imports without any new syntax.
Regarding working with the global flags: I don’t think the intent is that libraries work with the flags set arbitrarily. The idea behind the flags is that if you have some tightly controlled setup where you can somehow guarantee that everything works lazily/eagerly then you can use them, but in general you don’t touch them. There are plenty of perfectly legitimate coding practices that break if you force the import to be lazy or eager.
I don’t think we need functions that differentiate between “lazy but only if the global flag isn’t set” abd “actually always lazy”. If you’re using the flag you’re intentionally changing the semantics of what your libraries want. It might end up working fine in your use case, but it shouldn’t be encouraged to defensively code around that.
Ah, I see. It’s a long PEP, to be fair. 
That seems like a good solution.
The __lazy__ attribute could also fit this, assuming paths are resolved at lazy initialization eagerly, a module could have a __lazy__.py file that is eagerly executed, thus providing mitigability of eager checks and lazy heavy ops.
Also this
could be
assert hasattr(mod, '__lazy__')
They are not. See “Making lazy imports find the module without loading it” under the Rejected Ideas section for the detailed reasoning.
Just a short idea concerning library support of lazy imports.
If a library maintainer is aware of lazy imports and knows that he does not support them, is there an API planned to detect that the import was done lazily, so that he can issue a warning or rise an exception?
Something like
if is_imported_lazily():
raise RuntimeError("Lazy import not supported")
I’ll quote my earlier post on this topic (which, to be fair, was almost 300 posts ago!)
And if you’re worried about users turning on the “global lazy imports” option, the pep makes clear this is a “you broke it, you fix it” problem and not something library authors need to concern themselves with supporting.
(And the fact that you can check the type of imported modules without reifying gives you ways to do runtime checking of import types if you’re really paranoid.)
And the chance of causing unexpected problems when attempting to enforce those requirements programmatically is high enough to make such enforcement a bad idea, IMHO. Like how calling sys.exit(1) when user code imports an “internal-use only module” that’s not meant to be imported directly may seem like a good idea, but it crashes pydoc -k scans.
Dear PEP 810 authors. The Steering Council is happy to unanimously[1] accept “PEP 810, Explicit lazy imports”. Congratulations! We appreciate the way you were able to build on and improve the previously discussed (and rejected) attempt at lazy imports as proposed in PEP 690.
We have recommendations about some of the PEP’s details, a few suggestions for filling a couple of small gaps, and we have made decisions on the alternatives that you’ve left to the SC, all of which I’ll outline below. If you have any questions, please do reach out to the SC for clarification, either here, on the SC tracker, or in office hours.
Use lazy as the keyword. We debated many of the given alternatives (and some we came up with ourselves), and ultimately agreed with the PEP’s choice of the lazy keyword. The closest challenger was defer, but once we tried to use that in all the places where the term is visible, we ultimately didn’t think it was as good an overall fit. The same was true with all the other alternative keywords we could come up with, so… lazy it is!
What about from foo lazy import bar? Nope! We like that in both module imports and from-imports that the lazy keyword is the first thing on the line. It helps to visually recognize lazy imports of both varieties.
Leveraging a subclass of dict. We don’t see a need for this complicated alternative; please add this to the rejected ideas.
Allowing ’*’ in __lazy_modules__. We agree with the rationale for rejecting this idea; it can always be added later if needed.
One thing that the PEP does not mention is .pth files, which the site.py module processes, and which has some special handling for lines that begin with the string 'import' followed by a space or tab. It doesn’t make much sense for .pth files to support lazy imports, so we suggest that the PEP explicitly says that this special handling in .pth files will not be adapted to handle lazy imports.
There currently is no way to get the active filter mode, so please add a sys.get_lazy_imports() function. Also, do you think appending _mode to their names makes the purpose of these functions clearer? We leave that up to the PEP authors.
The PEP should be explicit about the precedence order between the different ways to set the mode, i.e. $PYTHON_LAZY_IMPORTS=<mode>, -X lazy_imports=<mode>, and sys.set_lazy_imports(). In all expectation, it will follow the same precedence order as other similar settings, but the PEP should be explicit.
We agree that the PEP should take no position on any style recommendations for sorting lazy imports. While we generally like the idea of grouping lazy imports together, let’s leave that up to the linters and auto-formatters to decide the details.
That should just about cover it. Again, thank you for your work on this, as it’s been a feature so many in the Python community have wanted for so long. Given the earlier attempts and existing workarounds, we think this strikes exactly the right balance.
-Barry, on behalf of the Python Steering Council
4 votes, as Pablo cannot vote ↩︎
Congrats! Looking forward to using this feature!