Support eager singleton

[richburdon]

There doesn't seem to be a way to bind eager singletons. The only reference is in the Module's provides method, but this is unreferenced.

def provides(interface, scope=None, eager=False):

[alecthomas]

Yes, I should remove that from the method signature. I don't think it will be added at this stage.

[richburdon]

Not added because it's not under active development or a time issue? Or not useful/practical?

BTW, are you still actively developing the project? Is there a better alternative?

Thanks.

[alecthomas]

Mostly the latter; not useful enough to warrant the effort IMO.

Yes, Injector is still being developed, it's just mostly stable so there's not much activity.

You can have a look at pinject, but it hasn't been touched in two years and IMO is not as nice as Injector anyway ;)

[jstasiak]

I can confirm that the lack of activity is due to the code just working.

[richburdon]

Fair enough :).

I find myself in a minority of Python developers who can't live without DI,
so thanks for this package.

I do however, think eager singletons are important. As would like to see
Sequence and Map keys support instantiation by class (rather than by
instance). I'll try to send a pull request for the workarounds that I have
created.

On Thu, Sep 17, 2015 at 6:14 PM Jakub Stasiak notifications@github.com
wrote:

I can confirm that the lack of activity is due to the code just working.

—
Reply to this email directly or view it on GitHub
#44 (comment).

[juharris]

On a similar note, I'd like to be able instantiate all singletons when my server starts, something analogous to Guice's Stage.PRODUCTION. Any particular reasons why it's not supported? Would you mind if I added it? I'd probably do it as a stage parameter to Injector.

[jstasiak]

I completely forgot this issue was still open :) I wouldn't be against it in principle, I'm just wondering what the actual implementation would look like.

[comtihon]

@jstasiak
It is used to catch configuration errors on startup, when spawning all singleton services.
F.e. I've introduced a breaking change (configuration/compilation error/broke all injections by binding unknown/wrong module).
Then I start the service and it is ok till any of injected services will be used.

Another issue is - run some things, like loading and configuring some RO model (~400MB) before spawning workers, so they won't make memory copies of this model.

[jstasiak]

Yeah I'm not saying it's not useful, it's the implementation I wondered about, the general case seems tricky. For example:

@singleton
SomeClass:
    pass


injector = Injector()
eagerly_instantiate_singletons(injector)

With auto_bind enabled (the default) Injector doesn't know anything about SomeClass until it's instantiated so the eager instantiation mechanism would need to actively look for classes (All loaded classes? Classes in a particular module?).

Depending on the application the easiest and the most helpful could be to do

for t in [list, of, some, known, singletons, that, can, cause, trouble]:
    injector.get(t)

at the start of your program.

BTW not sure if memory saving would work in the scenario you describe, see https://instagram-engineering.com/dismissing-python-garbage-collection-at-instagram-4dca40b29172

[juharris]

So for Guice it only knows about the singletons mentioned in the module or through dependencies. It would be a great start to instantiate those.

[Sytten]

I found myself wanting this feature quite badly. I use Injector to inject my service layer in my flask (flask-rebar to be precise) controllers. Some services need to fetch external dependencies, so the first call to the API takes a big hit in performances. I will try the trick of forcing the injector to get the type, but an eager parameter in the @singleton decorator would be much cleaner.

[levsa]

I would like to request a feature like this as well. I would like to be able to annotate singletons so that they are automatically instantiated upon creation of the injector.

The reason is that I have a module that is to be included by many injectors that have different set of modules, and it would be nice to not need to do injector.get(MySingleton) from all the places where injectors are created.

[delucca]

+1 for this feature