Lazy Provider

[xelandernt]

Discussed in #201

^{Originally posted by JCHacking October 10, 2025}
I have the following use case:

# ip.py
from that_depends import BaseContainer, ContextScopes, fetch_context_item, providers

from project import IpReader, IpDelete, FirewallContainer

class IpContainer(BaseContainer):
    reader: providers.Singleton[IpReader] = providers.Singleton(IpReader)
    
    delete: providers.Singleton[IpDelete] = providers.Singleton(IpDelete, reader=reader.cast, firewall_reader=FirewallContainer.reader.cast)

# firewall.py
from that_depends import BaseContainer, ContextScopes, fetch_context_item, providers

from project import FirewallReader, FirewallDelete, IpContainer

class FirewallContainer(BaseContainer):
    reader: providers.Singleton[FirewallReader] = providers.Singleton(FirewallReader)
    
    delete: providers.Singleton[FirewallDelete] = providers.Singleton(FirewallDelete, reader=reader.cast, ip_reader=IpContainer.reader.cast)

This generates a circular import, but if instead of doing it directly I do something like this:

# ip.py
from that_depends import BaseContainer, ContextScopes, fetch_context_item, providers

from project import IpReader, IpDelete, FirewallReader

async def _firewall_reader() -> FirewallReader:
    from project import FirewallContainer
    return await FirewallContainer.reader()


class IpContainer(BaseContainer):
    reader: providers.Singleton[IpReader] = providers.Singleton(IpReader)
    
    firewall_reader: providers.Singleton[FirewallReader] = providers.Singleton(_firewall_reader)
    
    delete: providers.Singleton[IpDelete] = providers.Singleton(IpDelete, reader=reader.cast, firewall_reader=firewall_reader.cast)

The other file a similar change

Could something like a lazy provider be created to do this?

For example, specifying it like this:

delete: providers.Singleton[IpDelete] = providers.Singleton(IpDelete, reader=reader.cast, firewall_reader=LazyProvider(“project.FirewallContainer.firewall_reader”).cast)

It would be good to have a way to avoid always importing containers for cases like these, where both depend on each other due to system requirements.

[xelandernt]

@JCHacking I am keeping this in experimental for now. I think the workaround that you mentioned is reasonable and is actually advantageous because you are not losing the typing information.

https://that-depends.readthedocs.io/experimental/lazy/

[JCHacking]

@xelandernt
Great!
I will use it, thank you very much