run_function return type specificity

[thundergolfer]

Describe your changes

context: https://modal-com.slack.com/archives/C06UKEPF3HR/p1730314009504369

• Provide Linear issue reference (e.g. MOD-1234) if available.

Backward/forward compatibility checks

Just a type-checking change and docs change.

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

[thundergolfer]

This is a narrowing but I can't think of a case where returning something is desired. These should just be side-effect functions.

[mwaskom]

I could imagine users doing something like this

def load_model_with_caching():
    return transformers.AutoModel.from_pretrained("whatever-bert")


image = modal.Image.debian_slim().run_function(load_model_with_caching)

...

@app.function()
def do_inference(input):
    model = load_model_with_caching()
    model(input)

Similar to our advertised "stack @modal.build / @modal.enter pattern, to take advantage of huggingface's caching during the Image build.

Still it's helpful to clarify that we don't use the return value. Maybe that could just be done with narrative documentation? Not sure.

[thundergolfer]

Yeh I think I've even done something similar myself. But the problem is that in the run_function case Modal's client will actually try pickle that return value and send it as an output, which is undesirable.

[mwaskom]

But the problem is that in the run_function case Modal's client will actually try pickle that return value and send it as an output, which is undesirable.

I guess it wouldn't be trivial not to do this since the container doesn't know it's running a "build function"?

[thundergolfer]

I guess it wouldn't be trivial not to do this since the container doesn't know it's running a

It might actually be easy enough. I think the container can access the Function definition which has something like is_build_function

[freider]

Good point, if it's not too hard structurally in the container entrypoint we should add something that just returns None as the result value when running a builder function! I've ran into this issue where builds take ages because of doing things like run_function(load_model_with_caching) like above to reuse the same model loading function without declaring a separate builder function