Re-working as_model

[zaxtax]

Since it's still new. The as_model decorator is nice, but for models where coords would change it could have better ergonomics. I've explored using a helper method add_coords

def add_coords(coords, lengths=None, model=None):
    model = pm.modelcontext(model)
    model.add_coords(coords=coords, lengths=lengths)

@pmx.as_model(name="linreg")
def linreg_model(x, y, coords):
    add_coords(coords)
    a = pm.Normal("a", 0, 3)
    b = pm.Normal("b", 0, 2)
    sigma = pm.HalfNormal("sigma", 2)
    
    pm.Normal("y", a + b * x, sigma, observed=y, dims="year")

m_train = linreg_model(x_train, y_train, coords={"year": [2019, 2020, 2021]})
m_test = linreg_model(x_test, y_test, coords={"year": [2022, 2023]})

But @ricardoV94 pointed out that we could return a function which took an additional coords or name argument instead

import pymc as pm
from functools import wraps

def as_model(*model_args, **model_kwargs):    
    def decorator(f):
        @wraps(f)
        def make_model(*args, **kwargs):
            coords = model_kwargs.pop("coords", {}) | kwargs.pop("coords", {})        
            with pm.Model(*model_args, coords=coords, **model_kwargs) as m:                    
                f(*args, **kwargs)
            return m
        return make_model
    return decorator

@pmx.as_model(name="linreg")
def linreg_model(x, y):
    a = pm.Normal("a", 0, 3)
    b = pm.Normal("b", 0, 2)
    sigma = pm.HalfNormal("sigma", 2)
    
    pm.Normal("y", a + b * x, sigma, observed=y, dims="year")

m_train = linreg_model(x_train, y_train, coords={"year": [2019, 2020, 2021]})
m_test = linreg_model(x_test, y_test, coords={"year": [2022, 2023]})

I'm curious, which do people prefer?

[twiecki]

Definitely the second, I don't think a user should have to remember to call the helper. In the previous we also deal with coords transparently.

[theorashid]

I prefer the second. As long as it works for coords_mutable (and whatever other args we need) too.

[ricardoV94]

You may have no need for coords_mutable with as_model since you can just build a different model with fixed coords for posterior predictive, as shown above with m_train and m_test

[theorashid]

Hmm, I think I'd rather keep the one model. Unless my current workflow is bad practice or I've missed the point:

m = model(data=data, coords=coords)
idata = pm.sample(
    model=m,
    return_inferencedata=True,
)

idata.extend(pm.sample_posterior_predictive(idata, model=m, predictions=False))

pm.set_data(new_data=data_predict, model=m)
idata.extend(pm.sample_posterior_predictive(idata, model=m, predictions=True))

tbf, I guess replacing the set_data line isn't a killer. Might be easier to keep track of one m though. Coming from R (sorry), I prefer one m: predict(m, newdata = new_data)

[fonnesbeck]

I'm starting to appreciate @ricardoV94 's approach of simply having a prediction model that uses the trace of the fitted model, and not have to worry about changing coords.

I'm confused by the example above -- m_train and m_test appear to be two entirely separate model instances, with no shared parameters. How is m_test getting the trained parameters?

[ricardoV94]

How is m_test getting the trained parameters?

That would happen when calling pm.sample_posterior_predictive with the idata from sampling

[fonnesbeck]

So the missing steps are:

trace = pm.sample(model=m_train)
pm.sample_posterior_predictive(trace, model=m_test, extend_inferencedata=True)

The nice thing here is that it avoids having to set up placeholder vars when you build a separate prediction model.

Yeah, I like the second one.

[twiecki]

Still in favor of the second.

[zaxtax]

Hmm, I think I'd rather keep the one model. Unless my current workflow is bad practice or I've missed the point:

m = model(data=data, coords=coords)
idata = pm.sample(
    model=m,
    return_inferencedata=True,
)

idata.extend(pm.sample_posterior_predictive(idata, model=m, predictions=False))

pm.set_data(new_data=data_predict, model=m)
idata.extend(pm.sample_posterior_predictive(idata, model=m, predictions=True))

tbf, I guess replacing the set_data line isn't a killer. Might be easier to keep track of one m though. Coming from R (sorry), I prefer one m: predict(m, newdata = new_data)

With either of the above workflows your code can look like this:

m = model(data=data, coords=coords)
idata = pm.sample(model=m)

pm.sample_posterior_predictive(idata, model=m, predictions=False, extend_inferencedata=True)
m_new = model(data=data_predict, coords=new_coords)
pm.sample_posterior_predictive(idata, model=m_new, predictions=True, extend_inferencedata=True)

[zaxtax]

Thanks everyone for your feedback!