Stan Using Modal

This is an example of running CmdStanPy using Modal on the Eight Schools, which examines coaching effects across eight schools.

import modal
from cmdstanpy import CmdStanModel

app = modal.App("stan-example")

cmdstan_image = (
    modal.Image.debian_slim(python_version="3.13")
    .pip_install("uv")
    .run_commands("uv pip install --system --compile-bytecode cmdstanpy")
    .run_commands("install_cmdstan")
)


@app.function(image=cmdstan_image)
def run_cmdstan_model():
    eight_schools_stan_code = """
    data {
        int<lower=0> J; // number of schools
        array[J] real y; // estimated treatment effect (school j)
        array[J] real<lower=0> sigma; // std err of effect estimate (school j)
    }
    parameters {
        real mu;
        array[J] real theta;
        real<lower=0> tau;
    }
    model {
        theta ~ normal(mu, tau);
        y ~ normal(theta, sigma);
    }
    """

    eight_schools_file = "eight_schools.stan"
    with open(eight_schools_file, "w") as f:
        f.write(eight_schools_stan_code)

    schools_data = {
        "J": 8,
        "y": [28, 8, -3, 7, -1, 1, 18, 12],
        "sigma": [15, 10, 16, 11, 9, 11, 10, 18],
    }

    eight_schools_model = CmdStanModel(stan_file=eight_schools_file)
    eight_schools_fit = eight_schools_model.sample(
        data=schools_data, chains=4, iter_sampling=1000
    )
    print(eight_schools_fit.summary())


@app.local_entrypoint()
def main():
    run_cmdstan_model.remote()