Python interface to Stan, a package for Bayesian inference

## Project description

PyStan provides a Python interface to Stan, a package for Bayesian inference using the No-U-Turn sampler, a variant of Hamiltonian Monte Carlo.

For more information on Stan and its modeling language, see the Stan User’s Guide and Reference Manual. PyStan has an interface similar to that of RStan. For an introduction to Stan visit http://mc-stan.org/.

PyStan aims to reproduce the functionality present in RStan. There are a few features present in RStan that have yet to be implemented in PyStan. If you find a feature missing that you use frequently please file a bug report so developers can better direct their efforts.

## Installation

NumPy and Cython (version 0.19.1 or greater) are required. matplotlib is optional.

PyStan and the required packages may be installed from the Python Package Index using pip.

```pip install numpy Cython
pip install pystan
```

Alternatively, if Cython (version 0.19 or greater) and NumPy are already available, PyStan may be installed from source with the following commands

```git clone https://github.com/stan-dev/pystan.git
cd pystan
python setup.py install
```

If you encounter an ImportError after compiling from source, try changing out of the source directory before attempting import pystan. For example, on Linux and OS X cd /tmp would work.

## Example

```import pystan
import numpy as np

schools_code = """
data {
int<lower=0> J; // number of schools
real y[J]; // estimated treatment effects
real<lower=0> sigma[J]; // s.e. of effect estimates
}
parameters {
real mu;
real<lower=0> tau;
real eta[J];
}
transformed parameters {
real theta[J];
for (j in 1:J)
theta[j] <- mu + tau * eta[j];
}
model {
eta ~ normal(0, 1);
y ~ normal(theta, sigma);
}
"""

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

fit = pystan.stan(model_code=schools_code, data=schools_dat,
iter=1000, chains=4)

print(fit)

eta = fit.extract(permuted=True)['eta']
np.mean(eta, axis=0)

# if matplotlib is installed (optional, not required), a visual summary and
# traceplot are available
fit.plot()
```

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