# Getting started¶

PyStan is the Python interface for Stan.

## Prerequisites¶

PyStan has the following dependencies:

PyStan also requires that a C++ compiler be available to Python during installation and at runtime. On Debian-based systems this is accomplished by issuing the command apt-get install build-essential.

## Installation¶

Note

Installing PyStan involves compiling Stan. This may take a considerable amount of time.

### Unix-based systems including Mac OS X¶

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

pip install pystan


Mac OS X users encountering installation problems may wish to consult the PyStan Wiki for possible solutions.

### Windows¶

PyStan on Windows requires Python 2.7/3.x and a working C++ compiler. If you have already installed Python and the MingW-w64 C++ compiler, running pip install pystan will install PyStan.

If you need to install a C++ compiler, you will find detailed installation instructions in PyStan on Windows.

## Using PyStan¶

The module’s name is pystan so we load the module as follows:

import pystan


### Example 1: Eight Schools¶

The “eight schools” example appears in Section 5.5 of Gelman et al. (2003), which studied coaching effects from eight schools.

schools_code = """
data {
int<lower=0> J; // number of schools
vector[J] y; // estimated treatment effects
vector<lower=0>[J] sigma; // s.e. of effect estimates
}
parameters {
real mu;
real<lower=0> tau;
vector[J] eta;
}
transformed parameters {
vector[J] theta;
theta = mu + tau * eta;
}
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]}

sm = pystan.StanModel(model_code=schools_code)
fit = sm.sampling(data=schools_dat, iter=1000, chains=4)


In this model, we let theta be transformed parameters of mu and eta instead of directly declaring theta as parameters. By parameterizing this way, the sampler will run more efficiently.

In PyStan, we can also specify the Stan model using a file. For example, we can download the file 8schools.stan into our working directory and use the following call to stan instead:

sm = pystan.StanModel(file='8schools.stan')
fit = sm.sampling(data=schools_dat, iter=1000, chains=4)


Once a model is compiled, we can use the StanModel object multiple times. This saves us time compiling the C++ code for the model. For example, if we want to sample more iterations, we proceed as follows:

fit2 = sm.sampling(data=schools_dat, iter=10000, chains=4)


The object fit, returned from function stan stores samples from the posterior distribution. The fit object has a number of methods, including plot and extract. We can also print the fit object and receive a summary of the posterior samples as well as the log-posterior (which has the name lp__).

The method extract extracts samples into a dictionary of arrays for parameters of interest, or just an array.

la = fit.extract(permuted=True)  # return a dictionary of arrays
mu = la['mu']

## return an array of three dimensions: iterations, chains, parameters
a = fit.extract(permuted=False)

print(fit)


If matplotlib and scipy are installed, a visual summary may also be displayed using the plot() method.

fit.plot()