sampled
Decorator for reusable models in PyMC3
Provides syntactic sugar for reusable models with PyMC3. This lets you separate creating a generative model from using the model.
Here is an example of creating a model:
import numpy as np import pymc3 as pm from sampled import sampled import theano.tensor as tt @sampled def linear_model(X, y): shape = X.shape X = pm.Normal('X', mu=tt.mean(X, axis=0), sd=np.std(X, axis=0), shape=shape) coefs = pm.Normal('coefs', mu=tt.zeros(shape[1]), sd=tt.ones(shape[1]), shape=shape[1]) pm.Normal('y', mu=tt.dot(X, coefs), sd=tt.ones(shape[0]), shape=shape[0])
Now here is how to use the model:
X = np.random.normal(size=(1000, 10)) w = np.random.normal(size=10) y = X.dot(w) + np.random.normal(scale=0.1, size=1000) with linear_model(X=X, y=y): sampled_coefs = pm.sample(draws=1000, tune=500) np.allclose(sampled_coefs.get_values('coefs').mean(axis=0), w, atol=0.1) # True
You can also use this to build graphical networks -- here is a continuous version of the STUDENT example from Koller and Friedman's "Probabilistic Graphical Models", chapter 3:
import pymc3 as pm from sampled import sampled import theano.tensor as tt @sampled def student(): difficulty = pm.Beta('difficulty', alpha=5, beta=5) intelligence = pm.Beta('intelligence', alpha=5, beta=5) SAT = pm.Beta('SAT', alpha=20 * intelligence, beta=20 * (1 - intelligence)) grade_avg = 0.5 + 0.5 * tt.sqrt((1 - difficulty) * intelligence) grade = pm.Beta('grade', alpha=20 * grade_avg, beta=20 * (1 - grade_avg)) recommendation = pm.Binomial('recommendation', n=1, p=0.7 * grade)
Observations may be passed into any node, and we can observe how that changes posterior expectations:
# no prior knowledge with student(): prior = pm.sample(draws=1000, tune=500) prior.get_values('recommendation').mean() # 0.502 # 99th percentile SAT score --> higher chance of a recommendation with student(SAT=0.99): good_sats = pm.sample(draws=1000, tune=500) good_sats.get_values('recommendation').mean() # 0.543 # A good grade in a hard class --> very high chance of recommendation with student(difficulty=0.99, grade=0.99): hard_class_good_grade = pm.sample(draws=1000, tune=500) hard_class_good_grade.get_values('recommendation').mean() # 0.705
References
- Koller, Daphne, and Nir Friedman. Probabilistic graphical models: principles and techniques. MIT press, 2009.