adding "var_names" parameter to filter the variables

src/chainconsumer/chain.py

@@ -12,6 +12,7 @@
 import logging
 from typing import TYPE_CHECKING, Any, TypeAlias
 
+import arviz as az
 import numpy as np
 import pandas as pd
 from pydantic import Field, field_validator, model_validator
@@ -395,40 +396,52 @@ def from_numpyro(
         cls,
         mcmc: numpyro.infer.MCMC,
         name: str,
+        var_names: list[str] = [],
         **kwargs: Any,
     ) -> Chain:
         """Constructor from numpyro samples
 
         Args:
             mcmc: The numpyro sampler
             name: The name of the chain
+            var_names: The names of the parameters to include in the chain. If the entries of var_names start with ~,
+            they are excluded from the variables. If empty, all parameters are included.
             kwargs: Any other arguments to pass to the Chain constructor.
 
         Returns:
             A ChainConsumer Chain made from numpyro samples
         """
-        df = pd.DataFrame.from_dict({key: np.ravel(value) for key, value in mcmc.get_samples().items()})
+
+        var_names = _filter_var_names(var_names, list(mcmc.get_samples().keys()))
+        df = pd.DataFrame.from_dict(
+            {key: np.ravel(value) for key, value in mcmc.get_samples().items() if key in var_names}
+        )
         return cls(samples=df, name=name, **kwargs)
 
     @classmethod
     def from_arviz(
         cls,
         arviz_id: arviz.InferenceData,
         name: str,
+        var_names: list[str] = [],
         **kwargs: Any,
     ) -> Chain:
         """Constructor from an arviz InferenceData object
 
         Args:
             arviz_id: The arviz inference data
             name: The name of the chain
+            var_names: The names of the parameters to include in the chain. If the entries of var_names start with ~,
+            they are excluded from the variables. If empty, all parameters are included.
             kwargs: Any other arguments to pass to the Chain constructor.
 
         Returns:
             A ChainConsumer Chain made from the arviz chain
         """
 
-        df = arviz_id.to_dataframe(groups="posterior").drop(columns=["chain", "draw"])
+        var_names = _filter_var_names(var_names, list(arviz_id.posterior.keys()))
+        reduced_id = az.extract(arviz_id, var_names=var_names, group="posterior")
+        df = reduced_id.to_dataframe().drop(columns=["chain", "draw"])
 
         return cls(samples=df, name=name, **kwargs)
 
@@ -444,3 +457,30 @@ class MaxPosterior(BetterBase):
     def vec_coordinate(self) -> np.ndarray:
         """The coordinate as a numpy array, in the order the columns were given."""
         return np.array(list(self.coordinate.values()))
+
+
+def _filter_var_names(var_names: list[str], all_vars: list[str]):
+    """
+    Helper function to return the var_names to allows filtering parameters names.
+    """
+
+    if not var_names:
+        return all_vars
+
+    elif var_names:
+        if not (all([var.startswith("~") for var in var_names]) or all([not var.startswith("~") for var in var_names])):
+            raise ValueError(
+                "all values in var_names must start with ~ to exclude a subset OR none of them to keep a subset"
+            )
+
+        if all([var.startswith("~") for var in var_names]):
+            # remove the ~ from the var names
+            var_names = [var[1:] for var in var_names]
+            var_names = [var for var in all_vars if var not in var_names]
+
+            return var_names
+
+        else:
+            # keep var_names as is but check if var is in all_vars
+            var_names = [var for var in all_vars if var in var_names]
+            return var_names

tests/test_translators.py

@@ -83,12 +83,29 @@ def test_arviz_translator(self):
 
         assert chain.samples.shape == (self.n_steps * self.n_chains, self.n_params + 1)  # +1 for weight column
 
+    def test_drop_on_arviz_translator(self):
+        numpyro_mcmc = run_numpyro_mcmc(self.n_steps, self.n_chains)
+        arviz_id = az.from_numpyro(numpyro_mcmc)
+        chain = Chain.from_arviz(arviz_id, "Arviz", var_names=["mu"])
+        assert ("mu" in chain.samples.columns) and "sigma" not in chain.samples.columns
+
+        chain = Chain.from_arviz(arviz_id, "Arviz", var_names=["~mu"])
+        assert ("mu" not in chain.samples.columns) and ("sigma" in chain.samples.columns)
+
     def test_numpyro_translator(self):
         numpyro_mcmc = run_numpyro_mcmc(self.n_steps, self.n_chains)
         chain = Chain.from_numpyro(numpyro_mcmc, "numpyro")
 
         assert chain.samples.shape == (self.n_steps * self.n_chains, self.n_params + 1)
 
+    def test_drop_on_numpyro_translator(self):
+        numpyro_mcmc = run_numpyro_mcmc(self.n_steps, self.n_chains)
+        chain = Chain.from_numpyro(numpyro_mcmc, "numpyro", var_names=["mu"])
+        assert ("mu" in chain.samples.columns) and "sigma" not in chain.samples.columns
+
+        chain = Chain.from_numpyro(numpyro_mcmc, "numpyro", var_names=["~mu"])
+        assert ("mu" not in chain.samples.columns) and ("sigma" in chain.samples.columns)
+
     def test_emcee_translator(self):
         emcee_sampler = run_emcee_mcmc(self.n_steps, self.n_chains)
         chain = Chain.from_emcee(emcee_sampler, ["mu", "sigma"], "emcee")