Require all step methods to return stats

The reason for this change is the resulting simplification of code,
including simpler branching and less type ambiguity.
At the same time it allowed for fixing of a lot of type hints
and method signatures on step methods.

Closes pymc-devs#6270

pymc/blocking.py

@@ -20,15 +20,18 @@
 from __future__ import annotations
 
 from functools import partial
-from typing import Callable, Dict, Generic, NamedTuple, TypeVar
+from typing import Any, Callable, Dict, Generic, List, NamedTuple, TypeVar
 
 import numpy as np
 
+from typing_extensions import TypeAlias
+
 __all__ = ["DictToArrayBijection"]
 
 
 T = TypeVar("T")
-PointType = Dict[str, np.ndarray]
+PointType: TypeAlias = Dict[str, np.ndarray]
+StatsType: TypeAlias = List[Dict[str, Any]]
 
 # `point_map_info` is a tuple of tuples containing `(name, shape, dtype)` for
 # each of the raveled variables.

pymc/sampling/mcmc.py

@@ -912,14 +912,10 @@ def _iter_sample(
                 step.iter_count = 0
             if i == tune:
                 step.stop_tuning()
-            if step.generates_stats:
-                point, stats = step.step(point)
-                strace.record(point, stats)
-                log_warning_stats(stats)
-                diverging = i > tune and stats and stats[0].get("diverging")
-            else:
-                point = step.step(point)
-                strace.record(point, [])
+            point, stats = step.step(point)
+            strace.record(point, stats)
+            log_warning_stats(stats)
+            diverging = i > tune and stats and stats[0].get("diverging")
             if callback is not None:
                 callback(
                     trace=strace,

pymc/sampling/parallel.py

@@ -173,7 +173,7 @@ def _start_loop(self):
 
             if draw < self._draws + self._tune:
                 try:
-                    point, stats = self._compute_point()
+                    point, stats = self._step_method.step(self._point)
                 except SamplingError as e:
                     e = ExceptionWithTraceback(e, e.__traceback__)
                     self._msg_pipe.send(("error", e))
@@ -191,14 +191,6 @@ def _start_loop(self):
             else:
                 raise ValueError("Unknown message " + msg[0])
 
-    def _compute_point(self):
-        if self._step_method.generates_stats:
-            point, stats = self._step_method.step(self._point)
-        else:
-            point = self._step_method.step(self._point)
-            stats = None
-        return point, stats
-
 
 def _run_process(*args):
     _Process(*args).run()

pymc/sampling/population.py

@@ -17,7 +17,7 @@
 import logging
 
 from copy import copy
-from typing import Iterator, List, Sequence, Union
+from typing import Iterator, List, Sequence, Tuple, Union
 
 import cloudpickle
 import numpy as np
@@ -31,7 +31,11 @@
 from pymc.model import modelcontext
 from pymc.stats.convergence import log_warning_stats
 from pymc.step_methods import CompoundStep
-from pymc.step_methods.arraystep import BlockedStep, PopulationArrayStepShared
+from pymc.step_methods.arraystep import (
+    BlockedStep,
+    PopulationArrayStepShared,
+    StatsType,
+)
 from pymc.util import RandomSeed
 
 __all__ = ()
@@ -224,7 +228,7 @@ def _run_secondary(c, stepper_dumps, secondary_end):
             _log.exception(f"ChainWalker{c}")
         return
 
-    def step(self, tune_stop: bool, population):
+    def step(self, tune_stop: bool, population) -> List[Tuple[PointType, StatsType]]:
         """Step the entire population of chains.
 
         Parameters
@@ -239,18 +243,18 @@ def step(self, tune_stop: bool, population):
         update : list
             List of (Point, stats) tuples for all chains
         """
-        updates = [None] * self.nchains
+        updates: List[Tuple[PointType, StatsType]] = []
         if self.is_parallelized:
             for c in range(self.nchains):
                 self._primary_ends[c].send((tune_stop, population))
             # Blockingly get the step outcomes
             for c in range(self.nchains):
-                updates[c] = self._primary_ends[c].recv()
+                updates.append(self._primary_ends[c].recv())
         else:
             for c in range(self.nchains):
                 if tune_stop:
                     self._steppers[c].stop_tuning()
-                updates[c] = self._steppers[c].step(population[c])
+                updates.append(self._steppers[c].step(population[c]))
         return updates
 
 
@@ -378,13 +382,9 @@ def _iter_population(
 
                 # apply the update to the points and record to the traces
                 for c, strace in enumerate(traces):
-                    if steppers[c].generates_stats:
-                        points[c], stats = updates[c]
-                        strace.record(points[c], stats)
-                        log_warning_stats(stats)
-                    else:
-                        points[c] = updates[c]
-                        strace.record(points[c])
+                    points[c], stats = updates[c]
+                    strace.record(points[c], stats)
+                    log_warning_stats(stats)
                 # yield the state of all chains in parallel
                 yield traces
     except KeyboardInterrupt:

pymc/step_methods/arraystep.py

@@ -14,22 +14,20 @@
 
 from abc import ABC, abstractmethod
 from enum import IntEnum, unique
-from typing import Dict, List, Tuple, TypeVar, Union
+from typing import Callable, Dict, List, Tuple, Union, cast
 
 import numpy as np
 
 from aesara.graph.basic import Variable
 from numpy.random import uniform
 
-from pymc.blocking import DictToArrayBijection, PointType, RaveledVars
+from pymc.blocking import DictToArrayBijection, PointType, RaveledVars, StatsType
 from pymc.model import modelcontext
 from pymc.step_methods.compound import CompoundStep
 from pymc.util import get_var_name
 
 __all__ = ["ArrayStep", "ArrayStepShared", "metrop_select", "Competence"]
 
-StatsType = TypeVar("StatsType")
-
 
 @unique
 class Competence(IntEnum):
@@ -49,7 +47,6 @@ class Competence(IntEnum):
 
 class BlockedStep(ABC):
 
-    generates_stats = False
     stats_dtypes: List[Dict[str, type]] = []
     vars: List[Variable] = []
 
@@ -103,7 +100,7 @@ def __getnewargs_ex__(self):
         return self.__newargs
 
     @abstractmethod
-    def step(point: PointType, *args, **kwargs) -> Union[PointType, Tuple[PointType, StatsType]]:
+    def step(self, point: PointType) -> Tuple[PointType, StatsType]:
         """Perform a single step of the sampler."""
 
     @staticmethod
@@ -146,35 +143,28 @@ def __init__(self, vars, fs, allvars=False, blocked=True):
         self.allvars = allvars
         self.blocked = blocked
 
-    def step(self, point: PointType):
+    def step(self, point: PointType) -> Tuple[PointType, StatsType]:
 
-        partial_funcs_and_point = [DictToArrayBijection.mapf(x, start_point=point) for x in self.fs]
+        partial_funcs_and_point: List[Union[Callable, PointType]] = [
+            DictToArrayBijection.mapf(x, start_point=point) for x in self.fs
+        ]
         if self.allvars:
             partial_funcs_and_point.append(point)
 
-        apoint = DictToArrayBijection.map({v.name: point[v.name] for v in self.vars})
-        step_res = self.astep(apoint, *partial_funcs_and_point)
-
-        if self.generates_stats:
-            apoint_new, stats = step_res
-        else:
-            apoint_new = step_res
+        var_dict = {cast(str, v.name): point[cast(str, v.name)] for v in self.vars}
+        apoint = DictToArrayBijection.map(var_dict)
+        apoint_new, stats = self.astep(apoint, *partial_funcs_and_point)
 
         if not isinstance(apoint_new, RaveledVars):
             # We assume that the mapping has stayed the same
             apoint_new = RaveledVars(apoint_new, apoint.point_map_info)
 
         point_new = DictToArrayBijection.rmap(apoint_new, start_point=point)
 
-        if self.generates_stats:
-            return point_new, stats
-
-        return point_new
+        return point_new, stats
 
     @abstractmethod
-    def astep(
-        self, apoint: RaveledVars, point: PointType, *args
-    ) -> Union[RaveledVars, Tuple[RaveledVars, StatsType]]:
+    def astep(self, apoint: RaveledVars, *args) -> Tuple[RaveledVars, StatsType]:
         """Perform a single sample step in a raveled and concatenated parameter space."""
 
 
@@ -198,30 +188,27 @@ def __init__(self, vars, shared, blocked=True):
         self.shared = {get_var_name(var): shared for var, shared in shared.items()}
         self.blocked = blocked
 
-    def step(self, point):
+    def step(self, point: PointType) -> Tuple[PointType, StatsType]:
 
         for name, shared_var in self.shared.items():
             shared_var.set_value(point[name])
 
-        q = DictToArrayBijection.map({v.name: point[v.name] for v in self.vars})
+        var_dict = {cast(str, v.name): point[cast(str, v.name)] for v in self.vars}
+        q = DictToArrayBijection.map(var_dict)
 
-        step_res = self.astep(q)
-
-        if self.generates_stats:
-            apoint, stats = step_res
-        else:
-            apoint = step_res
+        apoint, stats = self.astep(q)
 
         if not isinstance(apoint, RaveledVars):
             # We assume that the mapping has stayed the same
             apoint = RaveledVars(apoint, q.point_map_info)
 
         new_point = DictToArrayBijection.rmap(apoint, start_point=point)
 
-        if self.generates_stats:
-            return new_point, stats
+        return new_point, stats
 
-        return new_point
+    @abstractmethod
+    def astep(self, q0: RaveledVars) -> Tuple[RaveledVars, StatsType]:
+        """Perform a single sample step in a raveled and concatenated parameter space."""
 
 
 class PopulationArrayStepShared(ArrayStepShared):
@@ -281,7 +268,7 @@ def __init__(
 
         super().__init__(vars, func._extra_vars_shared, blocked)
 
-    def step(self, point):
+    def step(self, point) -> Tuple[PointType, StatsType]:
         self._logp_dlogp_func._extra_are_set = True
         return super().step(point)
 

pymc/step_methods/compound.py

@@ -17,9 +17,11 @@
 
 @author: johnsalvatier
 """
-from collections import namedtuple
 
-import numpy as np
+
+from typing import Tuple
+
+from pymc.blocking import PointType, StatsType
 
 
 class CompoundStep:
@@ -28,36 +30,23 @@ class CompoundStep:
 
     def __init__(self, methods):
         self.methods = list(methods)
-        self.generates_stats = any(method.generates_stats for method in self.methods)
         self.stats_dtypes = []
         for method in self.methods:
-            if method.generates_stats:
-                self.stats_dtypes.extend(method.stats_dtypes)
+            self.stats_dtypes.extend(method.stats_dtypes)
         self.name = (
             f"Compound[{', '.join(getattr(m, 'name', 'UNNAMED_STEP') for m in self.methods)}]"
         )
 
-    def step(self, point):
-        if self.generates_stats:
-            states = []
-            for method in self.methods:
-                if method.generates_stats:
-                    point, state = method.step(point)
-                    states.extend(state)
-                else:
-                    point = method.step(point)
-            # Model logp can only be the logp of the _last_ state, if there is
-            # one. Pop all others (if dict), or set to np.nan (if namedtuple).
-            for state in states[:-1]:
-                if isinstance(state, dict):
-                    state.pop("model_logp", None)
-                elif isinstance(state, namedtuple):
-                    state = state._replace(logp=np.nan)
-            return point, states
-        else:
-            for method in self.methods:
-                point = method.step(point)
-            return point
+    def step(self, point) -> Tuple[PointType, StatsType]:
+        stats = []
+        for method in self.methods:
+            point, sts = method.step(point)
+            stats.extend(sts)
+        # Model logp can only be the logp of the _last_ stats,
+        # if there is one. Pop all others.
+        for sts in stats[:-1]:
+            sts.pop("model_logp", None)
+        return point, stats
 
     def stop_tuning(self):
         for method in self.methods:

pymc/step_methods/hmc/base_hmc.py

@@ -23,7 +23,7 @@
 import numpy as np
 
 from pymc.aesaraf import floatX
-from pymc.blocking import DictToArrayBijection, RaveledVars
+from pymc.blocking import DictToArrayBijection, RaveledVars, StatsType
 from pymc.exceptions import SamplingError
 from pymc.model import Point, modelcontext
 from pymc.stats.convergence import SamplerWarning, WarningType
@@ -157,7 +157,7 @@ def _hamiltonian_step(self, start, p0, step_size) -> HMCStepData:
         Subclasses must overwrite this abstract method and return an `HMCStepData` object.
         """
 
-    def astep(self, q0: RaveledVars) -> tuple[RaveledVars, list[dict[str, Any]]]:
+    def astep(self, q0: RaveledVars) -> tuple[RaveledVars, StatsType]:
         """Perform a single HMC iteration."""
         perf_start = time.perf_counter()
         process_start = time.process_time()

pymc/step_methods/hmc/hmc.py

@@ -39,7 +39,6 @@ class HamiltonianMC(BaseHMC):
 
     name = "hmc"
     default_blocked = True
-    generates_stats = True
     stats_dtypes = [
         {
             "step_size": np.float64,

pymc/step_methods/hmc/nuts.py

@@ -97,7 +97,6 @@ class NUTS(BaseHMC):
     name = "nuts"
 
     default_blocked = True
-    generates_stats = True
     stats_dtypes = [
         {
             "depth": np.int64,