Source code for orion.algo.pbt.pbt

"""
Population Based Training
=========================

"""
from __future__ import annotations

import copy
import logging
import time
from typing import Any, ClassVar, Iterable, Sequence

import numpy

from orion.algo.base import BaseAlgorithm
from orion.algo.pbt.exploit import exploit_factory
from orion.algo.pbt.explore import explore_factory
from orion.algo.random import Random
from orion.algo.space import Fidelity, Space
from orion.core.utils.flatten import flatten
from orion.core.utils.tree import TreeNode
from orion.core.worker.transformer import TransformedDimension
from orion.core.worker.trial import Trial

logger = logging.getLogger(__name__)


SPACE_ERROR = """
PBT cannot be used if space does not contain a fidelity dimension.
For more information on the configuration and usage of Hyperband, see
https://orion.readthedocs.io/en/develop/user/algorithms.html#pbt
"""


def get_objective(trial: Trial) -> float:
    if trial.objective and trial.objective.value is not None:
        return trial.objective.value

    return float("inf")


def compute_fidelities(
    n_branching: int,
    low: numpy.ndarray | float,
    high: numpy.ndarray | float,
    base: float,
) -> list[float]:
    if base == 1:
        fidelities = numpy.linspace(low, high, num=n_branching + 1, endpoint=True)
    else:
        fidelities = numpy.logspace(
            numpy.log(low) / numpy.log(base),
            numpy.log(high) / numpy.log(base),
            n_branching + 1,
            base=base,
            endpoint=True,
        )

    return numpy.clip(fidelities, a_min=low, a_max=high).tolist()


[docs]class PBT(BaseAlgorithm): """Population Based Training algorithm Warning:PBT was broken in version v0.2.4. Make sure to use latest release. Population based training is an evolutionary algorithm that evolve trials from low fidelity levels to high fidelity levels (ex: number of epochs). For a population of size `m`, it first samples `m` trials at lowest fidelity level. When trials are completed, it decides based on the ``exploit`` configuration whether the trial should be promoted to next fidelity level or whether another trial should be selected instead and forked. When a trial is forked, new hyperparameters are selected based on the trials hyperparameters and the ``explore`` configuration. The original trial's working_dir is then copied over to the new trial's working_dir so that the user script can resume execution from model parameters of original trial. It is important that the weights of models trained for each trial are saved in the corresponding directory at path ``trial.working_dir``. The file name does not matter. The entire directory is copied to a new ``trial.working_dir`` when PBT selects a good model and explore new hyperparameters. The new trial can be resumed by the user by loading the weights found in the freshly copied ``new_trial.working_dir``, and saved back at the same path at end of trial execution. To access ``trial.working_dir`` from Oríon's commandline API, see documentation at https://orion.readthedocs.io/en/stable/user/script.html#command-line-templating. To access ``trial.working_dir`` from Oríon's Python API, set argument ``trial_arg="trial"`` when executing method :py:meth:`orion.client.experiment.ExperimentClient.workon`. The number of fidelity levels is determined by the argument ``generations``. The lowest and highest fidelity levels, and the distrubition, is determined by the search space's dimension that will have a prior ``fidelity(low, high, base)``, where ``base`` is the logarithm base of the dimension. Original PBT algorithm uses a base of 1. PBT will try to return as many trials as possible when calling ``suggest(num)``, up to ``num``. When ``population_size`` trials are sampled and more trials are requested, it will try to generate new trials by promoting or forking existing trials in a queue. This queue will get filled when calling ``observe(trials)`` on completed or broken trials. If trials are broken at lowest fidelity level, they are ignored and will not count in population size so that PBT can sample additional trials to reach ``population_size`` completed trials at lowest fidelity. If a trial is broken at higher fidelity, the original trial leading to the broken trial is examined again for ``exploit`` and ``explore``. If the broken trial was the result of a fork, then we backtrack to the trial that was dropped during ``exploit`` in favor of the forked trial. If the broken trial was a promotion, then we backtrack to the original trial that was promoted. For more information on the algorithm, see original paper at https://arxiv.org/abs/1711.09846. Jaderberg, Max, et al. "Population based training of neural networks." arXiv preprint, arXiv:1711.09846 (2017). Notes ----- It is important that the experiment using this algorithm has a working directory properly set. The experiment's working dir serve as the base for the trial's working directories. The trial's working directory is ``trial.working_dir``. This is where the weights of the model should be saved. Using ``trial.hash_params`` to determine a unique working dir for the trial will result in working on a different directory than the one copied by PBT, hence missing the copied model parameters. Parameters ---------- space: `orion.algo.space.Space` Optimisation space with priors for each dimension. seed: None, int or sequence of int Seed for the random number generator used to sample new trials. Default: ``None`` population_size: int, optional Size of the population. No trial will be continued until there are `population_size` trials executed until lowest fidelity. If a trial is broken during execution at lowest fidelity, the algorithm will sample a new trial, keeping the population of *non-broken* trials at `population_size`. For efficiency it is better to have less workers running than population_size. Default: 50. generations: int, optional Number of generations, from lowest fidelity to highest one. This will determine how many branchings occur during the execution of PBT. Default: 10 exploit: dict or None, optional Configuration for a ``pbt.exploit.BaseExploit`` object that determines when if a trial should be exploited or not. If None, default configuration is a ``PipelineExploit`` with ``BacktrackExploit`` and ``TruncateExploit``. explore: dict or None, optional Configuration for a ``pbt.explore.BaseExplore`` object that returns new parameter values for exploited trials. If None, default configuration is a ``PipelineExplore`` with ``ResampleExplore`` and ``PerturbExplore``. fork_timeout: int, optional Maximum amount of time in seconds that an attempt to mutate a trial should take, otherwise algorithm.suggest() will raise ``SuggestionTimeout``. Default: 60 """ requires_type: ClassVar[str | None] = None requires_dist: ClassVar[str] = "linear" requires_shape: ClassVar[str] = "flattened" def __init__( self, space: Space, seed: int | Sequence[int] | None = None, population_size: int = 50, generations: int = 10, exploit: dict | None = None, explore: dict | None = None, fork_timeout: int = 60, ): super().__init__(space) if exploit is None: exploit = { "of_type": "PipelineExploit", "exploit_configs": [ { "of_type": "BacktrackExploit", "min_forking_population": 5, "truncation_quantile": 0.9, "candidate_pool_ratio": 0.2, }, { "of_type": "TruncateExploit", "min_forking_population": 5, "truncation_quantile": 0.8, "candidate_pool_ratio": 0.2, }, ], } if explore is None: explore = { "of_type": "PipelineExplore", "explore_configs": [ {"of_type": "ResampleExplore", "probability": 0.2}, {"of_type": "PerturbExplore", "factor": 1.2, "volatility": 0.0001}, ], } self.random_search = Random(space) self._queue: list[Trial] = [] fidelity_index = self.fidelity_index if fidelity_index is None: raise RuntimeError(SPACE_ERROR) fidelity_dim = space[fidelity_index] while isinstance(fidelity_dim, TransformedDimension): fidelity_dim = fidelity_dim.original_dimension assert isinstance(fidelity_dim, Fidelity) self.fidelity_dim = fidelity_dim self.fidelities = compute_fidelities( generations, self.fidelity_dim.low, self.fidelity_dim.high, self.fidelity_dim.base, ) self.fidelity_upgrades = dict(zip(self.fidelities, self.fidelities[1:])) logger.info("Executing PBT with fidelities: %s", self.fidelities) self.exploit_func = exploit_factory.create(**exploit) self.explore_func = explore_factory.create(**explore) self.lineages = Lineages() self.seed = seed self.population_size = population_size self.generations = generations self.exploit = exploit self.explore = explore self.fork_timeout = fork_timeout if seed is not None: self.seed_rng(seed=seed) @property def space(self) -> Space: """Return transformed space of PBT""" return self.random_search.space @space.setter def space(self, space: Space) -> None: """Set the space of PBT and initialize it""" self.random_search.space = space @property def rng(self) -> numpy.random.RandomState: """Random Number Generator""" return self.random_search.rng
[docs] def seed_rng(self, seed: int | Sequence[int] | None) -> None: """Seed the state of the random number generator. Parameters ---------- seed: int Integer seed for the random number generator. """ self.random_search.seed_rng(seed)
@property def state_dict(self) -> dict: """Return a state dict that can be used to reset the state of the algorithm.""" state_dict: dict[str, Any] = super().state_dict state_dict["random_search"] = self.random_search.state_dict state_dict["lineages"] = copy.deepcopy(self.lineages) state_dict["queue"] = copy.deepcopy(self._queue) return state_dict
[docs] def set_state(self, state_dict: dict) -> None: """Reset the state of the algorithm based on the given state_dict""" super().set_state(state_dict) self.random_search.set_state(state_dict["random_search"]) self.lineages = state_dict["lineages"] self._queue = state_dict["queue"]
@property def _num_root(self) -> int: """Number of trials with lowest fidelity level that are not broken.""" return sum( int(lineage.root.item.status != "broken") for lineage in self.lineages ) @property def is_done(self) -> bool: """Is done if ``population_size`` trials at highest fidelity level are completed.""" n_completed = 0 final_depth = self._get_depth_of(self.fidelities[-1]) for trial in self.lineages.get_trials_at_depth(final_depth): n_completed += int(trial.status == "completed") return n_completed >= self.population_size
[docs] def register(self, trial: Trial) -> None: """Save the trial as one suggested or observed by the algorithm The trial is additionally saved in the lineages object of PBT. Parameters ---------- trial: ``orion.core.worker.trial.Trial`` Trial from a `orion.algo.space.Space`. """ super().register(trial) self.lineages.register(trial)
[docs] def suggest(self, num: int) -> list[Trial]: """Suggest a ``num`` ber of new sets of parameters. PBT will try to sample up to ``population_size`` trials at lowest fidelity level. If more trials are required, it will try to promote or fork trials based on the queue of available trials observed. Parameters ---------- num: int Number of points to suggest. The algorithm may return less than the number of points requested. Returns ------- list of trials A list of trials representing values suggested by the algorithm. """ assert num > 0 logger.debug( "PBT has %s pending or completed trials at root, %s broken trials.", self._num_root, len(self.lineages) - self._num_root, ) # Sample points until num is met, or population_size num_random_samples = min(max(self.population_size - self._num_root, 0), num) logger.debug("Sampling %s new trials", num_random_samples) trials = self._sample(num_random_samples) logger.debug("Sampled %s new trials", len(trials)) logger.debug( "After sampling, PBT has %s pending or completed trials at root, %s broken trials.", self._num_root, len(self.lineages) - self._num_root, ) # Then try branching based on observed_queue until num is met or queue is exhausted. num_fork_samples = max(num - len(trials), 0) logger.debug( "Attempting Forking %s trials, with %s trials queued available for forking", num_fork_samples, len(self._queue), ) forked_trials = self._fork_lineages(num_fork_samples) logger.debug("Forked %s new trials", len(forked_trials)) logger.debug( "After forking, PBT has %s pending or completed trials at root, %s broken trials.", self._num_root, len(self.lineages) - self._num_root, ) trials += forked_trials return trials
def _sample(self, num: int) -> list[Trial]: """Sample trials based on random search""" sampled_trials = self.random_search.suggest(num) trials = [] for trial in sampled_trials: branched_trial = trial.branch( params={self.fidelity_dim.name: self.fidelity_dim.low} ) # NOTE: We are using branching as a simple way to update the fidelity, we should not keep the # parent id since it is not a mutation from a parent trial. branched_trial.parent = None self.register(branched_trial) trials.append(branched_trial) return trials def _get_depth_of(self, fidelity: Any) -> int: """Get the depth of a fidelity in the lineages""" # NOTE: There is an issue with rounding, asking for fidelity of `10` but list has # `10.00000004`. This is a hacky bugfix. if fidelity in self.fidelities: return self.fidelities.index(fidelity) elif fidelity in numpy.round(self.fidelities, decimals=4): return numpy.round(self.fidelities, decimals=4).tolist().index(fidelity) raise RuntimeError( f"Fidelity {fidelity} not found in the fidelities {self.fidelities}." ) def _fork_lineages(self, num: int) -> list[Trial]: """Try to promote or fork up to ``num`` trials from the queue.""" branched_trials: list[Trial] = [] skipped_trials: list[Trial] = [] while len(branched_trials) < num and self._queue: trial = self._queue.pop(0) trial_to_branch, new_trial = self._generate_offspring(trial) if trial_to_branch is None: logger.debug("Skipping trial %s", trial) skipped_trials.append(trial) continue # NOTE: new_trial isn't None if trial_to_branch is not None. assert new_trial is not None self.lineages.fork(trial_to_branch, new_trial) if trial is not trial_to_branch: logger.debug("Dropped trial %s in favor of %s", trial, trial_to_branch) self.lineages.set_jump(trial, new_trial) logger.debug("Forking trial %s to %s", trial_to_branch, new_trial) branched_trials.append(new_trial) self.register(new_trial) self._queue = skipped_trials + self._queue return branched_trials def _generate_offspring( self, trial: Trial ) -> tuple[Trial, Trial] | tuple[None, None]: """Try to promote or fork a given trial.""" new_trial = trial if not self.has_suggested(new_trial): raise RuntimeError( "Trying to fork a trial that was not registered yet. This should never happen" ) attempts = 0 start = time.perf_counter() trial_to_branch: Trial | None = None while ( self.has_suggested(new_trial) and time.perf_counter() - start <= self.fork_timeout ): trial_to_explore = self.exploit_func( self.rng, trial, self.lineages, ) if trial_to_explore is None: return None, None elif trial_to_explore is trial: new_params = {} trial_to_branch = trial logger.debug("Promoting trial %s, parameters stay the same.", trial) else: new_params = flatten( self.explore_func(self.rng, self.space, trial_to_explore.params) ) trial_to_branch = trial_to_explore logger.debug( "Forking trial %s with new parameters %s", trial_to_branch, new_params, ) assert trial_to_branch is not None # note: implicitly assumed below. # Set next level of fidelity new_params[self.fidelity_index] = self.fidelity_upgrades[ trial_to_branch.params[self.fidelity_index] ] new_trial = trial_to_branch.branch(params=new_params) # TODO: Keep this? or not? assert new_trial.parent is not None logger.debug("Attempt %s - Creating new trial %s", attempts, new_trial) attempts += 1 if ( self.has_suggested(new_trial) and time.perf_counter() - start > self.fork_timeout ): trial_to_branch = None new_trial = None logger.info( f"Could not generate unique new parameters for trial {trial.id} in " f"less than {self.fork_timeout} seconds. Attempted {attempts} times." ) return trial_to_branch, new_trial def _triage(self, trials: list[Trial]) -> list[Trial]: """Triage observed trials and return those that may be queued.""" trials_to_verify: list[Trial] = [] # First try to resume from trials if necessary, then only push to queue leafs for trial in trials: if not self.has_suggested(trial): logger.debug("Ignoring unknown trial %s", trial) continue if not self.has_observed(trial) and trial.status in ["completed", "broken"]: logger.debug("Will verify trial %s for queue", trial) trials_to_verify.append(trial) self.register(trial) return trials_to_verify def _queue_trials_for_promotions(self, trials: list[Trial]) -> None: """Queue trials if they are completed or ancestor trials if they are broken.""" for trial in trials: if trial.status == "broken": # Branch again from trial that lead to this broken one. lineage_to_retry = self.lineages.get_lineage(trial).get_true_ancestor() if lineage_to_retry: logger.debug( "Trial %s is broken, queuing ancestor %s to re-attempt forking.", trial, lineage_to_retry.item, ) self._queue.append(lineage_to_retry.item) else: logger.debug( ( "Trial %s from initial generation is broken, " "new trials can be sampled at next suggest() call." ), trial, ) elif trial.status == "completed": if trial.params[self.fidelity_index] == self.fidelities[-1]: logger.debug( "Trial %s is completed at full fidelity (%s). Not forking.", trial, self.fidelities[-1], ) else: logger.debug( "Trial %s is completed, queuing it to attempt forking.", trial ) self._queue.append(trial)
[docs] def observe(self, trials: list[Trial]): """Observe the trials and queue those available for promotion or forking. Parameters ---------- trials: list of ``orion.core.worker.trial.Trial`` Trials from a `orion.algo.space.Space`. """ trials_to_verify = self._triage(trials) self._queue_trials_for_promotions(trials_to_verify)
[docs]class Lineages(Iterable["LineageNode"]): """Lineages of trials for workers in PBT This class regroup all lineages of trials generated by PBT for a given experiment. Each lineage is a path from a leaf trial (highest fidelity level) up to the root (lowest fidelity level). Multiple lineages can fork from the same root, forming a tree. A Lineages object may reference multiple trees of lineages. Iterating a Lineages object will iterate on the roots of these trees. """ def __init__(self): self._lineage_roots: list[LineageNode] = [] self._trial_to_lineages: dict[str, LineageNode] = {} def __len__(self) -> int: """Number of roots in the Lineages""" return len(self._lineage_roots) def __iter__(self): """Iterate over the roots of the Lineages""" return iter(self._lineage_roots)
[docs] def add(self, trial: Trial) -> LineageNode: """Add a trial to the lineages If the trial is already in the lineages, this will only return the corresponding lineage node. Otherwise, a new lineage node will be created and added as a root. Parameters ---------- trial: ``orion.core.worker.trial.Trial`` Trial from a `orion.algo.space.Space`. Returns ------- orion.algo.pbt.pbt.LineageNode The lineage node for the given trial. """ if trial.id in self._trial_to_lineages: return self._trial_to_lineages[trial.id] lineage = LineageNode(trial) self._lineage_roots.append(lineage) self._trial_to_lineages[trial.id] = lineage return lineage
[docs] def fork(self, base_trial: Trial, new_trial: Trial) -> LineageNode: """Fork a base trial to a new one. The base trial should already be registered in the Lineages Parameters ---------- base_trial: ``orion.core.worker.trial.Trial`` The base trial that will be the parent lineage node. new_trial: ``orion.core.worker.trial.Trial`` The new trial that will be the child lineage node. Raises ------ KeyError If the base trial is not already registered in the Lineages """ new_lineage = self._trial_to_lineages[base_trial.id].fork(new_trial) self._trial_to_lineages[new_trial.id] = new_lineage return new_lineage
[docs] def get_lineage(self, trial: Trial) -> LineageNode: """Get the lineage node corresponding to a given trial. Parameters ---------- trial: ``orion.core.worker.trial.Trial`` The trial for which the function should return the corresponding lineage node. Raises ------ KeyError If the base trial is not already registered in the Lineages """ return self._trial_to_lineages[trial.id]
[docs] def set_jump(self, base_trial: Trial, new_trial: Trial) -> None: """Set a jump between two trials This jump is set to represent the relation between the base trial and the new trial. This means the base trial was dropped during exploit and the new trial is the result of a fork from another trial selected during exploit. Both trials should already be registered in the Lineages. Parameters ---------- base_trial: ``orion.core.worker.trial.Trial`` The base trial that was dropped. new_trial: ``orion.core.worker.trial.Trial`` The new trial that was forked. Raises ------ KeyError If the base trial or the new trial are not already registered in the Lineages. """ self.get_lineage(base_trial).set_jump(self.get_lineage(new_trial))
[docs] def register(self, trial: Trial) -> LineageNode: """Add or save the trial in the Lineages If the trial is not already in the Lineages, it is added as root. Otherwise, the corresponding lineage node is updated with given trial object. Parameters ---------- trial: ``orion.core.worker.trial.Trial`` The trial to register. """ if trial.id not in self._trial_to_lineages: lineage = self.add(trial) else: lineage = self.get_lineage(trial) lineage.register(trial) return lineage
[docs] def get_elites(self, max_depth: int | None = None) -> list[Trial]: """Get best trials of each lineage Each lineage is a path from a leaf to the root. When there is a forking, the path followed is not from child (new trial) to parent (forked trial), but rather to base trial (trial dropped). This is to represent the path taken by the sequence of trial execution within a worker. This also avoids having duplicate elite trials on different lineages. Best trials may be looked for up to a ``max_depth``. Parameters ---------- max_depth: int or ``orion.core.worker.trial.Trial``, optional The maximum depth to look for best trials. It can be an int to represent the depth directly, or a trial, from which the depth will be inferred. If a trial, this trial should be in the Lineages. Default: None, that is, no max depth. """ if max_depth and not isinstance(max_depth, int): max_depth = self.get_lineage(max_depth).node_depth def get_parent_at_depth(node, depth): while node.node_depth > depth: node = node.parent return node trials: list[Trial] = [] for lineage in self._lineage_roots: nodes = lineage.leafs if max_depth is not None: trimmed_nodes = set() for node in nodes: node = get_parent_at_depth(node, max_depth) trimmed_nodes.add(node) nodes = list(trimmed_nodes) for node in nodes: if node.jumps and ( (max_depth is None) or (node.node_depth < max_depth) ): continue best_trial = node.get_best_trial() if best_trial is not None: trials.append(best_trial) return trials
[docs] def get_trials_at_depth(self, trial_or_depth: int | Trial) -> list[Trial]: """Returns the trials of all lineages at a given depth Parameters ---------- trial_or_depth: int or ``orion.core.worker.trial.Trial`` If an int, this represents the depth directly. If a trial, the depth will be inferred from it. This trial should be in the Lineages. Raises ------ KeyError If depth is inferred from trial but trial is not already registered in the Lineages """ if isinstance(trial_or_depth, int): depth = trial_or_depth else: depth = self.get_lineage(trial_or_depth).node_depth trials: list[Trial] = [] for lineage in self._lineage_roots: for trial_node in lineage.get_nodes_at_depth(depth): trials.append(trial_node.item) return trials
[docs]class LineageNode(TreeNode[Trial]): """ Lineage node The lineage node is based on :py:class:`orion.core.utils.tree.TreeNode`. It provides additional methods to help represent lineages for PBT, in particular, ``fork``, ``set_jump``, ``get_true_ancestor`` and ``get_best_trial``. A lineage node can be connected to a parent and children, like a typical TreeNode, but also to ``jumps`` and a ``base``. The jumps and base represent the connection between nodes when PBT drops a trial and rather fork another one. In such case, the dropped trial will refer to the new trial (the forked one) with ``jumps`` (it can refer to many if the new trials crashed and required rollback) and the forked trial will refer to the dropped one with ``base`` (it can only refer one). Parameters ---------- trial: ``orion.core.worker.trial.Trial`` The trial to represent with the lineage node. parent: LineageNode, optional The parent node for this lineage node. Default: None, that is, no parent. """ def __init__(self, trial: Trial, parent: LineageNode | None = None): super().__init__(copy.deepcopy(trial), parent=parent) self._jump: TreeNode[LineageNode] = TreeNode(self) @property def tree_name(self) -> str: """Name of the node for pretty printing.""" return str(self.item) @property def jumps(self) -> list[LineageNode]: """New trials generated from forks when dropping this node.""" return [node.item for node in self._jump.children] @property def base(self) -> LineageNode | None: """Base trial that was dropped in favor of this forked trial, if this trial resulted from a fork. """ return self._jump.parent.item if self._jump.parent else None
[docs] def register(self, trial: Trial) -> None: """Save the trial object. Register will copy the object so that any modifications on it externally will not impact the interval representation of the Lineage node. """ self.item = copy.deepcopy(trial)
[docs] def fork(self, new_trial: Trial) -> LineageNode: """Fork the trial to the new one. A new lineage node referring to ``new_trial`` will be created and added as a child to current node. Parameters ---------- new_trial: ``orion.core.worker.trial.Trial`` A new trial that is a child of the current one. Returns ------- LineageNode LineageNode referring to ``new_trial`` """ return LineageNode(new_trial, parent=self)
[docs] def set_jump(self, node: LineageNode) -> None: """Set the jump to given node This will also have the effect of setting ``node.base = self``. Parameters ---------- node: LineageNode Node to refer to as the jump targen for the current node. Raises ------ RuntimeError If the given node already has a base. """ if node._jump.parent is not None: raise RuntimeError( "Trying to jump to an existing node. Jumps to another lineage should only " "occur on new nodes." ) node._jump.set_parent(self._jump)
[docs] def get_true_ancestor(self) -> LineageNode | None: """Return the base if current trial is the result of a fork, otherwise return parent if is has one, otherwise returns None.""" if self.base is not None: return self.base if self.parent is not None: return self.parent return None
[docs] def get_best_trial(self) -> Trial | None: """Return best trial on the path from root up to this node. The path followed is through `true` ancestors, that is, looking at base if the current node is the result of a fork, otherwise looking at the parent. Only leaf node trials may not be completed. If there is only one node in the tree and the node's trial is not completed, ``None`` is returned instead of a trial object. Returns ------- ``None`` Only one node in the tree and it is not completed. ``orion.core.worker.trial.Trial`` Trial with best objective (lowest). """ parent_node = self.get_true_ancestor() if parent_node: parent_trial = parent_node.get_best_trial() if get_objective(parent_trial) <= get_objective(self.item): return parent_trial if self.item.status != "completed": return None return self.item