Today the following error appeared in my study when I try to optimize it after loading on Colab on Optuna 2.8.0. I save a study with joblib each time a trial begins or ends in a separate file for each trial. Never have I had this problem before and not sure what actually causes it.
Colab shows the following trace:
/usr/local/lib/python3.7/dist-packages/optuna/study.py in optimize(self, func, n_trials, timeout, n_jobs, catch, callbacks, gc_after_trial, show_progress_bar)
408 callbacks=callbacks,
409 gc_after_trial=gc_after_trial,
--> 410 show_progress_bar=show_progress_bar,
411 )
412
/usr/local/lib/python3.7/dist-packages/optuna/_optimize.py in _optimize(study, func, n_trials, timeout, n_jobs, catch, callbacks, gc_after_trial, show_progress_bar)
73 reseed_sampler_rng=False,
74 time_start=None,
---> 75 progress_bar=progress_bar,
76 )
77 else:
/usr/local/lib/python3.7/dist-packages/optuna/_optimize.py in _optimize_sequential(study, func, n_trials, timeout, catch, callbacks, gc_after_trial, reseed_sampler_rng, time_start, progress_bar)
160
161 try:
--> 162 trial = _run_trial(study, func, catch)
163 except Exception:
164 raise
/usr/local/lib/python3.7/dist-packages/optuna/_optimize.py in _run_trial(study, func, catch)
195 failed_trial_callback(study, failed_trial)
196
--> 197 trial = study.ask()
198
199 state: Optional[TrialState] = None
/usr/local/lib/python3.7/dist-packages/optuna/study.py in ask(self, fixed_distributions)
485 if trial_id is None:
486 trial_id = self._storage.create_new_trial(self._study_id)
--> 487 trial = trial_module.Trial(self, trial_id)
488
489 for name, param in fixed_distributions.items():
/usr/local/lib/python3.7/dist-packages/optuna/trial/_trial.py in __init__(self, study, trial_id)
55 self.storage = self.study._storage
56
---> 57 self._init_relative_params()
58
59 def _init_relative_params(self) -> None:
/usr/local/lib/python3.7/dist-packages/optuna/trial/_trial.py in _init_relative_params(self)
65 self.relative_search_space = self.study.sampler.infer_relative_search_space(study, trial)
66 self.relative_params = self.study.sampler.sample_relative(
---> 67 study, trial, self.relative_search_space
68 )
69
/usr/local/lib/python3.7/dist-packages/optuna/samplers/_tpe/sampler.py in sample_relative(self, study, trial, search_space)
327 self._raise_error_if_multi_objective(study)
328
--> 329 if self._group:
330 assert self._search_space_group is not None
331 params = {}
AttributeError: 'TPESampler' object has no attribute '_group'
PS. Interestingly, the problem with the study doesn't seem to exist on my local machine. I use version 2.5.0.
Just got the answer from Optuna developers:
A (private) attribute was introduced to the TPESampler in v2.8 and if you've pickled or serialized the sampler before this introduction, you'll encounter this AttributeError when unpickling it with v2.8.
The solution to this problem is re-instantiating the sampler object and substitute it to the study.sampler:
study = optuna.load_study(...)
sampler = optuna.samplers.TPESampler()
study.sampler = sampler
study.optimize(...)
PS. The same kind of problem may appear with a pruner. If so, re-instantiate also the pruner!