ImportError: cannot import name x from y

Question

I have a directory tree like so:

- main
  - training
    - run.py
    - utils.py
    - __init__.py

utils.py includes the following:

import numpy as np
from ray.rllib.algorithms.callbacks import DefaultCallbacks

# Guide: https://discuss.ray.io/t/log-or-record-custom-env-data-via-rllib/4674/2
class RewardLoggerCallback(DefaultCallbacks):
    def on_episode_start(
        self, *, worker, base_env, policies, episode, env_index, **kwargs
    ):
        episode.user_data = {
            'MainRew': 0
        }

    def on_episode_step(
        self, *, worker, base_env, episode, env_index, **kwargs
    ):
        # Running metrics -> keep all values
        # Final metrics -> only keep the current value
        info = episode.last_info_for()
        for k in episode.user_data.keys(): 
            episode.user_data[k].append(info[k])

    def on_episode_end(
        self, *, worker, base_env, policies, episode, env_index, **kwargs
    ):
        for name, value in episode.user_data.items():
            episode.custom_metrics[name + "_avg"] = np.mean(value)
            episode.custom_metrics[name + "_sum"] = np.sum(value)
            episode.hist_data[name] = value

However, when I try and add from main.training.utils import RewardLoggerCallback, I get an error of the form ImportError: cannot import name RewardLoggerCallback from main.training.utils. I thought it might be because I didn't add the import in the __init__.py file, but even adding from main.training.utils import RewardLoggerCallback there doesn't help.

Wondering what I might be missing? I have other files in the directory that I've been able to import fine before this way.