Airflow: no module named '...' problem

Once I encountered a mysterious error in the operator using some XCom variables:

ERROR - No module named 'commons'
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/airflow/models.py", line 1642, in _run_raw_task
    self.render_templates()
  File "/usr/local/lib/python3.6/site-packages/airflow/models.py", line 1998, in render_templates
    rendered_content = rt(attr, content, jinja_context)
  File "/usr/local/lib/python3.6/site-packages/airflow/models.py", line 2909, in render_template
    return self.render_template_from_field(attr, content, context, jinja_env)
  File "/usr/local/lib/python3.6/site-packages/airflow/models.py", line 2877, in render_template_from_field
    result = jinja_env.from_string(content).render(**context)
  File "/usr/local/lib/python3.6/site-packages/jinja2/asyncsupport.py", line 76, in render
    return original_render(self, *args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/jinja2/environment.py", line 1008, in render
    return self.environment.handle_exception(exc_info, True)
  File "/usr/local/lib/python3.6/site-packages/jinja2/environment.py", line 780, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.6/site-packages/jinja2/_compat.py", line 37, in reraise
    raise value.with_traceback(tb)
  File "