docs: def get_data_id_by_slug(self, slug):
          """Find data object ID for given slug.
  
          This method queries the Resolwe API and requires network access.
          """
          resolwe_host = os.environ.get('RESOLWE_HOST_URL')
          url = urllib.parse.urljoin(resolwe_host, '/api/data?slug={}&fields=id'.format(slug))
  
          with urllib.request.urlopen(url, timeout=60) as f:
              data = json.loads(f.read().decode('utf-8'))
  
          if len(data) == 1:
              return data[0]['id']
          elif not data:
              raise ValueError('Data not found for slug {}'.format(slug))
          else:
              raise ValueError('More than one data object returned for slug {}'.format(slug))