I don't know exactly the internals of beat but I wonder why the database view class needs to be this complicated? There may be some technical reasons behind this but I don't think this should be exposed to the users. These Views are just a container as far as I can see. So, I wonder if having something like:

class Train:
    """The training set"""

    def __init__(self, parameters=None, root_folder=None):
        self.parameters = parameters
        self.root_folder = root_folder
        # initialize the db interface here
        # e.g. load a csv file
        import pandas as pd

        self.df = pd.read_csv(...)

    def __len__(self):
        # return the total number of items in here
        return len(self.df)

    def __getitem__(self, index):
        # return the nth row in here
        row = self.df.iloc[index]
        # load the data here if necessary
        row.image = load(row.filename)
        return row

would not satisfy the requirements.