"""Explicit metadata union avoids dropping extension keys during Hub streaming."""
from pathlib import Path
from datasets import DatasetInfo,Value,load_dataset

def attach_schema(card,base_shard,extra_fields):
    features=load_dataset('webdataset',data_files={'train':[str(base_shard)]},split='train',streaming=True).features
    for name,dtype in extra_fields.items():
        target=features['json'];parts=name.split('.')
        for part in parts[:-1]:target=target[part]
        target[parts[-1]]=Value(dtype) if isinstance(dtype,str) else dtype
    schema=DatasetInfo(features=features)._to_yaml_dict()['features']
    card['dataset_info']=[dict(config_name=config['config_name'],features=schema) for config in card['configs']]
    return features