def flatten_dict(row, keys=[('title',),
                            ('street', 'city', 'postalCode')]):
    """Flatten a dict by concatenating string values
    of matching keys.

    Args:
        row (dict): Data to be flattened
    Returns:
        flat (str): Concatenated data.
    """
    flat = ''  # The output data
    for ks in keys:
        # If any keys are present, join the values
        if not any(k in row for k in ks):
            continue
        flat = '\n'.join(row[k] for k in ks
                         if k in row)
        break    
    assert len(flat) > 0  # Ensures that a key has been found, 
                          # otherwise you'll need to provide more keys
    return flat