def get_tot_pop(dwellings):
    """Get total population of all dwellings

    Return
    ------
    tot_pop : float or bool
        If population is not provided, return `None`,
        otherwise summed population of all dwellings
    """
    tot_pop = 0
    for dwelling in dwellings:
        if dwelling.population is None:
            return None
        else:
            tot_pop += dwelling.population
    return tot_pop
