<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def fillna(series_or_arr, missing_value=0.0):
 """Fill missing values in pandas objects and numpy arrays. Arguments --------- series_or_arr : pandas.Series, numpy.ndarray The numpy array or pandas series for which the missing values need to be replaced. missing_value : float, int, str The value to replace the missing value with. Default 0.0. Returns ------- pandas.Series, numpy.ndarray The numpy array or pandas series with the missing values filled. """


    if pandas.notnull(missing_value):
        if isinstance(series_or_arr, (numpy.ndarray)):
            series_or_arr[numpy.isnan(series_or_arr)] = missing_value
        else:
            series_or_arr.fillna(missing_value, inplace=True)

    return series_or_arr