[Tutor] question about descriptors

[Peter Otten]

Albert-Jan Roskam wrote:

>> class ReadColumn(object):
>>     def __init__(self, index):
>>         self._index = index
>>     def __get__(self, obj, type=None):
>>         return obj._row[self._index]
>>     def __set__(self, obj, value):
>>         raise AttributeError("oops")
> 
> This appears to return one value, whereas I wanted I wanted to return all
> values of a column, ie as many values as there are rows. But the logic
> probably won't change. 

Sorry, I overlooked that aspect. If you want a whole column you either have 
to iterate over the complete file and keep the data in memory or you need a 
separate file descriptor for every access of a column. Here's an 
implementation of the first:

def csv_columns(instream):
    reader = csv.reader(instream, delimiter=";")

    header = next(reader)
    return namedtuple("Columns", header)._make(tuple(zip(*reader)))