On 07/11/15 12:53, Albert-Jan Roskam wrote:
> Ok, now to my question.
> I want to create a class with read-only attribute access
> to the columns of a .csv file.
Can you clarify what you mean by that?
The csvreader is by definition read only.
So is it the in-memory model that you want read-only?
Except you don't really have an in-memory model that I can see?
> E.g. when a file has a column named 'a', that column should
> be returned as list by using instance.a.
That appears to be a separate issue to whether the returned
list is read-only or not? As ever the issue of dynamically
naming variables at run time and then figuring out how to
access them later raises its head. Its hardly ever a good plan.
> At first I thought I could do this with the builtin 'property'
> class, but I am not sure how.
To use property I think you'd need to know the names of your
columns in advance. (Or dynamically build your classes)
> I now tried to use descriptors (__get__ and __set__),
> which are also used by ' property'
> In the " if __name__ == '__main__'" section, [a] is supposed
> to be a shorthand for == equivalent to [b].
I have no idea what you mean by that sentence?
> class AttrAccess(object):
>
> def __init__(self, fileObj):
> self.__reader = csv.reader(fileObj, delimiter=";")
> self.__header = self.__reader.next()
> @property
> def header(self):
> return self.__header
>
> def __get_column(self, name):
> return [record[self.header.index(name)] for record in self.__reader] # generator expression might be better here.
You should only get the index once otherwise it could add a lot of time
for a long file(especially if there were a lot of columns)
def __get_column(self, name):
idx = self.header.index(name)
return [record[idx] for record in self.__reader]
> def __get__(self, obj, objtype=type):
> print "__get__ called"
> return self.__get_column(obj)
> #return getattr(self, obj)
>
> def __set__(self, obj, val):
> raise AttributeError("Can't set attribute")
If you want everything read-only should this not be __setattr__()?
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