Best way to do data source abstraction

[Larry Bates]

Arthur Pemberton wrote:
> What is the best way to do data source abtraction? For example have
> different classes with the same interface, but different
> implementations.
> 
> I was thinking of almost having classA as my main class, and have
> classA dynamically "absorb" classFood into to based on the extension
> of the input file received by classA. But this doesn't seem possible.
> 
> Please advise.
> 
> Thank you.
> 

The best method I've found is to have a class that abstracts
the data and presents the values from the data source via
class attributes.  If you also implement it as an iterator
(e.g. give it __iter__ and __next__ methods), you can easily
iterate over the resultset from each data source.  With this
method I've abstracted data in CSV files, fixed ASCII files,
SQL tables, Excel Spreadsheets, tab delimited files that are
members of a .ZIP archive, you name it.

Short example (not tested):
class foo:
    '''
    Class to abstract tab delimited files
    '''
    def __init__(self, filepath, columnnames):
        self.fp=open(filepath, 'r')
        self.columnnames=columnnames
        return

    def __iter__(self):
        return self

    def next(self):
        #
        # Try to get the next line from file
        #
        try: line=self.fp.next()
        except StopIteration:
            self.fp.close()
            raise

        #
        # Decode the tab delimited line into its parts
        #
        line=line.rstrip()
        if not line: raise StopIteration
        values=line.split('\t')
        print "values=", values
        l=zip(self.columnnames, values)
        print l
        for column, value in l:
            setattr(self, column, value)

        return

if __name__ == "__main__":
    obj=foo('abc.txt', ['name', 'address1', 'address2', 'city', 'state', 'zip'])

    for entry in obj:
        print ""
        print "Name........", obj.name
        print "Address1....", obj.address1
        print "Address2....", obj.address2
        print "City........", obj.city
        print "State.......", obj.state
        print "Zip.........", obj.zip


-Larry Bates