Hi Dave,
aha! Good thing I asked. ;-) I've indeed been thinking where this __add__
method should live. The program as it is now has a Generic class, a Reader
class and a Writer class. I thought an Append class was appropriate because it
uses Reader and Writer (and probably also Generic) methods and the data is from
multiple files. It reads a bunch of files (even though the term 'reading' is
more a conceptual term here, as none of the data will be held in memory),
appends them (__add__), and writes them to one merged file. Doesn't adding
__add__ change the responsibility from 'thou shallt read one and only one file'
into something less precise?
So if I understand you correctly, the following pseudocode is better?
merged = Reader.readFile(somefile1) + Reader.readFile(somefile2)
# ..which is the same as:
Reader.readFile(somefile1).__add__(Reader.readFile(somefile2))
for line in merged:
Writer.writerow(line)
Maybe this is why my 'top-down code' (what I posted earlier) and my 'bottom-up
code' (some code that I wrote earlier) don't add up (pun intended!). In the
bottom-up code there was no need for an Append class!
Regards,
Albert-Jan
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All right, but apart from the sanitation, the medicine, education, wine, public
order, irrigation, roads, a
fresh water system, and public health, what have the Romans ever done for us?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>________________________________
> From: Dave Angel <d...@davea.name>
>To: Albert-Jan Roskam <fo...@yahoo.com>
>Cc: Python Mailing List <tutor@python.org>
>Sent: Monday, March 5, 2012 9:36 PM
>Subject: Re: [Tutor] question about operator overloading
>
>On 03/05/2012 03:16 PM, Albert-Jan Roskam wrote:
>> Hi,
>>
>> I am extending a program for a hobby project where potentially huge spss
>> files are read. I would like to add functionality to append files. I thought
>> it would be nice and intuitive to overload + and += for this. The code below
>> is a gross simplification, but I want to get the basics right. Is this the
>> way how operator overloading is usually done?
>>
>>
>> class Append(object):
>>
>> def __init__(self, file1, file2=None):
>> """ file1 and file2 will actually be of a class of my own,
>> which has a readFile method that is a generator that returns
>> one record at a time """
>> self.file1 = file1
>> self.file2 = file2
>> self.merged = []
>>
>> def __add__(self):
>> self.file1.extend(self.file2)
>> return self.file1
>>
>> def __iadd__(self):
>> self.merged.extend(self.file1)
>> return self.merged
>> def writerows(self):
>> rows = self.file1
>> for row in rows:
>> yield row
>>
>> # overloading '+'
>> file1 = [[1, 2, 3], [4, 5, 6], [6, 6, 6]] file2 = [[1, 2, 3]]
>> app = Append(file1, file2)
>> merged = app.file1 + app.file2 # 'merged' will not actually hold data
>> for line in app.writerows():
>> print line
>>
>> # overloading '+='
>> files = [file1, file2]
>> for i, f in enumerate(files):
>> if i == 0:
>> app = Append(f)
>> app.merged = f
>> else:
>> app.merged += f
>> print app.merged
>>
>
>I hate to say it, but it's not even close.
>
>When you say app.file1 + app.file2, you're not calling either of your
>special methods you defined in Append. You're just adding the file1 and file2
>attributes. Since in your example these are lists, they do the usual thing.
>
>Similarly, your app.merged += f does NOT call your __iadd__() method.
>
>Just what kind of an object is an Append object supposed to be? Classes are
>usually for encapsulating data and behavior, not just to bundle up some
>functions.
>
>Normally, you should be defining the __add__() and __iadd__() methods in the
>class that file1 and file2 are instances of. So if you want to make a dummy
>example, start by defining a (single) class that holds just one of these.
>Then create two instances, and try adding and +='ing the two instances.
>
>
>
>DaveA
>
>
>--
>DaveA
>
>
>
>
_______________________________________________
Tutor maillist - Tutor@python.org
To unsubscribe or change subscription options:
http://mail.python.org/mailman/listinfo/tutor
