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
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