Proper way to return an instance of a class from a class method ?

Auré Gourrier aurelien.gourrier at
Fri Mar 5 11:35:42 CET 2010

Dear all,

I have what will probably sound as stupid questions for experienced Python users... which I am not, hence my seeking for some help.

I've implemeted a class which defines a container with 2 lists of data (with some specific format).
Some of the methods simply transform these data and therefore return a new container of 2 lists.
I want to be able to use these results without having to re-instanciate them, so I return them as instances of the class:

class MyClass:
    def __init__(self, data):
        #various checks for the format and content of data (data = [x,y] where x and y are 2 lists of floats) = data
    def transformdata(self):
        newdata = transform(data)
        return MyClass(newdata)

In this way I can do:

x = list1
y = list2
data = MyClass([x,y])
moddata = data.transformdata()
moddata2 = moddata.transformdata()

Is that the correct way to go (i.e. is this pythonic) ? What I want to avoid is:

moddata = data.transformdata()
moddata2 = MyClass(moddata).transformdata()

If I go this way, I have a second problem: if I create a new class which inherits from the previous, I would expect/like the methods from the initial class to return instances from the new class:

class MyClass2(MyClass):

    def othermethods(self):
        return MyClass2(whatever)

Then I get:

x = list1
y = list2
data = MyClass2([x,y])
finaldata = data.transformdata()

My problem is that finaldata is (of course) an instance of MyClass and not MyClass2. How do I deal with this ?

All help will be much appreciated...



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