Array references in Python class

Question

I created a class that has lots of numpy arrays inside. I created a __getitem__ function that attempts to return the class with the arrays indexed like so:

MyClass[i].array1 is equivalent to MyClass.array1[i]

I would like the _ _ getitem _ _ to return references, but they are returning copies, so that assignment doesn't work.

print(MyClass[i].array1)

returns 0

MyClass[i].array1 = 10

print(MyClass[i].array1)

still returns 0

This is the _get_item_ code I'm using:

def __getitem__(self, indices):
    g = copy.copy(self) # should be a shallow copy?
    for key,value in g.__dict__.iteritems():
        g.__dict__[key] = value[indices]
    return g

I've also tried:

def __getitem__(self, indices):
    g = MyClass()
    for key,value in self.__dict__.iteritems():
        g.__dict__[key] = value[indices]
    return g

and:

def __getitem__(self, indices):
    g = MyClass()
    g.__dict__ = self.__dict__
    for key,value in g.__dict__.iteritems():
        g.__dict__[key] = value[indices]
    return g

Note, this last instance does indeed seem to return references, but not the way I want. If I index my class using this last code, it performs indexing and truncating on the arrays in the original class, so:

g = MyClass[i].array1 truncates and overwrites the original array in MyClass to only have elements of index i like so:

print(len(MyClass.array1))

returns 128

print(MyClass[0].array1)

returns 0

now MyClass.array1 is a single float value, obviously not what I wanted.

I hope this is clear enough, and any help would be appreciated.

I found this but I wasn't quite sure if this applied to my problem.

They actually do return references - it's just that assigning to a variable rebinds the reference, instead of following the reference and mucking about with the object it refers to. You can write descriptors to get around this, but I'd first question whether it's a good idea. — user2357112
– user2357112, Commented Sep 12, 2013 at 16:28

Andrew Clark · Accepted Answer · 2013-09-12 16:55:15Z

1

This seems like a really bad idea, but it also seems like a fun problem so here is my crack at it:

class MyProxy(object):
    def __init__(self, obj, key):
        super(MyProxy, self).__setattr__('obj', obj)
        super(MyProxy, self).__setattr__('key', key)

    def __getattr__(self, name):
        return getattr(self.obj, name).__getitem__(self.key)

    def __setattr__(self, name, value):
        return getattr(self.obj, name).__setitem__(self.key, value)

class MyClass(object):
    def __init__(self, array_length):
        self.array_length = array_length

    def __getitem__(self, key):
        if key >= self.array_length:
            raise IndexError
        return MyProxy(self, key)

Example:

>>> obj = MyClass(4)  # replace 4 with the length of your arrays
>>> obj.array1 = [1, 2, 3, 4]
>>> obj.array2 = [5, 6, 7, 8]
>>> for c in obj:
...     print c.array1, c.array2
... 
1 5
2 6
3 7
4 8
>>> obj[1].array1
2
>>> obj[1].array1 = 5
>>> obj.array1
[1, 5, 3, 4]

edited Sep 12, 2013 at 16:55

answered Sep 12, 2013 at 16:28

Andrew Clark

210k36 gold badges286 silver badges310 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

user2773300 Over a year ago

That seems to mostly work, though it has trouble with iterating for c in MyClass: c.array1 = 0 ERROR: An unexpected error occurred while tokenizing input The following traceback may be corrupted or invalid The error message is: ('EOF in multi-line string', (33, 8)) --> 180 return getattr(self.obj, name).__setitem__(self.key, value) IndexError: index 128 is out of bounds for axis 0 with size 128

Andrew Clark Over a year ago

Are all your numpy arrays the same length? If not iteration will be tricky, I guess it could be based on the longest numpy array but this seems kind of weird.

user2773300 Over a year ago

Yes, they all will have the same length for their first dimension, though some will have an additional 2nd or 3rd dimension. Basically they are time series data and information where the time record is the 1st index.

Andrew Clark Over a year ago

It works now with iteration, you just need to pass the length of your arrays when creating the instance.

user2773300 Over a year ago

That might work, though I would have the length variable set based on the data that is read in during the init rather than passing it explicitly. One issue, though, is that indexing the class doesn't actually return a class object, so none of the methods are attached. Can we get around that by subclassing the original class with the proxy, like: class MyProxy(MyClass) ?

|

Collectives™ on Stack Overflow

Array references in Python class

1 Answer 1

7 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

7 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related