Save class upon deletion, load class upon instantiation: loaded class does not contain previously created attributes

Question

I have a python class representing grouped Dicom series from a particular subject, defined in a module dicom_parser.py. When instantiated, the class calls methods which read and sort Dicoms via UID.

import os
import pydicom as pdi

class DicomSeries():
    def __init__(self):
        self._dcm_unsort_headers = self._dicom_parse("/home/hamilton/dicoms/XYZ_fmri_2011")
        self.dcm_grouped = self._dicom_sort(self._dcm_unsort_headers)
        delattr(self, "_dcm_unsort_headers")

    @staticmethod
    def _dicom_parse(dcm_dir):
        # Find and parse all, return unsorted list
        return dcm_headers

    @staticmethod
    def _dicom_sort(dcm_headers):
        # Sort list of dcm_headers by UID
        return dcm_grouped

dicom_parser.py is imported and called from a main.py script.

I've written a save and load method, taken from bivouac0's answer on this post. save() is called via weakref.finalize, while load() is called if the saved file is present when __init__() is called.

import os
import pickle
from weakref import finalize

class DicomSeries():
    def __init__(self, filename = 'XYZ_dcms.pkl.gz'):
        if os.path.exists(filename) is True
            self = self.load()
        else:
            # Load and sort
        self._finalizer = finalize(self, self.save, filename = self.filename)
    def save(self, filename = None):
        if filename is None:
            filename = self.filename
        with gzip.open(filename, 'wb') as f:
            pickle.dump(self, f, protocol=pickle.HIGHEST_PROTOCOL)
    def load(self, filename = None):
        if filename is None:
            filename = self.filename
        with gzip.open(filename, 'rb') as f:
            old = pickle.load(f)
        return old

The class saves successfully upon deletion or script exit; however if the object is instantiated again in a new script call, the loaded object is void of all attributes declared in the previous instance (e.g. sub.dcm_grouped is not defined).

How might I go about fixing this and achieving the desired behavior of:

Save instance on object deletion or script exit (i.e. gc)
Load instance upon object instantiation if filename is present.

Assigning to `self` in a method has absolutely no effect outside of that method. If you want to do the loading in `__init__()`, you would have to copy all attributes from the loaded object into the existing `self` object. — jasonharper, Jan 12 '22 at 00:43

score 0 · Answer 1 · answered Jan 12 '22 at 20:45

In order to unpickle the saved class on each instantiation, I needed to modify __init__, replacing self = self.load() with self.__dict__ = self.load().__dict__

i.e.

class DicomSeries():
    def __init__(self, filename = 'XYZ_dcms.pkl.gz'):
        if os.path.exists(filename) is True
            self = self.load()

to

class DicomSeries():
    def __init__(self, filename = 'XYZ_dcms.pkl.gz'):
        if os.path.exists(filename) is True
            self.__dict__ = self.load().__dict__

This achieved the desired behavior, that is: reloading the saved, pickled class on re-instantiation.

Save class upon deletion, load class upon instantiation: loaded class does not contain previously created attributes

1 Answers1