Python object serialization

Question

I am trying to mimic a dict by using that as the base class. The objective is to meet these conditions:

If 2 arguments on the command line, set a key and value in the object's dictionary; if 1 argument on the command line, treat it as a key and show the value; if no arguments on the command line, show all keys and values.

Here is my code:

import pickle,os,sys class ConfigDict(dict): def __init__(self, filename): self._filename = filename if not os.path.exists(self._filename): with open(self._filename,"wb") as fh: pickle.dump({}, fh) with open(self._filename,"rb") as fh: self.update(pickle.load(fh)) def __setitem__(self, key, value): dict.__setitem__(self,key,value) with open(self._filename,"wb") as fh: pickle.dump(self, fh) def __getitem__(self,key): return dict.__getitem__(self,key) cd = ConfigDict('first.pickle') # if 2 arguments on the command line, # set a key and value in the object's dictionary if len(sys.argv) == 3: key, value = sys.argv[1], sys.argv[2] print('writing data: {0}, {1}'.format(key, value)) cd[key] = value # if 1 argument on the command line, treat it as a key and show the value elif len(sys.argv) == 2: print('reading a value') key = sys.argv[1] print('the value for {0} is {1}'.format(sys.argv[1], cd[key])) # if no arguments on the command line, show all keys and values else: print('keys/values:') for key in cd.keys(): print(' {0} = {1}'.format(key, cd[key]))

I am able to write to the file, however, when i try to retrive the value for a given key, i hit the error (only the end of stack trace shown):

 with open(self._filename,"wb") as fh: AttributeError: 'ConfigDict' object has no attribute '_filename'

But, i already set the _filename in __init__. What am i missing ?

avloss · Accepted Answer · 2019-04-19 12:54:14Z

Well, this is a tricky one - the problem seems to be with pickle.load(fh) and NOT with self.update( try this in two lines

... with open(self._filename,"rb") as fh: tmp = pickle.load(fh) self.update(tmp) ...

this would fail at tmp =, so it's the object you're un-pickling that's failing. An easy fix would be to do pickle.dump(dict(self), fh), when serialising your things. Though this whole approach seems "forced" to me. Fully working version:

import pickle,os,sys class ConfigDict(dict): def __init__(self, filename): self._filename = filename if not os.path.exists(self._filename): with open(self._filename,"wb") as fh: pickle.dump({}, fh) with open(self._filename,"rb") as fh: self.update(pickle.load(fh)) def __setitem__(self, key, value): dict.__setitem__(self,key,value) with open(self._filename,"wb") as fh: pickle.dump(dict(self), fh) def __getitem__(self,key): return dict.__getitem__(self,key) cd = ConfigDict('first.pickle') # if 2 arguments on the command line, # set a key and value in the object's dictionary if len(sys.argv) == 3: key, value = sys.argv[1], sys.argv[2] print('writing data: {0}, {1}'.format(key, value)) cd[key] = value # if 1 argument on the command line, treat it as a key and show the value elif len(sys.argv) == 2: print('reading a value') key = sys.argv[1] print('the value for {0} is {1}'.format(sys.argv[1], cd[key])) # if no arguments on the command line, show all keys and values else: print('keys/values:') for key in cd.keys(): print(' {0} = {1}'.format(key, cd[key]))

what is the need to typecast the object as seen in your working code ? pickle.dump(dict(self), fh) works but pickle.dump(self, fh) does not. Please help understand

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