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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