how to compare values in an existing dictionary and update the dictionary back to a file?

I am making an utility of sorts with dictionary. What I am trying to achieve is this:

for each XML file that I parse, the existing dictionary is loaded from a file (output.dict) and compared/updated for the current key and stored back along with existing values. I tried with has_key() and attributerr开发者_如何学Goor, it does not work.

Since I trying one file at a time, it creates multiple dictionaries and am unable to compare. This is where I am stuck.

def createUpdateDictionary(servicename, xmlfile):
   dictionary = {}
   if path.isfile == 'output.dict':
       dictionary.update (eval(open('output.dict'),'r'))


   for event, element in etree.iterparse(xmlfile):


      dictionary.setdefault(servicename, []).append(element.tag)

   f = open('output.dict', 'a')
   write_dict = str(dictionary2)
   f.write(write_dict)
   f.close()

(here the servicename is nothing but a split '.' of xmlfile which forms the key and values are nothing by the element's tag name)

def createUpdateDictionary(servicename, xmlfile):
   dictionary = {}
   if path.isfile == 'output.dict':
       dictionary.update (eval(open('output.dict'),'r'))

There is a typo, as the 'r' argument belongs to open(), not eval(). Furthermore, you cannot evaluate a file object as returned by open(), you have to read() the contents first.

   f = open('output.dict', 'a')
   write_dict = str(dictionary2)
   f.write(write_dict)
   f.close()

Here, you are appending the string representation to the file. The string representation is not guaranteed to represent the dictionary completely. It is meant to be readable by humans to allow inspection, not to persist the data.

Moreover, since you are using 'a' to append the data, you are storing multiple copies of the updated dictionary in the file. Your file might look like:

{}{"foo": []}{"foo": [], "bar":[]}

This is clearly not what you want; you won't even by able to eval() it later (syntax error!).

Since eval() will execute arbitrary Python code, it is considered evil and you really should not use it for object serialization. Either use pickle, which is the standard way of serialization in Python, or use json, which is a human-readable standard format supported by other languages as well.

import json

def createUpdateDictionary(servicename, xmlfile):
    with open('output.dict', 'r') as fp:
        dictionary = json.load(fp)

    # ... process XML, update dictionary ...

    with open('output.dict', 'w') as fp:
        json.dump(dictionary, fp)