Save objects into JSON or XML file in Python

Question

I create objects (using tkinter widgets) that I wish to save in a JSON or XML file, so that I can recover them after startup.

from Tkinter import *

class Texte:
    def __init__(self, ax, ay, txt):
        self.entry = Entry(root,bd=0,font=("Purisa",int(15)))
        self.entry.insert(0, txt)
        self.x = ax
        self.y = ay 
        self.entry.place(x=self.x,y=self.y)

root = Tk()

a = Texte(10, 20, 'blah')
b = Texte(20, 70, 'blah2')

# here the user will modify the entries' x, y, txt, etc.

L = [a,b]

# here save the list L (containing the Texte objects) into a JSON file or XML so that I can recover them after restart 

root.mainloop()

How can save and restore these objects with JSON or XML ?

(I'm a bit lost with http://docs.python.org/2/library/json.html right now.)

aIKid · Accepted Answer · 2013-12-12 10:32:07Z

6

It's mentioned in the docs, use json.dump.

Example of use:

import json

data = {'a':1, 'b':2}
with open('my_json.txt', 'w') as fp:
    json.dump(data, fp)

In your case, you can't convert the object itself to a json format. Save the information only:

data = {'a':(10, 20, 'blah'), 'b':(20, 70, 'blah2')
with open('my_json.txt', 'w') as fp:
     json.dump(data, fp)

And when you load it back:

with open('my_json.txt') as fp:
    data = json.loads(fp)
    a = Texte(*data['a'])
    b = Texte(*data['b'])

edited Dec 12, 2013 at 10:32

answered Dec 12, 2013 at 9:49

aIKid

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6 Comments

Basj Over a year ago

Is it so simple? (It seems that I need a write() for my object, do you have a clue of what it could be in my example with Entry widgets?)

aIKid Over a year ago

No, you don't need to write it. json.dump will automatically export what you need to the specified file.

Basj Over a year ago

I tried to paste your code at the end of mine and I get : AttributeError: 'str' object has no attribute 'write'

aIKid Over a year ago

Oh whoops, i was supposed to pass the file object. Sorry! Updated.

Basj Over a year ago

It runs now, but the resulting .txt file only contains {"a": 1, "b": 2} as text, and not the objects a and b's contents

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Andreas Klebinger · Accepted Answer · 2013-12-12 15:16:40Z

3

If you never modify these objects outside of your Application why the requirement for JSON or XML? If you don't change your data outside of your application you can use the pickle Module to serialize and deserialize the object into either binary data or a ASCII string and save these.

For details see: http://docs.python.org/2/library/pickle.html

There is also a 3rd party library which enables storage of classes as JSON. http://jsonpickle.github.io/

I haven't used it yet myself though so not sure how human readable the output is, but if all you want to do is store it as file and reload it after a application restart I don't see any advantage of JSON/XML over using Pickle.

Edit: As others pointed out you can use cPickle over pickle for better performance. The Algorithm is the same so you use them the exact same way.

edited Dec 12, 2013 at 15:16

answered Dec 12, 2013 at 10:43

Andreas Klebinger

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1 Comment

Jon Over a year ago

Personally, I always use cPickle instead of pickle.

Community · Accepted Answer · 2017-05-23 11:44:22Z

1

Please see update: json is simplejson.

json.dump(for file export) and json.dumps(for string export) can be very useful if you have a simple object. However, if you need to save a more complex data structure like a dictionary filled with dictionaries {'a':{...}, 'b':2} json from the standard library struggles.

For such cases a tool like simplejson can be useful. http://simplejson.readthedocs.org/en/latest/

>>> import simplejson as json
>>> json.dumps(['foo', {'bar': ('baz', None, 1.0, 2)}])
'["foo", {"bar": ["baz", null, 1.0, 2]}]'
>>> print(json.dumps("\"foo\bar"))
"\"foo\bar"
>>> print(json.dumps(u'\u1234'))
"\u1234"

As you need to save a complex list L to json - I would go with simplejson. Something like:

import simplejson as json
with open('Texte.json', 'w') as texte_file:
    json.dump(L, texte_file)`

Update: simplejson is json from the standard library (added in 2.6 to the standard library). For more details see: What are the differences between json and simplejson Python modules?.

edited May 23, 2017 at 11:44

CommunityBot

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answered Dec 12, 2013 at 9:58

Jon

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9 Comments

Basj Over a year ago

Hum, I think I am unable to decide if json or simplejson would be more adapted. How would you save the object in my example (do I require a write()) method in my class?

Basj Over a year ago

woud you have a hint with my simple object example (an object containing a widget, a x, a y, a string) ?

Jon Over a year ago

I edited my example for your cause. Please post a comment if you need more details.

aIKid Over a year ago

Where did you find out that the standard json library struggles when dealing with nested dictionaries?

Jon Over a year ago

@aIKid: Thanks for your advice (+1). simplejson is json - see stackoverflow.com/questions/712791/…

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