difflib to compare two python dictionaries

Question

I need help trying to use difflib to compare two dicts. My program takes 2 json files, converts them to python dicts. Then I would like to use difflib on the two dicts to display the differences between the two.

Whats the correct way of using difflib to go about this?

#!/usr/bin/env python2

import json
import collections
import difflib
import pprint

def get_json():
    file_name = raw_input("Enter name of JSON File: ")
    with open(file_name) as json_file:
        json_data = json.load(json_file)
        return json_data

def convert(data):
    if isinstance(data, basestring):
        return str(data)
    elif isinstance(data, collections.Mapping):
        return dict(map(convert, data.iteritems()))
    elif isinstance(data, collections.Iterable):
        return type(data)(map(convert, data))
    else:
        return data

def main():
    json1 = get_json()
    json2 = get_json()
    json1_dict = convert(json1)
    json2_dict = convert(json2)
    result = list(difflib.Differ.compare(json1_dict, json2_dict))
    pprint.pprint(result)

if __name__ == "__main__":
    main()

json example:

{
    "glossary": {
        "title": "example glossary",
        "GlossDiv": {
            "title": "S",
            "GlossList": {
                "GlossEntry": {
                    "ID": "SGML",
                    "SortAs": "SGML",
                    "GlossTerm": "Standard Generalized Markup Language",
                    "Acronym": "SGML",
                    "Abbrev": "ISO 8879:1986",
                    "GlossDef": {
                        "para": "A meta-markup language, used to create markup languages such as DocBook.",
                        "GlossSeeAlso": [
                            "GML",
                            "XML"
                        ]
                    },
                    "GlossSee": "markup"
                }
            }
        }
    }
}

And change the value of ID to "1234" in a second file

I wanted to compare the two and get and output of something like:

{
    "glossary": {
        "title": "example glossary",
        "GlossDiv": {
            "title": "S",
            "GlossList": {
                "GlossEntry": {
-                   "ID": "SGML",
+                   "ID": "1234",
                    "SortAs": "SGML",
                    "GlossTerm": "Standard Generalized Markup Language",
                    "Acronym": "SGML",
                    "Abbrev": "ISO 8879:1986",
                    "GlossDef": {
                        "para": "A meta-markup language, used to create markup languages such as DocBook.",
                        "GlossSeeAlso": [
                            "GML",
                            "XML"
                        ]
                    },
                    "GlossSee": "markup"
                }
            }
        }
    }
}

Differ.compare is an instance method. Is there any purpose to your convert function other than to get rid of unicode strings? Anyway, difflib works on sequences of lines, not on arbitrary objects like dictionaries. What sort of content do your json files have? What do you expect your program's output to be like? — Blckknght
– Blckknght, Commented Jan 25, 2014 at 3:08
I don't think this is what difflib is for; it's for producing differences between text files. What is the structure of your JSON files? Are they only 2 levels (or so) deep, or will you need a recursive solution? — dstromberg
– dstromberg, Commented Jan 25, 2014 at 3:11
The convert function is only there to get rid of the unicode, is there a better way to do that? I'll edit the question with examples of the json files and the expected output — user3230554
– user3230554, Commented Jan 25, 2014 at 3:15
@Blckknght The convert function is only there to get rid of the unicode, is there a better way to do that? I edited the question with examples of the json files and the expected output — user3230554
– user3230554, Commented Jan 25, 2014 at 3:23
@dstromberg So difflib is not the correct way for me to do this then, how would you suggest I get to the desired output? — user3230554
– user3230554, Commented Jan 25, 2014 at 3:25

Karl Knechtel · Accepted Answer · 2014-01-25 03:13:31Z

You have a few issues here. First off, you're trying to use the method difflib.Differ.compare, but you're calling it as a plain function - you have not actually created a difflib.Differ object.

Second, this compare method expects you to operate upon a sequence of strings (for each of the two things being compared). Your convert function is sometimes returning strings, sometimes dicts, sometimes other stuff... in general, you're not getting back sequences of strings.

The natural way to get what you want is to just compare the actual JSON data, because that's a string. However, there are two issues there:

you want a sequence of strings (line-by-line) instead of a single string with the whole JSON document, but that's trivial - just split it up into lines with the string .splitlines method.
your input might have differences in whitespace that you want to ignore. The simple way around this is to, after loading each JSON document into an object, re-create a string for it with dumps. The idea is that for both documents that you're comparing, you will dump with the same whitespace settings. You need to read the documentation and decide what settings you want to use.

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