I want to keep order of OrderedDict with ujson

Question

My function's return value type is OrderedDict,

and now I want to write this on the file:

Here's my code:

mainDict = OrderedDict([('a',1),('b',2),('c',3),('d',[4,5,6])])
with open(outFileName, 'w', encoding='utf-8') as outFile :
   outFile.write(ujson.dumps(mainDict, indent=4))

I expected it to keep the order of the dictionary in the file, but it got mixed up.

Is it because of using ujson.dumps? and how can I keep the order of an OrderedDict in the output file?

Answer 1

Use sort_keys parameter of ujson.dumps

Behaviour of ujson is the following:

• sort_keys=None (default if omitted) - dump dict keys in implementation-defined order, which may be different every launch (but it's fast)
• sort_keys=True - sort dict keys before dump
• sort_keys=False - preserve dict keys order provided by dict.items()

So to preserve ordering of OrderedDict with ujson, you need sort_keys=False.

---

Tests

This is how you can check it:

import sys
import ujson

order = None
if len(sys.argv) == 2:
    order = bool(int(sys.argv[1]))

mainDict = OrderedDict([('c',3),('b',2),('a',1)])
sys.stdout.write(ujson.dumps(mainDict, sort_keys=order))

Tests:

$ python order.py      # sort_keys=None
{"c":3,"a":1,"b":2}

$ python order.py      # sort_keys=None
{"b":2,"c":3,"a":1}

$ python order.py 1    # sort_keys=True
{"a":1,"b":2,"c":3}

$ python order.py 0    # sort_keys=False
{"c":3,"b":2,"a":1}

---

Notes

Note that unlike ujson, built-in json module preserves key order with sort_keys=None, as well as with sort_keys=False.

Also note that although it's possible to preserve keys order with these implementations (ujson and json), it's non-standard JSON. See json.org:

An object is an unordered set of name/value pairs.