Question

I am using Python and Django and messing around with returning JSON objects as Python dictonaries, but am not content because I can’t iterate through my dictionary’s elements in the order they were inserted.

If I create a dictionary as follows:

measurements = {
  'units': 'imperial',
  'fit': request.POST[ 'fit' ],
  'height': request.POST[ 'height' ],
  'weight': request.POST[ 'weight' ],
  'neck': request.POST[ 'neck' ],
  # further elements omitted for brevity
}

I can try iterating through it like:

for k,v in measurements.iteritems():
  print k, 'corresponds to ', v

The result is:

shoulders corresponds to  shoulders_val
weight corresponds to  weight_val
height corresponds to  height_val
wrist corresponds to  wrist_val
...

I also tried using sorted(), which iterates through my elements by key alphabetically

bicep corresponds to  bicep_val
chest corresponds to  chest_val
fit corresponds to  fit_val
height corresponds to  height_val
...

I am new to Python. I am hoping to find some way to both reference my dictionary elements by named keys like measurements[‘units’] but still be able to iterate through these elements in the order they were created. I am aware that there is an ordered dictionary module out there, but I would like to stay away from nonstandard packages. Would any other standard Python data structures ( lists, arrays, etc. ) allow me to iterate in insertion order and reference values by named keys?

Answer 1

You can use a collections.OrderedDict to preserve insertion order if you’re using py2.7 or newer. This is a part of the standard library. For older versions, there’s an activestate recipe floating around that you could copy and use as part of your package/module. Otherwise, there’s nothing in the standard library that will do it.

You could subclass dict yourself and make it so the it remembers the order things were inserted — storing the information in a list for instance — but that is overkill when something already exists in the standard library for newer versions and a recipe that you can copy/paste is readily available if you want to support old versions.

---

Note that dictionary methods which accept dictionaries (__init__, update) won’t be sorted properly if you pass dictionaries to them:

import collections
dd = collections.OrderedDict({
  'units': 'imperial',
  'fit': 'fit' ,
  'height': [ 'height' ],
  'weight': [ 'weight' ],
  'neck': [ 'neck' ],
})

print( dd )  #Order not preserved


#Pass an iterable of 2-tuples to preserve order.
ddd = collections.OrderedDict([
  ('units', 'imperial'),
  ('fit', 'fit') ,
  ('height', [ 'height' ]),
  ('weight', [ 'weight' ]),
  ('neck', [ 'neck' ]),
])

print( ddd ) #Order preserved