Converting dict to OrderedDict

Question

I am having some trouble using the collections.OrderedDict class. I am using Python 2.7 on Raspbian, the Debian distro for Raspberry Pi. I am trying to print two dictionaries in order for comparison (side-by-side) for a text-adventure. The order is essential to compare accurately. No matter what I try the dictionaries print in their usual unordered way.

Here's what I get when I do it on my RPi:

import collections

ship = {"NAME": "Albatross",
         "HP":50,
         "BLASTERS":13,
         "THRUSTERS":18,
         "PRICE":250}

ship = collections.OrderedDict(ship)

print ship
# OrderedDict([('PRICE', 250), ('HP', 50), ('NAME', 'Albatross'), ('BLASTERS', 13), ('THRUSTERS', 18)])

Obviously there is something not right because it is printing the function call and putting the keys and value groups into a nested list...

This is what I got by running something similar on my PC:

import collections

Joe = {"Age": 28, "Race": "Latino", "Job": "Nurse"}
Bob = {"Age": 25, "Race": "White", "Job": "Mechanic", "Random": "stuff"}

#Just for clarity:
Joe = collections.OrderedDict(Joe)
Bob = collections.OrderedDict(Bob)

print Joe
# OrderedDict([('Age', 28), ('Race', 'Latino'), ('Job', 'Nurse')])
print Bob
# OrderedDict([('Age', 25), ('Race', 'White'), ('Job', 'Mechanic'), ('Random', 'stuff')])

This time, it is in order, but it shouldn't be printing the other things though right? (The putting it into list and showing function call.)

Where am I making my error? It shouldn't be anything to do with the pi version of Python because it is just the Linux version.

Answer 1

You are creating a dictionary first, then passing that dictionary to an OrderedDict. For Python versions < 3.6 ^(*), by the time you do that, the ordering is no longer going to be correct. dict is inherently not ordered.

Pass in a sequence of tuples instead:

ship = [("NAME", "Albatross"),
        ("HP", 50),
        ("BLASTERS", 13),
        ("THRUSTERS", 18),
        ("PRICE", 250)]
ship = collections.OrderedDict(ship)

What you see when you print the OrderedDict is it's representation, and it is entirely correct. OrderedDict([('PRICE', 250), ('HP', 50), ('NAME', 'Albatross'), ('BLASTERS', 13), ('THRUSTERS', 18)]) just shows you, in a reproducable representation, what the contents are of the OrderedDict.

---

^(*): In the CPython 3.6 implementation, the dict type was updated to use a more memory efficient internal structure that has the happy side effect of preserving insertion order, and by extension the code shown in the question works without issues. As of Python 3.7, the Python language specification has been updated to require that all Python implementations must follow this behaviour. See this other answer of mine for details and also why you'd still may want to use an OrderedDict() for certain cases.