By Teifion


2009-02-08 18:03:46 8 Comments

Here is the dictionary I have

propertyList = {
    "id":           "int",
    "name":         "char(40)",

    "team":         "int",
    "realOwner":    "int",

    "x":            "int",
    "y":            "int",

    "description":  "char(255)",

    "port":         "bool",
    "secret":       "bool",
    "dead":         "bool",
    "nomadic":      "bool",

    "population":   "int",
    "slaves":       "int",
}

But when I print it out with "\n".join(myDict) I get this

name
nomadic
dead
port
realOwner
secret
slaves
team
y
x
population
id
description

I know that a dictionary is unordered but it comes out the same every time and I've no idea why.

3 comments

@CMS 2009-02-08 18:14:50

The specification for the built-in dictionary type disclaims any preservation of order, it is best to think of a dictionary as an unordered set of key: value pairs...

You may want to check the OrderedDict module, which is an implementation of an ordered dictionary with Key Insertion Order.

@Konrad Rudolph 2009-02-08 18:06:42

The real question should be “why not?” … an unordered dictionary is most probably implemented as a hash table (in fact, the Python documentation states this outright) where the order of elements is well-defined but not immediately obvious. Your observations match the rules of a hash table perfectly: apparent arbitrary, but constant order.

@llimllib 2009-02-09 06:05:37

It's well worth reading a comment from python's dictionary source file; I've already posted it in a stackoverflow post about "the best comment you've ever seen": is.gd/iSyN

@Maciej Gol 2013-10-02 09:07:09

A bit of dict's internal implementation can be found here laurentluce.com/posts/python-dictionary-implementation . There was a nice presentation from one of python's coders, but can't find it at the moment (I think it was from PyCon Australia).

@Miles 2009-02-10 09:37:21

The only thing about dictionary ordering you can rely on is that the order will remain the same if there are no modifications to the dictionary; e.g., iterating over a dictionary twice without modifying it will result in the same sequence of keys. However, though the order of Python dictionaries is deterministic, it can be influenced by factors such as the order of insertions and removals, so equal dictionaries can end up with different orderings:

>>> {1: 0, 2: 0}, {2: 0, 1: 0}
({1: 0, 2: 0}, {1: 0, 2: 0})
>>> {1: 0, 9: 0}, {9: 0, 1: 0}
({1: 0, 9: 0}, {9: 0, 1: 0})

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