Ordering in the printout of a dictionary

[Chris Angelico]

On Tue, Mar 18, 2014 at 11:32 AM, Mok-Kong Shen
<mok-kong.shen at t-online.de> wrote:
> Could someone kindly explain a phenomenon in the following where:
>
> (1) I first typed in a dictionary but got a printout in a reordered
> form.
>
> (2) I then typed in the reordered form but got a printout in the
> order that I typed in originally in (1).
>
> That is, there is no stable "standard" ordering. Why is that so?

A dictionary is simply a mapping from keys to values. It has no ordering.

> Is there a way to force a certain ordering of the printout or else
> somehow manage to get at least a certain stable ordering of the
> printout (i.e. input and output are identical)?

Yes; instead of simply printing it out (which calls repr()),
explicitly iterate over it, like this:

def display(d):
    return '{'+','.join('%r: %r'%(key,d[key]) for key in sorted(d))+'}'

>>> print(display({'label': 3, 'parent': 0, 'left child': 1, 'right child': 2}))
{'label': 3, 'left child': 1, 'parent': 0, 'right child': 2}

That will be consistent, and will also always be sorted, which will
probably be what you want for human-readable display. At least, it's
consistent as long as the keys all sort consistently, which they will
if you use simple strings. Other types of keys may not work, and in
fact mixing types may cause an exception:

>>> print(display({True:1,"Hello":2}))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in display
TypeError: unorderable types: str() < bool()

But for strings, this is the easiest way to get what you're looking for.

ChrisA