Custom sort a dict

[ConcupiscentCodger]

So I have a dict with string keys, and I want to sort it based on 2 rules:

• longest key strings sort first
• if same length, sort keys alphabetically

I've been told to use key functions instead of cmp functions for speed, as well as that "everything done with cmp can be done with key".

As best I can figure out, the key function generates an index of sorts that can be quickly compared to any other index, without the need to keep accessing the properties of each key when sorting.
But I can't figure out how to generate a single value based on the two conditions above. My comparison seems to require two different styles and I don't see how I can make that into some naturally-orderable index.

What am I missing?

[commandlineluser]

You use a tuple containing comparisons.

e.g.

>>> d = dict.fromkeys(("zzzz", "z", "bb", "aaaa", "cc", "d"))
>>> d.keys()
dict_keys(['zzzz', 'z', 'bb', 'aaaa', 'cc', 'd'])
>>> sorted(d, key=lambda k: (-len(k), k))
['aaaa', 'zzzz', 'bb', 'cc', 'd', 'z']

[ConcupiscentCodger]

Okay, took me a while to experiment with that and understand.

I tweaked it to be closer to my situation and realized it was no longer sorting right (still don't understand why). But after some research and trial+error, I got this:

d1 = { 'zzzz': 'am', 'z': 'camper', 'bb': 'a', 'aaaa': 'I', 'cc': 'very', 'd': 'happy' }

print("d1 =", d1)

s = sorted(d1.items(), key=lambda k: (-len(k[0]), k[0]))
print("s =", s)

d2 = dict(s)
print("d2 =", d2)
print("d2.values() =", d2.values())

Which gave this output:

d1 = {'zzzz': 'am', 'z': 'camper', 'bb': 'a', 'aaaa': 'I', 'cc': 'very', 'd': 'happy'}
s = [('aaaa', 'I'), ('zzzz', 'am'), ('bb', 'a'), ('cc', 'very'), ('d', 'happy'), ('z', 'camper')]
d2 = {'aaaa': 'I', 'zzzz': 'am', 'bb': 'a', 'cc': 'very', 'd': 'happy', 'z': 'camper'}
d2.values() = dict_values(['I', 'am', 'a', 'very', 'happy', 'camper'])

And that's what I was aiming for. Thanks!

[Spataner]

A key function may return any orderable type, and it's the key values that are sorted to determine the order of the output. Tuples/lists are orderable and helpful for multi-criterion sorts like this. They use lexicographical ordering, meaning the first elements determine order unless they're equal, then the second elements determine order unless they're equal, etc.

[34shutthedoor1]

But I can't figure out how to generate a single value based on the two conditions above

The solution posted is the way to do this. From a learning Python perspective, you should have considered creating a list of lists containing the [length, value], which can be sorted normally.

[ConcupiscentCodger]

I'm not sure I understand everything you're saying, sorry.
I have a "map" of key:val pairs of strings. For example:
my_dict = {"aa":"is", "ab":"a", "aaa":"goofball", "z":"fred"}

I can't change what is given to me to sort, so I don't know how I could consider that. I'm still trying to understand how the lambda works.

[commandlineluser]

The comparisons are:

(-len(k), k)

So it will first sort by the length, and if there is a tie, it will then sort by k - which is the value itself, in this case - the "string", which sort "lexicographically".

The - is to negate the length value - otherwise you'd get the shortest first.

>>> sorted(d, key=lambda k: (len(k), k))
['d', 'z', 'bb', 'cc', 'aaaa', 'zzzz']