Question

Here’s what I have so far:

def is_ordered(collection):
    if isinstance(collection, set):
        return False
    if isinstance(collection, list):
        return True
    if isinstance(collection, dict):
        return False

    raise Exception("unknown collection")

Is there a much better way to do this?

NB: I do mean ordered and not sorted.

Motivation:

I want to iterate over an ordered collection. e.g.

def most_important(priorities):
    for p in priorities:
        print p

In this case the fact that priorities is ordered is important. What kind of collection it is is not. I’m trying to live duck-typing here. I have frequently been dissuaded by from type checking by Pythonistas.

Answer 1

If the collection is truly arbitrary (meaning it can be of any class whatsoever), then the answer has to be no.

Basically, there are two possible approaches:

1. know about every possible class that can be presented to your method, and whether it’s ordered;
2. test the collection yourself by inserting into it every possible combination of keys, and seeing whether the ordering is preserved.

The latter is clearly infeasible. The former is along the lines of what you already have, except that you have to know about every derived class such as collections.OrderedDict; checking for dict is not enough.

Frankly, I think the whole is_ordered check is a can of worms. Why do you want to do this anyway?