Question

In Oracle SQL there is a feature to order as follow:

order by decode('carrot' = 2                ,'banana' = 1                ,'apple' = 3) 

What is the best way to implement this in python?

I want to be able to order a dict by its keys. And that order isn’t necessarily alphabetically or anything – I determine the order.

Answer 1

You can’t order a dict per se, but you can convert it to a list of (key, value) tuples, and you can sort that.

You use the .items() method to do that. For example,

>>> {'a': 1, 'b': 2} {'a': 1, 'b': 2} >>> {'a': 1, 'b': 2}.items() [('a', 1), ('b', 2)] 

Most efficient way to sort that is to use a key function. using cmp is less efficient because it has to be called for every pair of items, where using key it only needs to be called once for every item. Just specify a callable that will transform the item according to how it should be sorted:

sorted(somedict.items(), key=lambda x: {'carrot': 2, 'banana': 1, 'apple':3}[x[0]]) 

The above defines a dict that specifies the custom order of the keys that you want, and the lambda returns that value for each key in the old dict.