Custom sort order of list

Question

I have lists such as:

mylist1 = ['alpha', 'green']
mylist2 = ['blue', 'alpha', 'red']

I want to sort these two lists by this custom ordered list: ['red','blue','green','alpha']

so that mylist1 = ['green', 'alpha'] and mylist2 = ['red','blue','alpha']

How can i do this in Python?

Answer 1

Demonstration:

>>> mylist1 = ['alpha', 'green']
>>> mylist2 = ['blue', 'alpha', 'red']
>>> sort_order = ['red', 'blue', 'green', 'alpha']
>>> mylist1.sort(key=sort_order.index)
>>> mylist1
['green', 'alpha']
>>> mylist2.sort(key=sort_order.index)
>>> mylist2
['red', 'blue', 'alpha']

Explanation:

The key parameter in list.sort causes the list to determine the order by comparing key(element) instead of element. For example, to do case-insensitive sort, you can pass a key function which makes the string lowercase. The lowercase elements are compared, but the original elements are preserved:

>>> x = ["age", "Bonkers", "cheese"]
>>> x.sort()
>>> x
['Bonkers', 'age', 'cheese']
>>> str.lower("Bonkers")
'bonkers'    
>>> x.sort(key=str.lower)
>>> x
['age', 'Bonkers', 'cheese']

Using sort_order.index for the key uses the index that element has in the sort_order list to determine the order instead of the element itself. So 'red' uses 0, 'blue' uses 1, etc... the result is that the list to be sorted gets sorted according to where each element is in sort_order.