自定义排序的Python列表始终带有某些字符串？

Question

I am working in Python 2.7. I have a list of strings like this:

mylist = ['is_local', 'quantity_123', 'actual_cost_456', 
         'actual_cost_123', 'quantity_456', 'total_items_123', 
         'id', 'total_items_456', 'name', 'postcode']

The list will always have the id , name , postcode and is_local fields in it, but the other fields will vary.

I would like to sort the list so that it always starts with the set fields above, and then has the other fields in alphabetical order.

For example:

mylist.sort(custom_sort)
print mylist
['id', 'name', 'postcode', 'is_local', 'actual_cost_123', 
 'actual_cost_456', 'quantity_123', 'quantity_456' ...]

My problem is how to define the custom_sort function. I've tried this:

def custom_sort(a, b):
  if a == 'id':
    return 1
  elif a == 'name':
    return 1
  elif a == 'postcode':
    return 1
  elif a == 'is_dispensing':
    return 1
  elif a > b:
    return 1
  else:
    return -1

But then mylist.sort(custom_sort) gives me an error: TypeError: argument of type 'NoneType' is not iterable .

Answer 1

If you have not duplicate elements within mylist you can use set.difference method to get the difference between the custom list with mylist then sort and append it to your custom list :

>>> l=['id', 'name', 'postcode', 'is_local']
>>> l+sorted(set(mylist).difference(l))
['id', 'name', 'postcode', 'is_local', 'actual_cost_123', 'actual_cost_456', 'quantity_123', 'quantity_456', 'total_items_123', 'total_items_456']
>>> 

Else you can use a list comprehension :

>>> l+sorted([i for i in mylist if not i in l])
['id', 'name', 'postcode', 'is_local', 'actual_cost_123', 'actual_cost_456', 'quantity_123', 'quantity_456', 'total_items_123', 'total_items_456']
>>>