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pythondictionarykey
Behrouz Beheshti
Behrouz Beheshti

Reputation: 1143

How to change the order of keys in a Python 3.5 dictionary, using another list as a reference for keys?

I'm trying to change the order of 'keys' in a dictionary, with no success. This is my initial dictionary:

Not_Ordered={
  'item':'book',
  'pages':200, 
  'weight':1.0, 
  'price':25, 
  'city':'London'
}

Is there a chance for me to change the order according to a key-order list, like this:

key_order=['city', 'pages', 'item', 'weight', 'price']

note:

Upvotes: 10

Views: 24821

Answers (4)

Alejandro Lorefice
Alejandro Lorefice

Reputation: 889

Your problem here is that prior to Python 3.7 dictionaries did not have an insertion order so you could not "order" them. As you can see here: Official Python Docs on Dictionaries. If you provide more insight on whats the purpose of this we might try to solve it in other way.

Upvotes: 2

Chang Ye
Chang Ye

Reputation: 1215

no solution in Python 3.5

for python >= 3.6, just

Ordered_Dict = {k : Not_Ordered_Dict[k] for k in key_order}

Upvotes: 10

0x2bad
0x2bad

Reputation: 318

you still can use sorted and specify the items().

ordered_dict_items = sorted(Not_Ordered.items(), key=lambda x: key_order.index(x[0]))  
return dict((x, y) for x, y in ordered_dict_items) # to convert tuples to dict

or directly using lists:

ordered_dict_items = [(k, Not_Ordered[k]) for k in key_order]
return dict((x, y) for x, y in ordered_dict_items) # to convert tuples to

Upvotes: 2

Tim Peters
Tim Peters

Reputation: 70582

Dicts are "officially" maintained in insertion order starting in 3.7. They were so ordered in 3.6, but it wasn't guaranteed before 3.7. Before 3.6, there is nothing you can do to affect the order in which keys appear.

But OrderedDict can be used instead. I don't understand your "but it gives me a list" objection - I can't see any sense in which that's actually true.

Your example:

>>> from collections import OrderedDict
>>> d = OrderedDict([('item', 'book'), ('pages', 200),
...                  ('weight', 1.0), ('price', 25),
...                  ('city', 'London')])
>>> d # keeps the insertion order
OrderedDict([('item', 'book'), ('pages', 200), ('weight', 1.0), ('price', 25), ('city', 'London')])
>>> key_order= ['city', 'pages', 'item', 'weight', 'price'] # the order you want
>>> for k in key_order: # a loop to force the order you want
...     d.move_to_end(k)
>>> d # which works fine
OrderedDict([('city', 'London'), ('pages', 200), ('item', 'book'), ('weight', 1.0), ('price', 25)])

Don't be confused by the output format! d is displayed as a list of pairs, passed to an OrderedDict constructor, for clarity. d isn't itself a list.

Upvotes: 11

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