Python OrderedDict iteration

Question

Why does my python OrderedDict get initialized 'out of order'?

The solution here is less intriguing than the explanation. There's something here I just don't get, and perhaps an expanation would help others as well as me.

>>> from collections import OrderedDict  >>> spam = OrderedDict(s = (1, 2), p = (3, 4), a = (5, 6), m = (7, 8))  >>> spam OrderedDict([('a', (5, 6)), ('p', (3, 4)), ('s', (1, 2)), ('m', (7, 8))])  >>> for key in spam.keys(): ...    print key     ... #  this is 'ordered' but not the order I wanted.... a p s m  # I was expecting (and wanting): s p a m 

Answer 1

From the docs:

The OrderedDict constructor and update() method both accept keyword arguments, but their order is lost because Python’s function call semantics pass-in keyword arguments using a regular unordered dictionary.

So initialization loses the ordering, because it's basically calling a constructor with **kwargs.

Edit: In terms of a solution (not just an explanation)—as pointed out in a comment by the OP, passing in a single list of tuples will work:

>>> from collections import OrderedDict >>> spam = OrderedDict([('s',(1,2)),('p',(3,4)),('a',(5,6)),('m',(7,8))]) >>> for key in spam: ...     print(key) ... s p a m >>> for key in spam.keys(): ...     print(key) ... s p a m 

This is because it's only getting a single argument, a list.