OrderedDict comprehensions

Question

Can I extend syntax in python for dict comprehensions for other dicts, like the OrderedDict in collections module or my own types which inherit from dict?

Just rebinding the dict name obviously doesn't work, the {key: value} comprehension syntax still gives you a plain old dict for comprehensions and literals.

>>> from collections import OrderedDict
>>> olddict, dict = dict, OrderedDict
>>> {i: i*i for i in range(3)}.__class__
<type 'dict'>

So, if it's possible how would I go about doing that? It's OK if it only works in CPython. For syntax I guess I would try it with a O{k: v} prefix like we have on the r'various' u'string' b'objects'.

note: Of course we can use a generator expression instead, but I'm more interested seeing how hackable python is in terms of the grammar.

Answer 1

Sorry, not possible. Dict literals and dict comprehensions map to the built-in dict type, in a way that's hardcoded at the C level. That can't be overridden.

You can use this as an alternative, though:

OrderedDict((i, i * i) for i in range(3))

Addendum: as of Python 3.6, all Python dictionaries are ordered. As of 3.7, it's even part of the language spec. If you're using those versions of Python, no need for OrderedDict: the dict comprehension will Just Work (TM).