[Python-Dev] Guarantee ordered dict literals in v3.7? (original) (raw)

Paul G paul at ganssle.io
Sun Nov 5 13:57:19 EST 2017

Scientific applications want something like

{'a': 10, 'b': "foo", 'c': {'this': b'123'}} as an ordered initializer for unboxed or typed (or both) data. In general, if dicts are ordered, they can be used for example as initializers for (nested) C structs.

I can understand why you'd want an ordered container, I just don't see why it must be a dict. Why can't it be:

OrderedDict(a=10, b='foo', c=OrderedDict(this=b'123'))

Is it just that you don't want to type OrderedDict that many times? If it's so important to provide ordered dictionary literals, I would think it's a no-brainer to give them their own literal syntax (rather than re-defining dicts to have guaranteed order). e.g.:

o{'a': 10, 'b': 'foo', 'c': o{'this': b'123'}

Then there is no backwards incompatibility problem and users can express whether order does or does not matter to them when initializing a container.

On 11/05/2017 01:39 PM, Stefan Krah wrote:

On Sun, Nov 05, 2017 at 01:14:54PM -0500, Paul G wrote:

I'm not entirely sure I understand the full set of reasoning for this - I couldn't really tell what the problem with OrderedDict is from the link Stefan provided. It seems to me like a kind of huge change for the language to move from arbitrary-ordered to guaranteed-ordered dict. The problem I see is that this introduces a huge backwards compatibility burden on all implementations of Python.

2. Someone invents a new arbitrary-ordered container that would improve on the memory and/or CPU performance of the current dict implementation I would think this is very unlikely, given that the previous dict implementation has always been very fast. The new one is very fast, too. Stefan Krah

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