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> and I can't explain exactly how their ordered dicts work Traditionally you simply use a doubly linked list approach on the entries (each entry maintains two

> and I can't explain exactly how their ordered dicts work Traditionally you simply use a doubly linked list approach on the entries (each entry maintains two additional references to the previous and next entry) for that like LinkedHashMap: https://docs.oracle.com/javase//8/docs/api/java/util/LinkedHashMap.html https://docs.oracle.com/javase//8/docs/api/java/util/LinkedH... https://github.com/openjdk-mirror/jdk7u-jdk/blob/master/src/share/classes/java/util/LinkedHashMap.java#L318 https://github.com/openjdk-mirror/jdk7u-jdk/blob/master/src/... Which is also what Python seems to be doing: https://stackoverflow.com/a/34496644 https://stackoverflow.com/a/34496644 It's fairly intuitive. Do their new default (now also ordered?) dics do this differently?

kstrauser 2y ago
Note that OrderedDict is an implementation in Python. CPython's dict has a different implementation. There's more about it at https://docs.python.org/3.6/whatsnew/3.6.html#new-dict-implementation https://docs.python.org/3.6/whatsnew/3.6.html#new-dict-imple... and https://mail.python.org/pipermail/python-dev/2012-December/123028.html https://mail.python.org/pipermail/python-dev/2012-December/1... .
- quelltext 2y ago
  This implementation was used from 3.6, right? It's interesting that the idea mail mentions that nothing changes about the implementation (including order) but the memory layout. Which would imply insertion order was already preserved in older versions (not the case IIRC) or the idea underwent a few more changes that did in fact impact order. EDIT: I couldn't quite find an answer but https://softwaremaniacs.org/blog/2020/02/05/dicts-ordered/ https://softwaremaniacs.org/blog/2020/02/05/dicts-ordered/ mentions the behavior happens since then because the implementation only tracks indices in the hash table itself and relies on maintaining entries separately in a second array that gets expanded in insertion order. This would also seem straightforward but it raises a few questions such as how deletion is implemented (efficiently). EDIT2: Okay, the talk (https://youtu.be/p33CVV29OG8 https://youtu.be/p33CVV29OG8) mentions they just leave holes until the next resize (at around 42:00). Raymond also mentions there that his original idea didn't preserve ordering but happened due to an additional compacting optimization? Should probably watch the whole thing some time to get the history. Sounds like a fun talk.
  - kstrauser 2y ago
    Oh! Yeah, that was the talk, but repeated at PyCon. It’s a very clever design that wasn’t at all obvious to me.

kstrauser 2y ago

Note that OrderedDict is an implementation in Python. CPython's dict has a different implementation. There's more about it at https://docs.python.org/3.6/whatsnew/3.6.html#new-dict-implementation https://docs.python.org/3.6/whatsnew/3.6.html#new-dict-imple... and https://mail.python.org/pipermail/python-dev/2012-December/123028.html https://mail.python.org/pipermail/python-dev/2012-December/1... .

quelltext 2y ago
This implementation was used from 3.6, right? It's interesting that the idea mail mentions that nothing changes about the implementation (including order) but the memory layout. Which would imply insertion order was already preserved in older versions (not the case IIRC) or the idea underwent a few more changes that did in fact impact order. EDIT: I couldn't quite find an answer but https://softwaremaniacs.org/blog/2020/02/05/dicts-ordered/ https://softwaremaniacs.org/blog/2020/02/05/dicts-ordered/ mentions the behavior happens since then because the implementation only tracks indices in the hash table itself and relies on maintaining entries separately in a second array that gets expanded in insertion order. This would also seem straightforward but it raises a few questions such as how deletion is implemented (efficiently). EDIT2: Okay, the talk (https://youtu.be/p33CVV29OG8 https://youtu.be/p33CVV29OG8) mentions they just leave holes until the next resize (at around 42:00). Raymond also mentions there that his original idea didn't preserve ordering but happened due to an additional compacting optimization? Should probably watch the whole thing some time to get the history. Sounds like a fun talk.
- kstrauser 2y ago
  Oh! Yeah, that was the talk, but repeated at PyCon. It’s a very clever design that wasn’t at all obvious to me.