dict order is not kept in the same way as in CPython

Hello,

dict order after an update is inconsistent with CPython. Here is the result I have with CPython:

In [2]: a={1:1, 2:2}                                                                                                                                                                                                                                         

In [3]: a                                                                                                                                                                                                                                                    
Out[3]: {1: 1, 2: 2}

In [4]: a.update({1:3})                                                                                                                                                                                                                                      

In [5]: a                                                                                                                                                                                                                                                    
Out[5]: {1: 3, 2: 2}

In [6]: a.update({4:4})                                                                                                                                                                                                                                      

In [7]: a                                                                                                                                                                                                                                                    
Out[7]: {1: 3, 2: 2, 4: 4}

The same thing with Brython:

Brython 3.8.9 on Netscape 5.0 (X11)
>>> a = {1:1, 2:2}
>>> a
{1: 1, 2: 2}
>>> a.update({1:3})
>>> a
{2: 2, 1: 3}
>>> a.update({4:4})
>>> a
{2: 2, 1: 3, 4: 4}
>>> 

As you can see, the updating of key 1 is moving the key to the end, while is should keep its original location (first).

[PierreQuentel]

Thanks for the bug report, it was the same with string keys.

By the way, making what was a CPython implementation detail a language feature forced us, poor maintainers of alternative implementations, to rewrite the whole code for dictionaries 😦

[goffi-contrib]

This StackOverflow link is outdated, it predates Python 3.6

I don’t think there is any PEP for that, it’s a side effect of dict optimisation for Python 3.6, made official in Python 3.7, you can check changelog of Python 3.7 for that:

the insertion-order preservation nature of dict objects has been declared to be an official part of the Python language spec.

cf. Guido message at https://mail.python.org/pipermail/python-dev/2017-December/151283.html

It’s also clear in the language reference: https://docs.python.org/3/reference/datamodel.html#index-30

Dictionaries preserve insertion order, meaning that keys will be produced in the same order they were added sequentially over the dictionary. Replacing an existing key does not change the order, however removing a key and re-inserting it will add it to the end instead of keeping its old place.

(the emphasis is from me, as it is the topic of this Brython bug)