API: allow dependent assignment?

Since kwarg order will be guaranteed in python 3.6, we could allow this…though would only want to try with a 3.6+ version check, otherwise code could more or less randomly work/not work, depending on dict order.

In [44]: df = pd.DataFrame({'a': [1,2,3]})

In [45]: df.assign(b=1, c=lambda x: x['b'] * 2)

[TomAugspurger]

I think we should sort this out for 0.21. I propose

• No change to 2.7 - 3.5; order is sorted by key
• For 3.6+, we use the original order the user passed in
• We don’t allow dependent assignment yet

Maybe in the future, when there’s less 3.5 and lower code around, we can allow for dependent assignment.

[shoyer]

I would also be comfortable just changing this for Python 3.6+. My guess is that there aren’t that many folks using Python 3.6 in production yet, so this will probably cause minimal issues.