ak.cartesian(..., nested=True) results in behaviour expected from nested=False

[danielhundhausen]

Version of Awkward Array

2.5.1

Description and code to reproduce

In awkward 2 the behaviour from ak.cartesian is unexpected and deviant from behaviour in v1.

>>> one = ak.zip({"pt": [[1, 2], [1]], "eta": [[1.1, 2.1], [1]]})
>>> two = ak.zip({"pt": [[2.1], [2.21, 2.22]], "eta": [[2.11], [2.221, 2.222]]})
>>> ak.cartesian({"foo": one, "bar": two}, nested=False)["foo"].show()
[[{pt: 1, eta: 1.1}, {pt: 2, eta: 2.1}],
 [{pt: 1, eta: 1}, {pt: 1, eta: 1}]]
ak.cartesian({"foo": one, "bar": two}, nested=True)["foo"].show()
[[{pt: 1, eta: 1.1}, {pt: 2, eta: 2.1}],
 [{pt: 1, eta: 1}, {pt: 1, eta: 1}]]

In v1 (1.9.0) the behaviour was:

>>> for x in ak.cartesian({"foo": one, "bar": two}, nested=True)["foo"]:
>>>     print(x)
[[{pt: 1, eta: 1.1}], [{pt: 2, eta: 2.1}]]
[[{pt: 1, eta: 1}, {pt: 1, eta: 1}]]
>>> for x in ak.cartesian({"foo": one, "bar": two}, nested=False)["foo"]:
>>>     print(x)
[{pt: 1, eta: 1.1}, {pt: 2, eta: 2.1}]
[{pt: 1, eta: 1}, {pt: 1, eta: 1}]

Am I misinterpreting what ak.cartesian(..., nested=True) should do or is this a bug?

Thanks for considering this comment!

[agoose77]

This behaviour has changed in awkward 2.x, but not intentionally; it does behave as described in 2.1.0. This code was recently refactored, I'll take a look and see what was dropped!