Possible bug? dpdata.LabeledSystem.to() method unable to dump selected frame_idx

[qchempku2017]

Dear community,
I'm trying to dump selected frames read from vasp/outcar into deepmd/npy format for further training. The system has 63 frames, as follows:

Data Summary
Labeled System
Frame Numbers : 63
Atom Numbers : 104
Including Virials : Yes
Element List :
Li Fe P O
8 16 16 64

Now I want to dump only the last frame into deepmd format. However, when I followed these instructions, i.e., to add frame_idx=-1 kwarg in dpdata.LabeledSystem.to() method, dpdata still dumped all 63 frames:

data.to('deepmd/npy', 'testdump', frame_idx=-1); np.load("./testdump/set.000/coord.npy")

gave me:

array([[-1.57977326e-03, -2.34016510e-03, 1.79843686e-04, ..., 6.09007077e+00, 1.04698606e+01, 5.81606287e+00], [-1.91966367e-03, -2.85024358e-03, 2.09768799e-04, ..., 6.09167291e+00, 1.04744094e+01, 5.82075400e+00], [-2.59938316e-03, -3.86044725e-03, 2.89575864e-04, ..., 6.09487653e+00, 1.04835070e+01, 5.83013631e+00], ..., [-1.16243966e-01, -8.44958327e-02, -2.78206678e-02, ..., 6.15496586e+00, 1.03983518e+01, 5.97138997e+00], [-1.16474492e-01, -8.44157124e-02, -2.81099389e-02, ..., 6.15492763e+00, 1.03983904e+01, 5.97130743e+00], [-1.16826572e-01, -8.43043710e-02, -2.88392941e-02, ..., 6.15523225e+00, 1.03981569e+01, 5.97101773e+00]], shape=(63, 312))

while using command:
data.sub_system([-1]).to('deepmd/npy', 'testdump')

worked just the way I wanted:

np.load("./testdump/set.000/coord.npy").shape

(1, 312)

Since the original documentation did not explicitly tell the expected output of LabeledSystem.to(), I'm not sure if the behaviors I had can be defined as a bug. Can someone help explaining this issue? If it can be identified as a bug, I can create a formal issue to be fixed. Appreciated!

[njzjz]

frame_idx is not an argument of to. It's the argument for specific formats such as lammps/lmp as given in the example.

[qchempku2017]

I see. Appreciate that.