Adding number to ket unexpectedly modifies ket data

[vikramkashyap]

Bug Description

Adding (or subtracting) a number from a ket modifies the data of the ket even without an assignment. In the code below, I would expect that a is not modified when the expression a+1 is run, but it is. That is to say, a+1 has the same effect as a+=1. In contrast, the data of the QObj does not change when adding a number to an operator or when multiplying a ket by a number.

Code to Reproduce the Bug

from qutip import basis
a = basis(2)
print(a)
print()
print(a+1)
print()
print(a)

Code Output

Quantum object: dims = [[2], [1]], shape = (2, 1), type = ket
Qobj data =
[[1.]
 [0.]]

Quantum object: dims = [[2], [1]], shape = (2, 1), type = ket
Qobj data =
[[2.]
 [0.]]

Quantum object: dims = [[2], [1]], shape = (2, 1), type = ket
Qobj data =
[[2.]
 [0.]]

Expected Behaviour

First of all, adding a number to a ket is not a well-defined operation, so perhaps it should throw an error. However, I think that the value of a+1 is not entirely unexpected, so it could be kept. The real issue is that the data of a is modified without the code ever explicitly reassigning a. This is definitely unexpected. I would expect the output to be:

`Quantum object: dims = [[2], [1]], shape = (2, 1), type = ket
Qobj data =
[[1.]
[0.]]

Quantum object: dims = [[2], [1]], shape = (2, 1), type = ket
Qobj data =
[[2.]
[0.]]

Quantum object: dims = [[2], [1]], shape = (2, 1), type = ket
Qobj data =
[[1.]
[0.]]
`

Your Environment

QuTiP Version:      4.7.2
Numpy Version:      1.23.5
Scipy Version:      1.10.0
Cython Version:     0.29.33
Matplotlib Version: 3.6.2
Python Version:     3.10.9
Number of CPUs:     4
BLAS Info:          OPENBLAS
OPENMP Installed:   False

Additional Context

No response

[Ericgig]

Thank you for reporting.

I will fix by having ket + scalar raising an error.

[AGaliciaMartinez]

Closing the issue, it was solved in #2209. Thanks for reporting the issue @vikramkashyap and for solving it @Ericgig !