Repository: qutip/qutip (python)

Issue:
Strange behavior with list-based time-dependent Hamilitonian and mesolve
### Bug Description

When I add a zero matrix with time dependence (even if the time dependence is constant) my Hamiltonian, it changes the way mesolve behaves.
Am I misunderstanding the way H is constructed?

### Code to Reproduce the Bug

```shell
from qutip import *

N = 4
a = destroy(N)
psi0 = fock(N, 0)
omega = 1
kappa = 1

def coeff_1(t, args): 
    return 1

def coeff_2(t, args):
    return 1

H0 = omega * a.dag() * a
H1 = a.dag() + a
H2 = 0 * (a.dag() + a)

coeff_test1 = coefficient(coeff_1)
coeff_test2 = coefficient(coeff_2)

H = QobjEvo([H0, [H1, coeff_test1], H2])
H_prime = QobjEvo([H0, [H1, coeff_test1], [H2, coeff_test2]])

times = np.linspace(0, 1, 1000)

result1 = mesolve(H, psi0, times, [kappa * a], [], args={})
result2 = mesolve(H_prime, psi0, times, [kappa * a], [], args={})

n1 = expect(a.dag() * a, result1.states[-1])
n2 = expect(a.dag() * a, result2.states[-1])

print('diff: ' + str(n1 - n2))
assert n1 == n2
```

### Code Output

```shell
diff: -1.2532650784482082
Traceback (most recent call last):
  File "/test.py", line 50, in <module>
    assert n1 == n2
           ^^^^^^^^
AssertionError
```

### Expected Behaviour

Adding time-dependence to a zero component of H shouldn't change anything.

### Your Environment

```shell
QuTiP Version:      5.0.2
Numpy Version:      1.26.4
Scipy Version:      1.13.1
Cython Version:     None
Matplotlib Version: 3.9.0
Python Version:     3.12.4
Number of CPUs:     12
BLAS Info:          Generic
INTEL MKL Ext:      False
Platform Info:      Darwin (arm64)
Installation path:  /Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/site-packages/qutip
```

### Additional Context

_No response_

Pull request description:
Fix isequal_dia for unequal number of diags
**Description**
Fix a bug in `isequal_dia` for case where the number of diagonals doesn't match.

**Related issues or PRs**
Fix #2669

Patch:
diff --git a/doc/changes/2669.bugfix b/doc/changes/2669.bugfix
new file mode 100644
index 000000000..ced3b1adc
--- /dev/null
+++ b/doc/changes/2669.bugfix
@@ -0,0 +1,1 @@
+Fix equality operators for diagonal format Qobj.
diff --git a/qutip/core/data/properties.pyx b/qutip/core/data/properties.pyx
index 2ab25ca54..e3a82637e 100644
--- a/qutip/core/data/properties.pyx
+++ b/qutip/core/data/properties.pyx
@@ -371,6 +371,18 @@ cpdef bint isequal_dia(Dia A, Dia B, double atol=-1, double rtol=-1):
                         return False
                 ptr_b += size
                 diag_b += 1
+        while diag_a < A.num_diag:
+            for i in range(size):
+                if not _feq(0., ptr_a[i], atol, rtol):
+                    return False
+            ptr_a += size
+            diag_a += 1
+        while diag_b < B.num_diag:
+            for i in range(size):
+                if not _feq(0., ptr_b[i], atol, rtol):
+                    return False
+            ptr_b += size
+            diag_b += 1
     return True
 
 
diff --git a/qutip/core/numpy_backend.py b/qutip/core/numpy_backend.py
index ad8516c52..39fa1d094 100644
--- a/qutip/core/numpy_backend.py
+++ b/qutip/core/numpy_backend.py
@@ -2,46 +2,8 @@ from ..settings import settings
 
 
 class NumpyBackend:
-    __slots__ = (
-        "_qt_np",
-        "abs",
-        "angle",
-        "arccos",
-        "array",
-        "ceil",
-        "conj",
-        "cos",
-        "cumsum",
-        "diff",
-        "e",
-        "histogram",
-        "imag",
-        "inf",
-        "inner",
-        "linalg",
-        "log",
-        "log2",
-        "max",
-        "maximum",
-        "ones",
-        "pi",
-        "prod",
-        "real",
-        "searchsorted",
-        "sin",
-        "sort",
-        "sqrt",
-        "sum",
-        "where",
-        "zeros",
-        "zeros_like",
-    )
-
     def _qutip_setting_backend(self, np):
         self._qt_np = np
-        for slot in self.__slots__:
-            if slot != "_qt_np":
-                setattr(self, slot, getattr(np, slot))
 
     def __getattr__(self, name):
         return getattr(self._qt_np, name)


Test patch:
diff --git a/qutip/tests/core/data/test_properties.py b/qutip/tests/core/data/test_properties.py
index e90b23a4c..4999779b4 100644
--- a/qutip/tests/core/data/test_properties.py
+++ b/qutip/tests/core/data/test_properties.py
@@ -1,6 +1,7 @@
 import numpy as np
 import pytest
 
+import qutip
 from qutip import data as _data
 from qutip import CoreOptions
 from . import conftest
@@ -243,3 +244,13 @@ class TestIsEqual:
         A = conftest.random_diag(shape, 0.5, False)
         B = clean_dia(A)
         assert _data.isequal(A, B)
+
+    def test_dia_mismatch_structure(self):
+        true_zeros = qutip.qzero(5, dtype="dia")
+        diag_of_zeros = qutip.qdiags([0]*5)
+        id = qutip.qeye(5, dtype="dia")
+        assert true_zeros == diag_of_zeros
+        assert id != diag_of_zeros
+        assert id != true_zeros
+        assert qutip.destroy(3) + qutip.create(3) != qutip.destroy(3)
+        assert qutip.destroy(3) + qutip.create(3) != qutip.create(3)
diff --git a/qutip/tests/core/test_options.py b/qutip/tests/core/test_options.py
index 8d1a26072..2b1a1eb13 100644
--- a/qutip/tests/core/test_options.py
+++ b/qutip/tests/core/test_options.py
@@ -1,21 +1,14 @@
 import pytest
 import numpy
+from unittest.mock import Mock
 
 import qutip
 from qutip.core.numpy_backend import np
 from qutip import CoreOptions, settings
 
-
 # Mocking JAX to demonstrate backend switching
-class Mock_JAX:
-    def sum(*a, **kw):
-        return "jax_sum"
-
-    def __getattr__(self, _):
-        return "other"
-
-
-mock_jax = Mock_JAX()
+mock_jax = Mock
+mock_jax.sum = Mock(return_value="jax_sum")
 mock_np = numpy