Qobj: Deprecate Qobj @ numpy.array by mudit06mah · Pull Request #2829 · qutip/qutip

mudit06mah · 2026-03-05T12:26:45Z

Description
This PR deprecates Qobj @ numpy.array operation.

Related issues or PRs
fixes #2547

Steps to test
Code:

import qutip
import numpy as np

A = qutip.Qobj([[0, 1], [1, 0]])
C = np.array([[0, 1], [1, 0]])

print(A @ C)

Output:

/home/mochi/Projects/qutip/temp.py:7: DeprecationWarning: Support for Qobj @ numpy.array has been deprecated and will be removed in qutip 5.4. Please use Qobj(A) @ B instead.
  print(A @ C)
Quantum object: dims=[[2], [2]], shape=(2, 2), type='oper', dtype=Dense, isherm=True
Qobj data =
[[1. 0.]
 [0. 1.]]

pmenczel

Thank you for the contribution. Other than the comment below, this seems like a good addition. (@Ericgig is there any reason not to add this / why it hasn't been added yet?)

Note also, the changelog file should not have the ".rst" ending; that's why the documentation build is failing.

pmenczel · 2026-03-06T02:57:07Z

        )

+    def __rmatmul__(self, other) -> Qobj:
+        if not isinstance(other, Qobj):


This condition should always be true: if other were a Qobj, then it would have tried other.__matmul__(self) first. I would suggest to just remove the condition.

Ericgig · 2026-03-06T18:08:17Z

Do we want to encourage / officially support operations between Qobj and ndarray?

If we do, what about Qobj + ndarray, Qobj - ndarray, Qobj & ndarray, etc.
Also tests should be added for all new officially supported operations...

If not, should we remove it from __matmul__?

pmenczel · 2026-03-10T09:33:14Z

Do we want to encourage / officially support operations between Qobj and ndarray?

It's an a bit difficult question. In principle, I would be for removing it altogether, but I don't know if that could be a breaking change for users. However, having it only for the one order of operands is definitely weird. @Ericgig what do you think is best?

(And sorry @mudit06mah that #2547 was maybe somewhat misleading for an open issue.)

mudit06mah · 2026-03-10T10:25:38Z

(And sorry @mudit06mah that #2547 was maybe somewhat misleading for an open issue.)

No worries, Since there is still need for changes (whether to add support or remove it completely) so I don't think it was invalid to keep it open.
Once a decision has been made I'll make changes accordingly 👾

mudit06mah · 2026-03-17T11:07:22Z

@Ericgig @pmenczel Any Updates on this?

Ericgig · 2026-03-17T14:08:52Z

Personally I am for not supporting any operations with numpy array with the present Qobj.

But for this kind of design decision, it will be better to discuss with the other qutip developers, we will meet in a few weeks.

veronikakurth · 2026-04-07T02:07:14Z

@@ -0,0 +1 @@
+Added a __rmatmul__() function to Qobj class to successfully handle numpy.ndarray @ Qobj operations


Would it be considered a bug fix or a feature, though?

veronikakurth · 2026-04-08T04:37:38Z

Personally I am for not supporting any operations with numpy array with the present Qobj.

But for this kind of design decision, it will be better to discuss with the other qutip developers, we will meet in a few weeks.

I added a new label (core-design-review) to this PR and a related issue for us to track such topics that require a core maintainers discussion before the actual work starts or changes get merged. So that we don't lose them out of sight.

hodgestar · 2026-04-14T05:05:47Z

@mudit06mah: Thank you for raising this issue. As mentioned in #2547 we decided to deprecate support for numpy arrays in @ and ask users to do Qobj(A) @ B if A is a numpy array.

You're welcome to make a PR for the deprecation if you like, otherwise one of the admin team will get to it.

mudit06mah · 2026-04-14T05:54:17Z

You're welcome to make a PR for the deprecation if you like, otherwise one of the admin team will get to it.

Sure, I'd change this PR only to deprecate it instead :)

mudit06mah · 2026-04-16T02:46:38Z

@hodgestar @Ericgig @pmenczel @veronikakurth I have changed this PR to deprecate the operation instead :D PTAL

veronikakurth · 2026-04-17T02:13:29Z

+        if isinstance(other, np.ndarray):
+            warnings.warn(
+                "Support for Qobj @ numpy.array has been deprecated "
+                "and will be removed in qutip 5.4. "


It might be a bit of a short timeline for deprecation; @Ericgig would you say it would be better to communicate that the support will be removed in "qutip 5.5 or later"?

I have changed the warning message, Could you please review it again?

Ericgig · 2026-04-20T13:10:52Z

+                "Support for Qobj @ numpy.array has been deprecated "
+                "and will be removed in qutip 5.5 or later. "
+                "Please use Qobj(A) @ B instead.",
+                DeprecationWarning,


It's better to use FutureWarning, DeprecationWarning are hidden per default so the average user will miss them.

I see, I changed the stack level to 2 to make the DeprecationWarning visible earlier. I have changed it to FutureWarning now.

pmenczel reviewed Mar 6, 2026

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mudit06mah requested a review from pmenczel March 8, 2026 19:30

veronikakurth reviewed Apr 7, 2026

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veronikakurth added the core-design-review Affects core QuTiP design and needs prior maintainer discussion before work or merge label Apr 8, 2026

Qobj: Deprecate Qobj @ numpy.array

4eda31b

mudit06mah force-pushed the fix/__rmatmul()__ branch from 5f7bd08 to 4eda31b Compare April 15, 2026 19:30

mudit06mah changed the title ~~fix: qobj: Add __rmatmul__ function for consistency~~ Qobj: Deprecate Qobj @ numpy.array Apr 15, 2026

Fix failing test cases

b0482b1

Fix warning

7a80aae

veronikakurth reviewed Apr 17, 2026

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Change warning message

4be3bea

Ericgig reviewed Apr 20, 2026

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Change warning type

0735c4f

Ericgig approved these changes Apr 20, 2026

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Ericgig added this to the QuTiP 5.3 milestone Apr 20, 2026

pmenczel approved these changes Apr 21, 2026

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Ericgig merged commit f020adb into qutip:master Apr 22, 2026
16 of 17 checks passed

		@@ -0,0 +1 @@
		Added a __rmatmul__() function to Qobj class to successfully handle numpy.ndarray @ Qobj operations

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mudit06mah commented Mar 5, 2026 •

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