Error when creating qnode with backprop and finite-shots #1588

albi3ro · 2021-08-24T15:30:48Z

Fixes Issue #1481

Right now, you can create a qnode that uses finite shots and backpropagation. This is fine on qnode execution on the forward pass, but will always fail and raise some sort of unhelpful error message of the backward pass/ gradient computation.

For example:

dev = qml.device('default.qubit.autograd', shots=10, wires=1)

@qml.qnode(dev, interface='autograd', diff_method='backprop')
def circ(x):
    qml.RX(x, wires=0)
    return qml.expval(qml.PauliZ(0))

## works fine
circ(0.1)

# incomprehensible error
qml.grad(circ)(0.1)

For context, that error is:

ValueError: setting an array element with a sequence.

With this change, during QNode creation, a UserWarning is raise. I decided to use a warning instead of a Error, as users may still want to calculate finite shot forward passes on an interface device. This way, they at least have an explanation for the strange errors they will end up getting.

github-actions · 2021-08-24T15:31:06Z

Hello. You may have forgotten to update the changelog!
Please edit .github/CHANGELOG.md with:

A one-to-two sentence description of the change. You may include a small working example for new features.
A link back to this PR.
Your name (or GitHub username) in the contributors section.

antalszava · 2021-08-25T14:20:14Z

[ch8003]

…I/pennylane into backprop_finite_shots

codecov · 2021-08-26T11:04:39Z

Codecov Report

Merging #1588 (5f3eee2) into master (67a5daf) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1588   +/-   ##
=======================================
  Coverage   99.14%   99.14%           
=======================================
  Files         196      196           
  Lines       14245    14247    +2     
=======================================
+ Hits        14123    14125    +2     
  Misses        122      122

Impacted Files	Coverage Δ
pennylane/qnode.py	`98.69% <100.00%> (+<0.01%)`	⬆️

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antalszava

Hi @albi3ro, thanks! 💯 Had one major comment for a test case, the rest is looking good (minor suggestions).

tests/tape/test_qnode.py

antalszava · 2021-08-26T14:10:27Z

tests/tape/test_qnode.py

+        dev = qml.device("default.qubit", wires=1, shots=3)
+
+        with pytest.raises(qml.QuantumFunctionError, match="Devices with finite shots"):
+            QNode._validate_backprop_method(dev, "doesnt matter")


Wondering if it's worth inputting valid interface names here. Just by looking at the test case, the error might arise due to the invalid interface. Contrary to that, we'd like to make sure that we get an error with every valid interface. So maybe good to parameterize the test.

pennylane/qnode.py

.github/CHANGELOG.md

Co-authored-by: antalszava <antalszava@gmail.com>

antalszava · 2021-09-08T13:08:59Z

Hi @albi3ro what's the status on this?

I decided to use a warning instead of a Error, as users may still want to calculate finite shot forward passes on an interface device.

Could they simply just evaluate the QNode without trying to differentiate it? If so, maybe we could refer to that in the error message.

…I/pennylane into backprop_finite_shots

rmoyard

Great, I've left some comments. 💯 I think we should add a new test, because a deleted test that was previously covering some jax functionalities was deleted.

rmoyard · 2021-09-09T20:39:59Z

pennylane/qnode.py

@@ -342,6 +342,12 @@ def _validate_backprop_method(device, interface):
            qml.QuantumFunctionError: if the device does not support backpropagation, or the
            interface provided is not compatible with the device
        """
+        if device.shots is not None:


rmoyard · 2021-09-09T20:41:20Z

tests/tape/test_qnode.py

+
+        dev = qml.device("default.qubit", wires=1, shots=3)
+
+        with pytest.raises(qml.QuantumFunctionError, match="Devices with finite shots"):


Nice test 💯

rmoyard · 2021-09-09T20:49:11Z

tests/devices/test_default_qubit_tf.py

-            return qml.sample(qml.PauliZ(0))
-
-        a = tf.Variable(0.54)
+        with pytest.raises(qml.QuantumFunctionError, match="Devices with finite shots"):


Should we keep this test? Maybe changing the description to better fit the error message?

Since we test elsewhere that this error is raised, I'm deleting this test.

albi3ro · 2021-09-10T09:47:19Z

@rmoyard what do we need to add?

rmoyard · 2021-09-10T13:09:35Z

@albi3ro I think one the test using jax was deleted. default_qubit_jax.py line 269 more precisely we should add something covering this line.

tests/devices/test_default_qubit_jax.py

rmoyard

Great, it seems good to me 💯

bugfix: warn when backprop with finite-shots

a9b1802

albi3ro added the bug 🐛 Something isn't working label Aug 24, 2021

albi3ro requested a review from trbromley August 24, 2021 15:31

add changelog

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albi3ro mentioned this pull request Aug 24, 2021

Allow QNodes to support diff_method=None option #1274

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antalszava and others added 4 commits August 25, 2021 10:20

Merge branch 'master' into backprop_finite_shots

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Merge branch 'master' into backprop_finite_shots

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get tests to pass, black

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Merge branch 'backprop_finite_shots' of https://github.com/PennyLaneA…

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…I/pennylane into backprop_finite_shots

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albi3ro and others added 6 commits August 26, 2021 15:25

Apply suggestions from code review

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Co-authored-by: antalszava <antalszava@gmail.com>

test over interfaces

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Merge branch 'master' into backprop_finite_shots

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Merge branch 'master' into backprop_finite_shots

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Merge branch 'master' into backprop_finite_shots

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Merge branch 'master' into backprop_finite_shots

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albi3ro requested review from thisac and removed request for trbromley August 31, 2021 18:06

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albi3ro force-pushed the backprop_finite_shots branch from ac05674 to c6fc3aa Compare September 9, 2021 11:16

albi3ro and others added 3 commits September 9, 2021 12:19

Merge branch 'master' into backprop_finite_shots

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use diff_method=None in tests

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albi3ro and others added 3 commits September 9, 2021 18:41

Merge branch 'master' into backprop_finite_shots

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Merge branch 'backprop_finite_shots' of https://github.com/PennyLaneA…

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…I/pennylane into backprop_finite_shots

albi3ro requested a review from rmoyard September 9, 2021 18:00

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remove unnecessary test

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Update tests/devices/test_default_qubit_jax.py

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albi3ro requested a review from rmoyard September 10, 2021 13:38

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Merge branch 'master' into backprop_finite_shots

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albi3ro merged commit 4b82c83 into master Sep 10, 2021

albi3ro deleted the backprop_finite_shots branch September 10, 2021 15:48

antalszava mentioned this pull request Sep 15, 2021

Specify diff_method=None when using qml.sample with default.qubit.jax PennyLaneAI/qml#332

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antalszava mentioned this pull request Sep 28, 2021

Add notes to the docs about the unsupported measurements for devices PennyLaneAI/pennylane-sf#76

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albi3ro mentioned this pull request Jan 19, 2022

Raise error upon request of backpropagation on device with finite shots #2114

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Error when creating qnode with backprop and finite-shots #1588

Error when creating qnode with backprop and finite-shots #1588

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		dev = qml.device("default.qubit", wires=1, shots=3)

		with pytest.raises(qml.QuantumFunctionError, match="Devices with finite shots"):

Error when creating qnode with backprop and finite-shots #1588

Error when creating qnode with backprop and finite-shots #1588

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