# KeyError: ('torch', 'linalg.eigh') when using PennyLane with PyTorch

**URL:** https://discuss.pennylane.ai/t/keyerror-torch-linalg-eigh-when-using-pennylane-with-pytorch/7624
**Category:** PennyLane Help
**Created:** [November 21, 2024, 9:35pm UTC](https://discuss.pennylane.ai/t/keyerror-torch-linalg-eigh-when-using-pennylane-with-pytorch/7624 "2024-11-21T21:35:28Z")
**Posts on this page:** 4
**Page:** 1

<div class="post-metadata">

### Author: ![Lazarus](https://avatars.discourse-cdn.com/v4/letter/l/4da419/32.png) [@Lazarus](https://discuss.pennylane.ai/u/Lazarus)
#### Post date: [November 21, 2024, 9:35pm UTC](https://discuss.pennylane.ai/t/keyerror-torch-linalg-eigh-when-using-pennylane-with-pytorch/7624/1 "2024-11-21T21:35:28Z")

</div>

**Description** :

I’m encountering an issue where importing PyTorch causes PennyLane to throw a `KeyError`. When I use PennyLane on its own, everything works fine, but after installing PyTorch, I receive an error when running a simple PennyLane script.

**Steps to Reproduce** :

1. **Create a conda environment and install packages** :

```auto
conda create --name pennylane_test python=3.9

conda activate pennylane_test

conda install pennylane

conda install pytorch torchvision lightning -c pytorch

```

Pennylane on his own will work fine, I tested this on a dummy pennylane python exemple as well as my own project. The problems start to arise after I installed pytorch…

1. **Create a Test Script (`test.py`)**:

```auto
import pennylane as qml

# Define a simple quantum device
dev = qml.device("default.qubit", wires=1)

@qml.qnode(dev)
def circuit():
    qml.PauliX(wires=0)
    return qml.expval(qml.PauliZ(0))

print(circuit())

```

1. **Run the Script** :

**Expected Behavior** :

The script should output `-1.0` without any errors.

**Actual Behavior** :

I receive the following error when running the script:

```auto
File " ______", line 1, in <module>
  import pennylane as qml
File " ______\pennylane\lib\site-packages\pennylane\__ init__.py", line 29, in <module>
  import pennylane.kernels
File " ______\pennylane\lib\site-packages\pennylane\kernels\__ init__.py", line 18, in <module>
  from .cost_functions import (
File " ______ \pennylane\lib\site-packages\pennylane\kernels\cost_functions.py", line 19, in <module>
File " ______\pennylane\lib\site-packages\pennylane\math\__ init__.py", line 36, in <module>
  from .multi_dispatch import (
File " ______ \pennylane\lib\site-packages\pennylane\math\multi_dispatch.py", line 24, in <module>
  from . import single_dispatch # pylint:disable=unused-import
File " ______ \pennylane\lib\site-packages\pennylane\math\single_dispatch.py", line 329, in <module>
  del ar.autoray._FUNCS["torch", "linalg.eigh"]
KeyError: ('torch', 'linalg.eigh')

```

**Additional Information** :

- **Operating System** : Windows 10
- **Python Version** : 3.9.20  
I tried different version of python: 3.9, 3.10, 3.12, 3.13
- **Conda Version** : 24.9.2
- **PennyLane Version** : 0.23.0
- **Pytorch Version** : 2.3.1

Is this a known issue with compatibility between PennyLane and PyTorch? Also, do you have any suggestions on how to resolve this issue?

**Thank you for your assistance!**

---

<div class="post-metadata">

### Author: ![CatalinaAlbornoz](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/catalinaalbornoz/32/1196_2.png) [@CatalinaAlbornoz](https://discuss.pennylane.ai/u/CatalinaAlbornoz)
#### Post date: [November 22, 2024, 9:00pm UTC](https://discuss.pennylane.ai/t/keyerror-torch-linalg-eigh-when-using-pennylane-with-pytorch/7624/2 "2024-11-22T21:00:07Z")

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Hi @Lazarus, welcome to the Forum!

I noticed a few things. I’ll add them as a list here for easier reference:

1. Please use Python 3.10-3.12 since these are the currently supported versions.
2. Please use the latest PennyLane version (v0.39 at the moment).
3. While we recommend creating a new virtual environment with venv, Conda, or Miniconda, we generally recommend to install all libraries with `pip` instead of `conda`. This is because Conda will sometimes mess up package installations and versions so `pip` is better for this.  
Eg. the code below should be enough

```auto
conda create --name pennylane_test python=3.10
conda activate pennylane_test
python -m pip install pennylane torch

```

1. I noticed you’re installing `lightning`. Note that the `lightning.qubit` device already comes pre-installed with PennyLane so you don’t need to install any additional library to use it.

Let me know if this solves your issues!

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<div class="post-metadata">

### Author: ![Lazarus](https://avatars.discourse-cdn.com/v4/letter/l/4da419/32.png) [@Lazarus](https://discuss.pennylane.ai/u/Lazarus)
#### Post date: [November 25, 2024, 11:07pm UTC](https://discuss.pennylane.ai/t/keyerror-torch-linalg-eigh-when-using-pennylane-with-pytorch/7624/3 "2024-11-25T23:07:52Z")

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Hi Catalina,

The “lightning” I installed refers to PyTorch Lightning, not `lightning.qubit`. You can find more about it here: [PyTorch Lightning](https://lightning.ai/docs/pytorch/stable/starter/introduction.html).

Thanks for your response and suggestions! I wanted to share my findings regarding the issue:

- It’s odd that Conda installed an older version of PennyLane [0.23]… I didn’t specify any version, so I assumed it would install the latest one. To investigate, I tried installing PennyLane on its own via Conda in a new environment, to see if the package manager was responsible for the downgrade. It still installed version 0.23. When I tried to force version 0.39, Conda couldn’t find it on `conda-forge` or any other channel I tried.
- Using pip instead worked perfectly. Pip installed the latest version (0.39), and everything is now working as expected.

Thanks again for your help!

---

<div class="post-metadata">

### Author: ![CatalinaAlbornoz](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/catalinaalbornoz/32/1196_2.png) [@CatalinaAlbornoz](https://discuss.pennylane.ai/u/CatalinaAlbornoz)
#### Post date: [November 26, 2024, 2:17am UTC](https://discuss.pennylane.ai/t/keyerror-torch-linalg-eigh-when-using-pennylane-with-pytorch/7624/4 "2024-11-26T02:17:43Z")

</div>

Thanks for sharing your findings @Lazarus !

Let us know if you have any other questions.