# FAQ¶

Frequently Asked Questions about pyhf and its use.

## Questions¶

### Where can I ask questions about pyhf use?¶

If you have a question about the use of pyhf not covered in the documentation, please ask a question on Stack Overflow with the [pyhf] tag, which the pyhf dev team watches.

If you believe you have found a bug in pyhf, please report it in the GitHub Issues.

### Is it possible to set the backend from the CLI?¶

Yes. Use the --backend option for pyhf cls to specify a tensor backend. The default backend is NumPy. For more information see pyhf cls --help.

### Does pyhf support Python 2?¶

No. Like the rest of the Python community, as of January 2020 the latest releases of pyhf no longer support Python 2. The last release of pyhf that was compatible with Python 2.7 is v0.3.4, which can be installed with

python -m pip install pyhf~=0.3


### I only have access to Python 2. How can I use pyhf?¶

It is recommended that pyhf is used as a standalone step in any analysis, and its environment need not be the same as the rest of the analysis. As Python 2 is not supported it is suggested that you setup a Python 3 runtime on whatever machine you’re using. If you’re using a cluster, talk with your system administrators to get their help in doing so. If you are unable to get a Python 3 runtime, versioned Docker images of pyhf are distributed through Docker Hub.

Once you have Python 3 installed, see the Installation page to get started.

### I validated my workspace by comparing pyhf and HistFactory, and while the expected CLs matches, the observed CLs is different. Why is this?¶

Make sure you’re using the right test statistic ($$q$$ or $$\tilde{q}$$) in both situations. In HistFactory, the asymptotics calculator, for example, will do something more involved for the observed CLs if you choose a different test statistic.

### I ran validation to compare HistFitter and pyhf, but they don’t match exactly. Why not?¶

pyhf is validated against HistFactory. HistFitter makes some particular implementation choices that pyhf doesn’t reproduce. Instead of trying to compare pyhf and HistFitter you should try to validate them both against HistFactory.

### How is pyhf typeset?¶

As you may have guessed from this page, pyhf is typeset in all lowercase. This is largely historical, as the core developers had just always typed it that way and it seemed a bit too short of a library name to write as PyHF. When typesetting in LaTeX the developers recommend introducing the command

\newcommand{\pyhf}{\texttt{pyhf}}


If the journal you are publishing in requires you to use textsc for software names it is okay to instead use

\newcommand{\pyhf}{\textsc{pyhf}}


## Troubleshooting¶

• import torch or import pyhf causes a Segmentation fault (core dumped)

This is may be the result of a conflict with the NVIDIA drivers that you have installed on your machine. Try uninstalling and completely removing all of them from your machine

# On Ubuntu/Debian
sudo apt-get purge nvidia*