call pytest with python3 -m
This runs pytest as
```sh
python3 -m pytest  # ...
```
This allows running the tests in a venv. (Otherwise, if pytest is not installed in the venv, it falls back to the system pytest, leading to some surprises down the road.)
Hi @nschloe ,

> if pytest is not installed in the venv

why would it not be installed in the venv? If someone has followed the contributing guide, then it should be there > If someone has followed the contributing guide, then it should be there

Indeed. It's just a safety net for "anyone" :smile: who missed an installation step. Did this happen to you?

If they miss this installation step, then they will also miss all the other dev dependencies > Did this happen to you?

Indeed. I thought I contribute this as it hits you on the head if you made a mistake, but feel free to close the PR if you don't think it's useful. > Indeed. I thought I contribute this as it hits you on the head if you made a mistake

I'd just like to understand how it happened - which set of instructions from [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_environment.html) did you follow, and which step did you skip? If you start and Python repl and write `import pandas`, does it work fine? > I'd just like to understand how it happened - which set of instructions from here did you follow, and which step did you skip?

[This one](https://pandas.pydata.org/pandas-docs/dev/development/contributing_environment.html#creating-a-python-environment-pip), and i skipped installing `requirements-dev.txt`.

> If you start and Python repl and write import pandas, does it work fine?

I don't know what "repl" means, but yeah, it imports fine. Ah OK, thanks! Then you'll also be missing all the other dev dependencies, and this script will still not work even with `python -m`. If you install `requirements-dev.txt`, does this script work as it is? If so, I'd suggest keeping it as is Sure.