More informative error message

[cbrnr]

My project MNELAB depends on qtpy because I would like to support both PyQt5 and PySide2. However, if no bindings are installed, the displayed error message could be more informative and tell users what to do (see e.g. the problem reported in cbrnr/mnelab#182). I've added explicit instructions how to install either PyQt5 or PySide2.

[cbrnr]

@goanpeca @altendky do you have time to review this minor change?

[altendky]

I have no authority here but... I would tend to avoid explicit commands. If people don't know to run pip to install stuff then they probably won't be remembering to use venv/bin/pip (you do correctly do everything in an env, right? :]) or --user or apt install python3-pyqt5 or... etc.

[cbrnr]

I would assume that people know how to pip install stuff, but currently the problem is that they don't know that a package is missing.

[altendky]

In which case they don't need to be given a command. I understand the interest in saying more than just 'not found'. Maybe something like This is usually caused by not having installed a compatible Qt wrapper. Maybe even include the list of compatible wrappers. (#217 happens to add a more explicit list (dict) of the supported wrappers) Or maybe link to https://github.com/spyder-ide/qtpy#requirements? Though then you can have a disconnect between the site and the installed version.

As a side note, I presently add extras_requires for the qt wrappers so people can install my packages like package[pyside2] to allow my package to specify whatever version restrictions etc it has for pyside2. Or package[pyqt5] for that option.

[cbrnr]

I agree, a command is not necessary, but I think the message should mention the wrappers explicitly to get people on the right track. Also, I would restrict the wrappers to PySide2 and PyQt5, because PyQt4 is old and deprecated.

Thanks for mentioning the extras_require thing, I haven't thought about that – that's a very good idea!

[cbrnr]

OK, I've removed the part about pip from the message.