Integration with qtpy

[rth]

The support of multiple PyQt / PySide backends is a recurrent problem for any project working with Qt in Python. In pytest-qt it is solved by providing a compatibility _QtApi interface in qt_compat.py (that is not in the public interface). At the same time there are packages that specialize in providing such a compatibility interface, namely spyder-ide/qtpy and mottosso/Qt.py. For project that use one of those packages (e.g. matplotlib is considering it), and wanting to test it with with pytest-qt it would mean that they would not, technically be testing the project in the same conditions (and it would also mean setting differently named environment variables twice PYTEST_QT_API and QT_API or QT_PREFERRED_BINDING.)

I think the integration could be straightforward: e.g. to integrate with qtpy, the following should almost work,

At the end of pytestqt/qt_compat.py

try:
    if 'QT_API' not in os.environ and 'PYTEST_QT_API' in os.environ:
        os.environ['QT_API'] = os.environ['PYTEST_QT_API']
    import qtpy
    qt_api = qtpy
    qt_api.pytest_qt_api = qtpy.API
except ImportError:
     # fallback to the built-in compatibility layer..
    qt_api = _QtApi()

And maybe one extra environment in TravisCI for testing..

What do you think?

[nicoddemus]

Hi @rth,

I wouldn't mind ditching the current API layer if qtpy fits the bill, it would mean less code to maintain.

Does qtpy support lazy loading?

My preference would be to completely remove the compatibility API and use qtpy directly, if we can maintain backward compatibility with PYTEST_QT_API configuration and lazy loading.

[rth]

Does qtpy support lazy loading?

Aww right, that's another challenge. No qtpy doesn't and expect explicit imports e.g. import qtpy.QtCore. Qt.py (!=qtpy the names are just not confusing) seems to import everything by default if I understood correctly. Maybe one could just wrap qtpy in something like this example mottosso/Qt.py#152 (comment) as a way around it?

Though this would add one extra layer of indirection for the wrapper around the wrapper around Qt :/

[The-Compiler]

There's already some discussion about this at #130 (comment) FWIW.

[machinekoder]

python_qt_bindings is another wrapper provided by the ROS project: http://wiki.ros.org/python_qt_binding

[nicoddemus]

Thanks for the pointer @machinekoder.

But as commented in #130 (comment) it is probably not worth to use a wrapper, the qt_compat module provides lazy loading and doesn't really add too much to maintenance because we don't need to import the entire API anyway, just a few symbols.