The situation for Qt has changed several times in the last few years through the various acquisitions, so yeah, I might be a bit out of date because it doesn't really matter for PyQt itself -- that's just GPL or commercial.

Exactly. The LGPL (which Qt is licensed under) permits this. It also permits subclassing. What toyg wrote would be true if Qt were licensed under the GPL.

Qt is under the LGPL, but PyQt isn't. So if you want to develop Qt applications with python and PyQt you have to abide by the GPL, not just the LGPL, or buy a commercial license.

You can use the LGPL version of Qt and the commercial version of PyQT (which is much cheaper than the commercial version of Qt)

pyside recently came back under the Qt umbrella, so it should be viable option again.