Operating system: Ubuntu 20.04, Windows 10
wxPython version & source: 4.2.1
Python version & source: 3.8.10 (Ubuntu), 3.10.0 (Windows)
Description of the problem:

InformFirstDirection() must be overridden by classes that derive from Sizer in order to properly implement any non-trivial sizer, as indicated by this comment:

https://github.com/wxWidgets/wxWidgets/blob/be772c284f20969e7c2a5b62d3d43db56cdc5a6b/include/wx/sizer.h#L636-L642

However, wxPython forbids this method from being overridden in Python classes that derive from Sizer. The only methods that can be overridden are CalcMin() and RepositionChildren():

This means it is not possible to implement a non-trivial custom Sizer in Python. Even the simple BoxSizer and WrapSizer need InformFirstDirection() to properly calculate their size.

I am trying to implement a custom Sizer in Python similar to WrapSizer that uses a box packing algorithm to more efficiently layout its children. Unfortunately, without this method being virtual, it is impossible to make it work properly.

(As a workaround I am having my custom sizer store a reference to its containing ScrolledPanel so it can call GetClientSize() on it during its CalcMin(). This is an ugly hack that should not be necessary.)