If you want to construct these objects directly, you can explicitly pass None for one of the parameters, e.g.

In [4]: lines.AxLine(xy1=(0, 0), xy2=(1, 1), slope=None)
Out[4]: <matplotlib.lines.AxLine at 0x7fb8639a09d0>

However, the usual way to create these objects is through the axline method which does have the relevant defaults. Do you have a use-case that requires constructing the AxLine directly? Or is there scope to improve our documentation to make it clearer how these things are expected to be handled?

@rcomer Would any changes in lines.AxLine() solve this issue? Let me know if I can work on it if it needs to be done.

The idea use case is I'd like to not only draw a line on a chart with AxLine, but I wanted to update it as part of a loop, as part of an animation or a live plot. However, since Axes.AxLine() returns a Line2D object which doesn't support updating slope, I thought I'd create a lines.AxLine artist myself and add it to the axes. If axes.AxLine returned a lines.AxLine instead of a lines.Line2D, that would also work.

So, I can just pass one of the arguments as 'None' and that will work. Maybe axes.AxLine() should also return an AxLine instead of a Line2D?

Axes.axline returns an AxLine object, which has setters for xy1, xy2, and slope. https://matplotlib.org/stable/api/_as_gen/matplotlib.lines.AxLine.html

It does look like the documentation for axes.axline is incorrect about the return type, and that should get fixed.

@jklymak see #27504 :-)

OK, so we can close this?

We maybe we should also add a note to the __init__ that you must pass both but one must be None?

you must pass both but one must be None

This is not the most elegant API. If I would design this now, I‘d make one of the variants a factory class method, but migrating is not worth it and better documentation should be enough.