Paraview python annotation filter: Get name of file of a PVD reader as its input

I have a pipeline with a PVDReader as a source, and a filter on it of type PythonAnnotation.
Selecting the filter,

>>> flt = GetActiveSource()
>>> flt
<paraview.servermanager.PythonAnnotation object at 0x7f4190035eb0>
>>> inp = flt.Input
>>> inp
<paraview.servermanager.PVDReader object at 0x7f4190035fa0>

The reader inp got the data from file dir1/dir2/dir3/mydata.pvd.

I also have a RenderView1 where I plot data from the reader.
I want to add the annotation dir2 (not fixed, but obtained dynamically from the path above).
I set the PythonAnnotation filter to obtain such data.
But I don’t know the appropriate expression.
I could not figure this out from the documentation.
What should that be?

Note: if I use input for flt.Expression, I get a different object as compared to inp above. I am copying an image of the result, since I cannot copy the result of the annotation.