Extracting feature attributes/geometry using PyQGIS

Question

I am trying to extract co-ordinate and height data from a contour layer imported from a .shp file. I have loaded the file into QGIS and am attempting to access with Python the height values of each feature - Closest vertex Z or Interpolated Z in the picture below.

I am eventually aiming for something similar to what is achieved in this answer https://gis.stackexchange.com/a/41922/172876, but I can't find a way to adapt their solution to work for me. I have looked through several forum posts, but many rely on using rasters to extract z values, or are using points not contours.

I have been able to select features in the contour layer, and access their attributes using:

layer = iface.activeLayer()
for feature in layer.selectedFeatures():
    attrs = feature.attributes()
    print(attrs)

But this only accesses the attributes pictured below, and none of the attributes under the (Derived) dropdown in the picture above, which is where the Z values are.

How do I access and extract these using PyQGIS?

Answer 1

I had the same problem. I had a bunch of linear features that stored z values, with each single line having the same z value.

I was able to create a new field using the field calculator, as per the following expression.

z(line_interpolate_point($geometry, 1))

This basically generations a point a specific distance along the line (which is $geometry, the distance, I just picked 1). This will return a geometry.

z($geometry) will return the z value of that geometry - In this instance it will be the point geometry.