Selecting a face in Python

1. I think the added lines are not necessary and actually incorrect as written. "Selection" should be imported from NXOpen, not NXOpen.UF. Likewise, "UFConstants" should be imported from NXOpen.UF instead of NXOpen. You can shorten up some lines of code with these imports, but they are not required.

2. I don't think the Utilities import is needed.

3. This style of declaring variables is called "type hinting". This is optional in python; the interpreter ignores it, but can be useful in the IDE or with 3rd party tools (code checkers and linters).
https://docs.python.org/3.8/library/typing.html

The code below will allow you to select a face and report the density of the solid body that owns the face.

import NXOpen
import NXOpen.UF
import NXOpen.Features
from NXOpen import Selection
from NXOpen.UF import UFConstants

theSession = NXOpen.Session.GetSession()
workPart = theSession.Parts.Work
theUI = NXOpen.UI.GetUI()
theUfSession = NXOpen.UF.UFSession.GetUFSession()
lw = theSession.ListingWindow
dispObj1 = None
minPt = None

def select_face():
dispObj1 = None

# Variablen fuer Auswahl-Dialoge (Selection Dialogs)
cue = "Bitte waehle die Flaeche, auf der gelasert wird, aus."
title = "Kosten- und CO2-Rechner"
scope = Selection.SelectionScope.WorkPart
action = Selection.SelectionAction.ClearAndEnableSpecific
includeFeatures = False
keepHighlighted = False
maskArray = [None] * 1
maskArray[0] = Selection.MaskTriple(UFConstants.UF_face_type, 0, 0)
response, selObj1, cursor1 = theUI.SelectionManager.SelectTaggedObject(cue, title, scope, action, includeFeatures, keepHighlighted, maskArray)

if response == Selection.Response.ObjectSelected or response == Selection.Response.ObjectSelectedByName:
if isinstance(selObj1, NXOpen.Face):
dispObj1 = selObj1
return dispObj1

def main():
lw.Open()

if theSession.Parts.Work is None:
lw.WriteLine("No work part for processing")
return

partBodiesArray = list(workPart.Bodies)
theBody = None
myMeasure = theSession.Parts.Display.MeasureManager
displayPart = theSession.Parts.Display
dispObj1 = select_face()

if dispObj1 is None:
# Face selection canceled
return

#minPt = PointOnSelectedFace(dispObj1)

# Achtung: partBodiesArray() speichert verschiedene Koerpers

# Pruefen den Flaechentypen
lw.WriteLine(str(dispObj1.SolidFaceType))
lw.WriteLine("Number of BODIES in part= " + str(len(partBodiesArray)))

theBody = dispObj1.GetBody()
# for theBody in partBodiesArray:
# attributes = theBody.GetUserAttributes()
# # Lese Materialtypen aus (Read material types)
# for attribute in attributes:
# # Equivalent of lw.WriteLine(attribute.Title & " = " & attribute.StringValue) in Python
# lw.WriteLine(f"{attribute.Title} = {attribute.StringValue}")

bodyDensity = theBody.Density # kilograms per cubic meter, 1 kg / (m3) = 1.0 * 10-6 g / (mm3)
lw.WriteLine('density: ' + str(bodyDensity))

lw.Close()

if __name__ == "__main__":
main()