How to test for 'if arcpy.exists' with RemoveFeatureClassFromTopology stand-alone script?

Question

I have some dataset topologies that I want to loop through, but I need to test for the assignment of feature classes so the scripts don't fail. (Using Win 7, ArcGIS 10.2, Python 2.7.5 and a file GDB)

My research hasn't turned up too much but using the 10.2 "Remove Feature Class From Topology (Data Management)", using ArcCatalog I can remove an existing fc from the topo set. If however I run the script 'stand alone' and the fc isn't currently assigned to the topology, the script fails. (Error shown below script)

I read this post (ArcMap remove feature-class from feature-dataset) as well but I don't want to delete the fc, I only want to remove it from the topo if it exists as I need it again later.

Am I going down the wrong path?

Using ArcCatalog, this succeeds if the fc exists;
topo = "C:/Data/Terrain_36/TOPO_EXPORT_2015_09_09_1_2.gdb/ZONES_1_2/TOPO_1_2"
fc = "RL_10"
arcpy.RemoveFeatureClassFromTopology_management(topo, fc)

# Running the script below as 'stand alone', fails if the fc isnt currently assigned;
# import modules
import arcpy, sys, os, datetime, traceback, string, time
from os import sep, listdir
from arcpy import env  as env

# This scripts name
script_name = os.path.basename(sys.argv[0])
# Create file prefix for unique file naming
prefix = datetime.datetime.now().strftime('%Y-%m-%d %H%M')
# Create file suffix for avoiding filenames staring with numbers
suffix = datetime.datetime.now().strftime('%Y-%m-%d %H%M')

# Set variables
Top_Dir = "C:/Data/Terrain_36"
td = Top_Dir
print "Top_Dir set to: '" +td+ "'" + '\n'
# Out_Dir = td+sep+ 'Exports'
# print "Out_Dir set to: '" +Out_Dir+ "'" + '\n'
Log_Dir = td+sep+ 'zz_logs'
print "Log_Dir set to: '" +Log_Dir+ "'" + '\n'
GDB_Nam = 'TOPO_EXPORT_2015_09_09_1_2.gdb'
print "GDB_Nam set to: '" +GDB_Nam+ "'" + '\n'
GDB_Home = os.path.join(td, GDB_Nam)
print  "GDB_Home set to: '" +GDB_Home+ "'" + '\n'

# Set the workspace environment
env.workspace = os.path.join(Top_Dir, GDB_Nam)
ws = env.workspace
print "Env.workspace set to: '" +ws+ "'" '\n'
topo = "C:/Data/Terrain_36/TOPO_EXPORT_2015_09_09_1_2.gdb/ZONES_1_2/TOPO_1_2"
fc = "RL_10"
if arcpy.Exists (fc):
    arcpy.RemoveFeatureClassFromTopology_management(topo, fc)

# My error messages
Log: 2015-09-14 11:48:32.722000
Begin process: Topo_script.py
 Process started at 2015-09-14 11:48:32.722000
 PYTHON ERRORS:
 Traceback info:
   File "C:\Data\Terrain_36\Data_quality\_Scripts\Topo_script.py", line 62, in <module> arcpy.RemoveFeatureClassFromTopology_management(topo, fc)
 ArcPy ERRORS:
 ERROR 999999: Error executing function.
 Cannot register as versioned on this database.
 Cannot register as versioned on this database.
 Failed to execute (RemoveFeatureClassFromTopology).

Answer 1

Why not use arcpy.Describe on the topology itself and see if the featureclass is in the featureClassNames attribute?

Something simple like:

if fc in arcpy.Describe(topo).featureClassNames:
    arcpy.RemoveFeatureClassFromTopology_management(topo, fc)
else:
    print "{0} wasn't in the topology...".format(fc)