listing gis files on disc

I have found this script to list shapefiles/geodatabases/dbase files/coverages on a disc.
The script runs well, BUT when i run it a second time, datalines are addded,; so my listing is getting larger every time.
I suppose it is because I use that yield command? Is there a way to avoid this? I am a new user of python/arc python. Can anyoen help me a little bit. I have already read that yield is difficult to understand for new users.
Thanks in advance,
Bart De Bruyn
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import os
import arcpy

workspace = r"T:\test"
output = r"H:\testlist.txt"
outFile = open(output, "w")
lijn=""

def inventory_data(workspace, datatypes):
"""
Generates full path names under a catalog tree for all requested
datatype(s).
Parameters:
workspace: string
The top-level workspace that will be used.
datatypes: string | list | tuple
Keyword(s) representing the desired datatypes. A single
datatype can be expressed as a string, otherwise use
a list or tuple. See arcpy.da.Walk documentation
for a full list.
"""
for path, path_names, data_names in arcpy.da.Walk(
workspace, datatype=datatypes):
for data_name in data_names:
yield os.path.join(path, data_name)

for feature_class in inventory_data(r"T:\test", "Any"):

lijn = lijn + feature_class + "\n"
outFile.write(lijn)

outFile.close()