I tested it with your sample data, but I don't know how well it will work with more a more complex dataset. Note that the output field that lists all of the ItemID values has to have a limit, which I arbitrarily set to 255. Of course, you'll also need to change the paths and names accordingly.
def main():
import arcpy
import os
# Local variables
mygdb = r"N:\TechTemp\BlakeT\Work\TEMP.gdb"
mytable = os.path.join(mygdb, "SegmentSequence")
# Read input table into dictionary
itemsdict = {}
with arcpy.da.SearchCursor(mytable, ["ItemID", "Begin", "End"]) as s_cursor:
for item, begin, end in s_cursor:
itemsdict[item] = (begin, end)
# Identify segments
allsegments = [i[0] for i in itemsdict.values()] + [i[1] for i in itemsdict.values()]
segments = tuple(sorted(set(allsegments)))
del allsegments
# Create output table
arcpy.CreateTable_management(mygdb, "SegmentSequence_output")
mytable_out = os.path.join(mygdb, "SegmentSequence_output")
arcpy.AddField_management(mytable_out, "Begin", "SHORT")
arcpy.AddField_management(mytable_out, "End", "SHORT")
arcpy.AddField_management(mytable_out, "SegmentItems", "TEXT", field_length=255)
# Write segment items to output table
with arcpy.da.InsertCursor(mytable_out, ["Begin", "End", "SegmentItems"]) as i_cursor:
for enum, seg in enumerate(segments):
try:
begin = seg
end = segments[enum + 1]
except IndexError:
break
seg_items = [k for k, v in itemsdict.items() if v[0] <= begin and v[1] >= end]
row = (begin, end, str(seg_items).strip('[]'))
i_cursor.insertRow(row)
if __name__ == '__main__':
main()
