Doesn't explain it but what if you try:
for tb in Document.Data.Tables:
for col in tb.Columns:
# initialize column variable
ces = ""
# get calculated expression and cast to string
try:
ces = str(col.Properties.CalculatedExpression)
exception:
ces="Error:Calculated Expression Not Read"
instead?
If that doesn't work look into the unicode() function.
str() docs - https://ironpython-test.readthedocs.org/en/latest/library/functions.html#str
unicode() docs - https://ironpython-test.readthedocs.org/en/latest/library/functions.html#unicode
I'd test it myself if there was example data given.