Make the address field active.
put the function in the code block.
the expression in the expression box
and calculate away
def my_func(val, fld1, fld2)
""" NS_Address if the Prefix attribute is N or S
EW_Address if the Prefix attribute is E or W
"""
if val in ('N', 'S'):
return fld1
elif val in ('E', 'W'):
return fld2
else:
return None
expression
my_func(!Prefix!, !NS_Address!, !EW_Address! )