How about this (in you don't want an one liner) ?
for row in df.collect():
d = row.asDict()
s = "%d\t%s\t%s\n" % (d["int_column"], d["string_column"], d["string_column"])
f.write(s)
f is a opened file descriptor. Also the separator is a TAB char, but it's easy to change to whatever you want.