How do I determine if a Python sqlite UPDATE worked?

For a slightly more complete answer for if you want to handle error and success:

c.execute('update students set gpa=3.5 where stuid=123')

if c.rowcount < 1:
    #error
else:
    #success

cursor.rowcount will be 1 if the update was successful (affecting 1 row) or 0 if it failed.