How to create a cumulative sum on update in mysql-python [Migrated from Stackoverflow]

I have the following statement in Python for MySQL Driver, that increments counter by one, on update with the same language.

add_lang = ("INSERT INTO Languages (lang, length, counter)\
            VALUES (%s, %s, 1)\
            ON DUPLICATE KEY UPDATE\
            counter = counter + 1;")


Now, what I want to achieve is a cumulative sum. My analogy suggests:

add_lang = ("INSERT INTO Languages (lang, length, counter)\
            VALUES (%s, 0, 1)\
            ON DUPLICATE KEY UPDATE\
            counter = counter + 1;\
            length = length + new_length")


How to differentiate between inserted new values, and current value before update ? Notice new_length and length. this is just to show my aim.
Try this:

add_lang = ("INSERT INTO Languages (lang, length, counter)\ 
VALUES (%s, %s, 1)\ 
ON DUPLICATE KEY UPDATE\ 
counter = counter + 1,\ 
length = length + VALUES(length);")


NB: edited to apply parameter replacement to the VALUES