I have a table Mapping that looks like this

Mapping
+---------+---------+
| user_id | prod_id |
+---------+---------+
|     41  |  N12ad  |
|      2  |  nd5Ed  |
|     73  |  bDe4d  |
|     25  |  M87he  |
+---------+---------+

I would like to add a column ranking that numbers the rows in the order that they are stored in the database. So that it looks like this

+---------+---------+---------+
| user_id | prod_id | ranking |
+---------+---------+---------+
|     41  |  N12ad  |      1  |
|      9  |  nd5Ed  |      2  |
|     73  |  bDe4d  |      3  |
|     25  |  M87he  |      4  |
+---------+---------+---------+

In Microsoft SQL, I can do

SELECT user_id, prod_id, ROW_NUMBER() OVER(PARTITION BY (SELECT NULL)) AS ranking
FROM Mapping

What would be the pandas equivalent of ROW_NUMBER() OVER(PARTITION BY (SELECT NULL))? Thank you for your help.