Generate random number in csv file with python

I am struggling with an issue regarding CSV files and Python. How would I generate a random number in a csv file row, based of a condition in that row.

Essentially, if the value in the first column is 'A' I want a random number between (1 and 5). If the value is B I want a random number between (2 and 6) and if the value is C, and random number between (3 and 7).

CSV File

Letter	Color	Random Number
A	Green
C	Red
B	Red
B	Green
C	Blue

Thanks in advance

The only thing I have found was creating a new random number dataframe. But I need to create a random number for an existing df.