Subtracting a constant value from one column when condition on another column holds

I have a Pandas data frame that has the following columns: foo and bar. foo values are integers and bar values are strings. For each row, if the value of bar is some particular value, say, ‘ABC’, then I want to set the value of the foo column (for that row) to its current value minus one.

For example, I want to convert this data frame:

converted to this:

How is this done?

>Solution :

You can do this via the .loc on dataframe:

CONSTANT_VAL = 1
df.loc[df['bar'] == 'ABC', 'foo'] -= CONSTANT_VAL