pandas DataFrame, how to apply function to a specific column?

The answer is,

df['A'] = df['A'].map(addOne)

and maybe you would be better to know about the difference of map, applymap, apply.

but if you insist to use apply, you could try like below.

def addOne(v):
    v['A'] += 1
    return v

df.apply(addOne, axis=1)

One simple way would be:

df['A'] = df['A'].apply(lambda x: x+1)

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