df-iterate-test.py

import pandas as pd

data = [
        [1,2],
        [3,4],
        [5,6]
        ]

cols = ['a','b']

df = pd.DataFrame(data,columns=cols)

print df

def my_fn(x,y):
    return x*y

l = list(df['a'])

tmp = []

for x in l:
    tmp.append(my_fn(3,x))

df['a'] = pd.Series(tmp, index=df.index)

print df