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Function mapping in a pandas dataframe

Hello,

after creating a pandas dataframe consisting of historical stock prices I am trying to add a new column that would include the volume for each day if the stock price went down from previous day or 0 otherwise.

My problem is that mapping a function doesn't return desired results. This is my code:

import pandas as pd
import pandas.io.data as web

import numpy as np


def adjustedVolume(change, volume):
    if change < 0:
        return volume
    else:
        return 0

ticker = 'TSLA'
source = 'google'

start = '1/1/2015'
end = '7/4/2016'

df = web.DataReader(ticker, source, start, end)

df['50DMA'] = pd.rolling_mean(df['Close'], 50)
df['Difference'] = df['Close'].diff()

df['Adjusted volume'] = map(adjustedVolume, df['Difference'], df['Volume'])

print(df.tail())

The script does create a new column, however the rows do not contain any numbers. What I see in each line is the following message:

<map object at 0x05567630>

Any idea what I do wrong? Thank you very much!

Mike

df['Adjusted volume'] = list(map(adjustedVolume, df['Difference'], df['Volume']))