data.csv

pandas_example.py

import pandas as pd

df = pd.DataFrame.from_csv("data.csv")

# calculate sum of areas per feature
sums = df.groupby("feature").area.sum()

df["sums"] = df.feature.apply(
    # look up pre-computed sum, fill in every row
    lambda x: sums[x])

df["pct"] = df.apply(
    # calculate percentage of each area from its sum
    lambda row: "{:.2%}".format(row["area"] / row["sums"]),
    # apply to rows
    axis=1)

# print a CSV file
print df.to_csv()