I'm learning Pandas. I have a project where I need to munge some CSV files and resave them as CSV. I can use dictionaries and CSV module, but decided to use DataFrames to get more exposure and practice with Pandas.

The task is to sum() values for some keys while others are not summed. My solution was to add a column called "Line" where items with the same line number would be summed.

sales = [{"Line": 1, "KEY": "Apples", "AMOUNT": 3.99},
         {"Line": 1, "KEY": "Apples", "AMOUNT": 3.99},
         {"Line": 2, "KEY": "Oranges", "AMOUNT": 5.99},
         {"Line": 3, "KEY": "Pears", "AMOUNT": 2.99},
         {"Line": 3, "KEY": "Pears", "AMOUNT": 2.99},
         {"Line": 4, "KEY": "Carrots", "AMOUNT": .99},
         {"Line": 5, "KEY": "Carrots", "AMOUNT": .99},

df = pd.DataFrame(sales)

Now, Pandas groupby might seem like a perfect solution, but since I need to export the munged file as CSV, this seemed like something I could do only to have to undo.

# Find duplicate Line entries
# Subset df into just duplicate `Line` values
df_tmp = df[df.duplicated(subset="Line", keep=False)]

# Save a list of Line numbers to sum
line_dups = df_tmp['Line'].unique()

for x in line_dups:
    # Sum every line in the DF; one value sum is unchanged
    # asum = df.loc[df['Line'] == x, 'AMOUNT'].sum()
    # or
    # Subset the subset
    df_tmp2 = df_tmp[df_tmp["Line"] == x]
    # sum the sub-subset
    asum = df_tmp2['AMOUNT'].sum()
    # set the value of all keys with the same Line value
    df.loc[df['Line'] == x, 'AMOUNT'] = asum
    # take the inverse of the duplicate subset on the original df
    # Keep only the first duplicate line
    df2 = df[~df.duplicated(subset="Line", keep='first')]

The solution is effective, df2:

  AMOUNT    KEY     Line
0   7.98    Apples  1
2   5.99    Oranges 2
3   5.98    Pears   3
5   0.99    Carrots 4
6   0.99    Carrots 5

And yet, this solution is sort of hairy with all of the temp dataframes. Maybe a more seasoned Pandas user will have something to add which might help me better understand Pandas?

  • \$\begingroup\$ What do you mean that you need to undo the groupby to save your data to csv? \$\endgroup\$
    – mochi
    Jun 4, 2018 at 4:58
  • \$\begingroup\$ I see, that was a strange note--the problem is really a file conversion problem and can not produce new columns. From the groupby examples I've seen I expected changes to the index which I would need to undo. Maybe I was incorrect--if the same results could be obtained. \$\endgroup\$
    – xtian
    Jun 6, 2018 at 9:04
  • \$\begingroup\$ Is it important to maintain the index information (0, 2, 3, 5, 6)? Or do you really just want KEY and AMOUNT? I'm unclear about why exactly groupby doesn't work. \$\endgroup\$
    – mochi
    Jun 7, 2018 at 1:42

1 Answer 1


If you have the Line column in your CSV itself, you can replace what you have with:

df.groupby(['Line', 'KEY']).agg({'AMOUNT': 'sum'}).reset_index()

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