pandas to_csv arguments float_format and decimal not working for index column

Question

Background

I am doing some simulations resp. a system analysis by variing parameters (in this case rpm only) and append every last line of a results dataframe results_df to a summarizing dataframe df containing giving the baviour of my system in depencence of the varied rpm.

In order to get an appropriate index for plotting and data analysis I converted the varied values (here rpm) from the list into a pandas series ser and concat this series with the summarizing dataframe df containing the results I am interested in.

Since the results of each calculation I am interested in is only last line of each calculation I am extracting this data from the results dataframe results_df by using .tail(1).

What I have done so far is shown in the following snippet:

rpm = [0.25, 0.3, 0.5, 0.75, 1.0, 1.5, 2.0]

ser = pd.Series(rpm, name='rpm')
df = pd.DataFrame()
df_list = list()

for i, val in enumerate(rpm):
    results_df = get_some_data_from_somwhere()
    df_list.append(results_df.tail(1))

df = df.append(df_list, ignore_index=True)
df = pd.concat([df, ser], axis=1)
df.set_index('rpm', inplace=True)


with open('foo.csv', 'w') as f:
    data.to_csv(f, index=True, header=True, decimal=',', sep=' ', float_format='%.3f')

Problem

This csv-file what I get has the follwing format:

rpm cooling_inner heating_inner cooling_outlet heating_outlet
0.25 303,317 323,372 302,384 324,332

However, I expected having three decimal digits and a comma as decimal sign on my index column, like shown here:

rpm cooling_inner heating_inner cooling_outlet heating_outlet
0,250 303,317 323,372 302,384 324,332

So it seems that the index and decimal sign options are not applied to the index column when exporting dataframes to csv-files using the .to_csv command.

How could I achieve this behaviour since the index option is set True and all values (with exception to the index column) have the right format and decimal sign?

Do I have to handle the index column somehow seperate?

Answer 1

I would rewrite your two bottom lines:

with open('foo.csv', 'w') as f:
    data.to_csv(f, index=True, header=True, decimal=',', sep=' ', float_format='%.3f')

Into

data.reset_index().to_csv('foo.csv', index=False, header=True, decimal=',', sep=' ', float_format='%.3f')

This is a bit of a workaround, but as you have noticed, the keyword arguments decimal= and float_format= only work on data columns, not on the index.

What I do instead is to put the index into the dataframe with reset_index and then I tell to_csv(index=False not to save the index to the file (since it is now in the data).

Also, opening a file stream yourself (with open('foo.csv', 'w') as f:) is better left to pandas, which does this by itself when you just give it a string 'foo.csv' as first argument.