How to append multiple pandas DataFrames in a loop?

Question

I've been banging my head on this python problem for a while and am stuck. I am for-looping through several csv files and want one data frame that appends the csv files in a way that one column from each csv file is a column name and sets a common index of a date_time.

There are 11 csv files that look like this data frame except for different value and pod number, but the time_stamp is the same for all the csvs.

data

    pod time_stamp  value
0   97  2016-02-22  3.048000
1   97  2016-02-29  23.622001
2   97  2016-03-07  13.970001
3   97  2016-03-14  6.604000
4   97  2016-03-21  NaN

And this is the for-loop that I have so far:

import glob
import pandas as pd

filenames = sorted(glob.glob('*.csv'))

new = []

for f in filenames:
    data = pd.read_csv(f)

    time_stamp = [pd.to_datetime(d) for d in time_stamp]

    new.append(data)

my_df = pd.DataFrame(new, columns=['pod','time_stamp','value'])

What I want is a data frame that looks like this where each column is the result of value from each of the csv files.

time_stamp  97        98       99 ...
2016-02-22  3.04800   4.20002  3.5500
2016-02-29. 23.62201  24.7392  21.1110
2016-03-07 13.97001   11.0284  12.0000

But right now the output of my_df is very wrong and looks like this. Any ideas of where I went wrong?

    0
0   pod time_stamp value 0 22 2016-...
1   pod time_stamp value 0 72 2016-...
2   pod time_stamp value 0 79 2016-0...
3   pod time_stamp value 0 86 2016-...
4   pod time_stamp value 0 87 2016-...
5   pod time_stamp value 0 88 2016-...
6   pod time_stamp value 0 90 2016-0...
7   pod time_stamp value 0 93 2016-0...
8   pod time_stamp value 0 95 2016-...

You convert time_stamp but don't assign it back. Furthermore, you create a new variable rain but are not doing anything with it. — cs95
– cs95, Commented Nov 10, 2017 at 4:50

cs95 · Accepted Answer · 2017-11-10 05:00:04Z

2

I'd recommend first concatenating all your dataframes together with pd.concat, and then doing one final pivot operation.

filenames = sorted(glob.glob('*.csv'))

new = [pd.read_csv(f, parse_dates=['time_stamp']) for f in filenames]
df = pd.concat(new) # omit axis argument since it is 0 by default

df = df.pivot(index='time_stamp', columns='pod')

Note that I'm forcing read_csv to parse time_stamp when loading the dataframe, so parsing after loading is no longer required.

MCVE

df

   pod  time_stamp      value
0   97  2016-02-22   3.048000
1   97  2016-02-29  23.622001
2   97  2016-03-07  13.970001
3   97  2016-03-14   6.604000
4   97  2016-03-21        NaN

df.pivot(index='time_stamp', columns='pod')

                value
pod                97
time_stamp           
2016-02-22   3.048000
2016-02-29  23.622001
2016-03-07  13.970001
2016-03-14   6.604000
2016-03-21        NaN

edited Nov 10, 2017 at 5:00

answered Nov 10, 2017 at 4:54

cs95

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4 Comments

JAG2024 Over a year ago

This is really clean. Excellent. Thanks a lot.

cs95 Over a year ago

@JAG2024 Thanks, it should've been axis=0. Typo.

piRSquared Over a year ago

I like it . . .

cs95 Over a year ago

@piRSquared Thank you!

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