Question

I have a csv file which contains date and time stamps as two of the columns. I am using pandas read_csv to read the contents into a dataframe. My ultimate goal is to plot time series graphs from the data.

!head vmstat.csv
wait_proc,sleep_proc,swapped_memory,free_memory,buffered_memory,cached_memory,swapped_in,swapped_out,received_block,sent_block,interrups,context_switches,user_time,sys_time,idle_time,wait_io_time,stolen_time,date,time
0,0,10896,3776872,380028,10284052,0,0,6,16,7716,4755,3,1,96,0,0,2012-11-01,08:59:27
0,0,10896,3776500,380028,10284208,0,0,0,40,7471,4620,0,0,99,0,0,2012-11-01,08:59:32
0,0,10896,3749840,380028,10286864,0,0,339,19,7479,4704,20,2,77,1,0,2012-11-01,08:59:37
0,0,10896,3747536,380028,10286964,0,0,17,118,7488,4638,0,0,99,0,0,2012-11-01,08:59:42
0,0,10896,3747452,380028,10287148,0,0,0,24,7489,4676,0,0,99,0,0,2012-11-01,08:59:47


df = read_csv("vmstat.csv", parse_dates=[['date','time']])
f = DataFrame(df, columns=[ 'date_time',  'user_time', 'sys_time', 'wait_io_time'])

In [3]: f
Out[3]:
date_time               user_time  sys_time     wait_io_time
0  2012-11-01 08:59:27          3         1             0
1  2012-11-01 08:59:32          0         0             0
2  2012-11-01 08:59:37         20         2             1
3  2012-11-01 08:59:42          0         0             0
4  2012-11-01 08:59:47          0         0             0

So far, we could read the data correctly and date_time is combined in the DataFrame. There are issues if I try to used the date_time from df as index. Specifying index = df.date_time gives all NaN values:

dindex = f['date_time']
print dindex
g = DataFrame(f, columns=[ 'user_time', 'sys_time', 'wait_io_time'], index=dindex)

In [7]: g
Out[7]:
0    2012-11-01 08:59:27
1    2012-11-01 08:59:32
2    2012-11-01 08:59:37
3    2012-11-01 08:59:42
4    2012-11-01 08:59:47
Name: date_time  <---- dindex
g:
                 user_time  sys_time  wait_io_time
date_time                                             
2012-11-01 08:59:27        NaN       NaN           NaN
2012-11-01 08:59:32        NaN       NaN           NaN
2012-11-01 08:59:37        NaN       NaN           NaN
2012-11-01 08:59:42        NaN       NaN           NaN
2012-11-01 08:59:47        NaN       NaN           NaN

As you see, the column values are coming out as all NaNs. How do I get correct values as in the intermediate f frame?

Answer 1

You want to use set_index:

df1 = df.set_index('date_time')

Which selects the column 'date_time' as an index for the new DataFrame.

.

Note: The behaviour you are coming across in the DataFrame constructor is demonstrated as follows:

df = pd.DataFrame([[1,2],[3,4]])
df1 = pd.DataFrame(df, index=[1,2])

In [3]: df1
Out[3]: 
    0   1
1   3   4
2 NaN NaN