Writing multiple header lines in pandas.DataFrame.to_csv

Question

I am putting my data into NASA's ICARTT format for archvival. This is a comma-separated file with multiple header lines, and has commas in the header lines. Something like: ``` 46, 1001 lastname, firstname location instrument field mission 1, 1 2011, 06, 21, 2012, 02, 29 0 Start_UTC, seconds, number_of_seconds_from_0000_UTC 14 1, 1 -999, -999 measurement name, units measurement name, units column1 label, column2 label, column3 label, column4 label, etc. ``` I have to make a separate file for each day that data were collected, so I will end up creating around thirty files in all. When I create a csv file via pandas.DataFrame.to_csv I cannot (as far as I know) simply write the header lines to the file before writing the data, so I have had to trick it to doing what I want via ``` # assuming <df> is a pandas dataframe df.to_csv('dst.ict',na_rep='-999',header=True,index=True,index_label=header_lines) ``` where "header_lines" is the header string What this give me is exactly what I want, e…

Accepted Answer

You can, indeed, just write the header lines before the data. `pandas.DataFrame.to_csv` takes a `path_or_buf` as its first argument, not just a pathname:

`pandas.DataFrame.to_csv(path_or_buf, *args, **kwargs)`

path_or_buf : string or file handle, default None

File path or object, if None is provided the result is returned as a string.

Here's an example:

```
#!/usr/bin/python2

import pandas as pd
import numpy as np
import sys

# Make an example data frame.
df = pd.DataFrame(np.random.randint(100, size=(5,5)),
                  columns=['a', 'b', 'c', 'd', 'e'])

header = '
'.join(
    # I like to make sure the header lines are at least utf8-encoded.
    [unicode(line, 'utf8') for line in 
        [ '1001',
        'Daedalus, Stephen',
        'Dublin, Ireland',
        'Keys',
        'MINOS',
        '1,1',
        '1904,06,16,1922,02,02',
        'time_since_8am', # Ends up being the header name for the index.
        ]
    ]
)

with open(sys.argv[1], 'w') as ict:
    # Write the header lines, including the index variable for
    # the last one if you're letting Pandas produce that for you.
    # (see above).
    for line in header:
        ict.write(line)

# Just write the data frame to the file object instead of
    # to a filename. Pandas will do the right thing and realize
    # it's already been opened.
    df.to_csv(ict)
```

The result is just what you wanted - to write the header lines, and then call `.to_csv()` and write that:

```
$ python example.py test && cat test
1001
Daedalus, Stephen
Dublin, Ireland
Keys to the tower
MINOS
1, 1
1904, 06, 16, 1922, 02, 02
time_since_8am,a,b,c,d,e
0,67,85,66,18,32
1,47,4,41,82,84
2,24,50,39,53,13
3,49,24,17,12,61
4,91,5,69,2,18
```

Sorry if this is too late to be useful. I work in archiving these files (and use Python), so feel free to drop me a line if you have future questions.

Writing multiple header lines in pandas.DataFrame.to_csv

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