Write comments in CSV file with pandas

df.to_csv accepts a file object. So you can open a file in a mode, write you comments and pass it to the dataframe to_csv function.

For example:

In [36]: df = pd.DataFrame({'a':[1,2,3], 'b':[1,2,3]})In [37]: f = open('foo', 'a')In [38]: f.write('# My awesome comment\n')In [39]: f.write('# Here is another one\n')In [40]: df.to_csv(f)In [41]: f.close()In [42]: more foo# My awesome comment# Here is another one,a,b0,1,11,2,22,3,3

An alternative approach @Vor's solution is to first write the comment to a file, and then use mode='a' with to_csv() to add the content of the data frame to the same file. According to my benchmarks (below), this takes about as long as opening the file in append mode, adding the comment and then passing the file handler to pandas (as per @Vor's answer). The similar timings make sense considering that this is what pandas in doing internally (DataFrame.to_csv() calls CSVFormatter.save(), which uses _get_handles() to read in the file via open().

On a separate note, it is convenient work with file IO via with statement which ensures that opened files close when you're done with them and leave the with statement. See examples in the benchmarks below.

Read in test data

import pandas as pd# Read in the iris data frame from the seaborn GitHub locationiris = pd.read_csv('https://raw.githubusercontent.com/mwaskom/seaborn-data/master/iris.csv')# Create a bigger data framewhile iris.shape[0] < 100000:    iris = iris.append(iris)# `iris.shape` is now (153600, 5)

1. Append with the same file handler

%%timeit -n 5 -r 5# Open a file in append mode to add the comment# Then pass the file handle to pandaswith open('test1.csv', 'a') as f:    f.write('# This is my comment\n')    iris.to_csv(f)

972 ms ± 31.9 ms per loop (mean ± std. dev. of 5 runs, 5 loops each)

2. Reopen the file with to_csv(mode='a')

%%timeit -n 5 -r 5# Open a file in write mode to add the comment# Then close the file and reopen it with pandas in append modewith open('test2.csv', 'w') as f:    f.write('# This is my comment\n')iris.to_csv('test2.csv', mode='a')

949 ms ± 19.3 ms per loop (mean ± std. dev. of 5 runs, 5 loops each)