Writing large Pandas Dataframes to CSV file in chunks

Question

How do I write out a large data files to a CSV file in chunks?

I have a set of large data files (1M rows x 20 cols). However, only 5 or so columns of the data files are of interest to me.

I want to make things easier by making copies of these files with only the columns of interest so I have smaller files to work with for post-processing. So I plan to read the file into a dataframe, then write to csv file.

I've been looking into reading large data files in chunks into a dataframe. However, I haven't been able to find anything on how to write out the data to a csv file in chunks.

Here is what I'm trying now, but this doesn't append the csv file:

with open(os.path.join(folder, filename), 'r') as src:     df = pd.read_csv(src, sep='\t',skiprows=(0,1,2),header=(0), chunksize=1000)     for chunk in df:         chunk.to_csv(os.path.join(folder, new_folder,                                   "new_file_" + filename),                                    columns = [['TIME','STUFF']]) 

Answer 1

Solution:

header = True for chunk in chunks:      chunk.to_csv(os.path.join(folder, new_folder, "new_file_" + filename),         header=header, cols=[['TIME','STUFF']], mode='a')      header = False 

Notes:

• The mode='a' tells pandas to append.
• We only write a column header on the first chunk.