PySpark: spit out single file when writing instead of multiple part files

Question

Is there a way to prevent PySpark from creating several small files when writing a DataFrame to JSON file?

If I run:

 df.write.format('json').save('myfile.json')

or

df1.write.json('myfile.json')

it creates the folder named myfile and within it I find several small files named part-***, the HDFS way. Is it by any means possible to have it spit out a single file instead?

Solution

Well, the answer to your exact question is coalesce function. But as already mentioned it is not efficient at all as it will force one worker to fetch all data and write it sequentially.

df.coalesce(1).write.format('json').save('myfile.json')

P.S. Btw, the result file is not a valid json file. It is a file with a json object per line.

Comments

[the.malkolm]

df.coalesce(1).write.json('myfile.json') works fine

[mar tin]

About the non-validity of the JSON, this happens in any case, even when spitting out several files.

[the.malkolm]

@martina, yep. It is confusing sometimes to see .json extension and no valid json file inside :D

[Zahiduzzaman]

remember you do need to concat the parts files after it is produced. spark.apache.org/docs/latest/api/python/…

[Matěj Račinský]

for me, this line created directory named myfile.json with one part file inside (using spark 2.4)