CSV file written with Python has blank lines between each row

The simple answer is that csv files should always be opened in binary mode whether for input or output, as otherwise on Windows there are problems with the line ending. Specifically on output the csv module will write \r\n (the standard CSV row terminator) and then (in text mode) the runtime will replace the \n by \r\n (the Windows standard line terminator) giving a result of \r\r\n.

Fiddling with the lineterminator is NOT the solution.

The csv.writer module directly controls line endings and writes \r\n into the file directly. In Python 3 the file must be opened in untranslated text mode with the parameters 'w', newline='' (empty string) or it will write \r\r\n on Windows, where the default text mode will translate each \n into \r\n.

#!python3
with open('/pythonwork/thefile_subset11.csv', 'w', newline='') as outfile:
    writer = csv.writer(outfile)

In Python 2, use binary mode to open outfile with mode 'wb' instead of 'w' to prevent Windows newline translation. Python 2 also has problems with Unicode and requires other workarounds to write non-ASCII text. See the Python 2 link below and the UnicodeReader and UnicodeWriter examples at the end of the page if you have to deal with writing Unicode strings to CSVs on Python 2, or look into the 3rd party unicodecsv module:

#!python2
with open('/pythonwork/thefile_subset11.csv', 'wb') as outfile:
    writer = csv.writer(outfile)

Documentation Links

• https://docs.python.org/3/library/csv.html#csv.writer
• https://docs.python.org/2/library/csv.html#csv.writer

Opening the file in binary mode "wb" will not work in Python 3+. Or rather, you'd have to convert your data to binary before writing it. That's just a hassle.

Instead, you should keep it in text mode, but override the newline as empty. Like so:

with open('/pythonwork/thefile_subset11.csv', 'w', newline='') as outfile: