pandas - Solving error "delimiter must be a 1-character string" while writing a dataframe to a csv file

Question

Using this question: Pandas writing dataframe to CSV file as a model, I wrote the following code to make a csv file:

df.to_csv('/Users/Lab/Desktop/filteredwithheading.txt', sep='s+', header=True)

But it returns the following error:

TypeError: "delimiter" must be an 1-character string

I have looked up the documentation for this here http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.to_csv.html but I can't figure out what I am missing, or what that error means. I also tried using (sep='s') in the code, but got the same error.

Answer 1

Note that the although the solution to this error was using a string charcter instead of regex, pandas also raises this error when using from __future__ import unicode_literals with valid unicode characters. As of 2015-11-16, release 0.16.2, this error is still a known bug in pandas:
"to_csv chokes if not passed sep as a string, even when encoding is set to unicode" #6035

For example, where df is a pandas DataFrame:

# -*- coding: utf-8 -*-
from __future__ import unicode_literals
import pandas as pd

df.to_csv(pdb_seq_fp, sep='', encoding='utf-8')

TypeError: "delimiter" must be an 1-character string

Using a byte lteral with the specified encoding (default utf-8 with Python 3) -*- coding: utf-8 -*- will resolve this in pandas 0.16.2: (b'') —I haven't tested with previous versions or 0.17.0.

# -*- coding: utf-8 -*-
from __future__ import unicode_literals
import pandas as pd

df.to_csv(pdb_seq_fp, sep=b'', encoding='utf-8')

(Note that with versions 0.13.0 - ???, it was necessary to use pandas.compat import u; but by 0.16.2 the byte literal is the way to go.)