177

I want to cast data like [1,2,'a','He said "what do you mean?"'] to a CSV-formatted string.

Normally one would use csv.writer() for this, because it handles all the crazy edge cases (comma escaping, quote mark escaping, CSV dialects, etc.) The catch is that csv.writer() expects to output to a file object, not to a string.

My current solution is this somewhat hacky function:

def CSV_String_Writeline(data):
    class Dummy_Writer:
        def write(self,instring):
            self.outstring = instring.strip("\r\n")
    dw = Dummy_Writer()
    csv_w = csv.writer( dw )
    csv_w.writerow(data)
    return dw.outstring

Can anyone give a more elegant solution that still handles the edge cases well?

Edit: Here's how I ended up doing it:

def csv2string(data):
    si = StringIO.StringIO()
    cw = csv.writer(si)
    cw.writerow(data)
    return si.getvalue().strip('\r\n')
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2
  • 2
    In Python 3, StringIO() is in the io library. Commented Mar 12, 2019 at 15:05
  • Since "\r" and "\n" are considered whitespace, you can actually just use "strip" with no argument: return si.getvalue().strip()--unless for some reason you need to preserve spaces at the end. Commented Aug 25, 2020 at 14:43

7 Answers 7

Reset to default
268

In Python 3:

>>> import io
>>> import csv
>>> output = io.StringIO()
>>> csvdata = [1,2,'a','He said "what do you mean?"',"Whoa!\nNewlines!"]
>>> writer = csv.writer(output, quoting=csv.QUOTE_NONNUMERIC)
>>> writer.writerow(csvdata)
59
>>> output.getvalue()
'1,2,"a","He said ""what do you mean?""","Whoa!\nNewlines!"\r\n'

Some details need to be changed a bit for Python 2:

>>> output = io.BytesIO()
>>> writer = csv.writer(output)
>>> writer.writerow(csvdata)
57L
>>> output.getvalue()
'1,2,a,"He said ""what do you mean?""","Whoa!\nNewlines!"\r\n'
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11 Comments

Should be output = StringIO.StringIO(), io.StringIO() will raise TypeError: string argument expected, got 'str'.
@Marboni: StringIO is gone in Python 3 (which is what my solution is written in), and I can't reproduce that error in Python 2.7.3 - although I do get a TypeError in the writer.writerow(...) line (unicode argument expected, got 'str'). Will look into this.
@Marboni: Thanks for the heads-up: I found the problem with the help of StackOverflow. In Python 2, you need io.BytesIO() instead of io.StringIO().
@Marboni: In Python 2.7.9 it works with StringIO.StringIO() or io.BytesIO().
@nz_21: I just re-tested it - works perfectly fine. The second example (written for Python 2, as explicitly stated) will throw that error, of course.
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73

You could use StringIO instead of your own Dummy_Writer:

This module implements a file-like class, StringIO, that reads and writes a string buffer (also known as memory files).

There is also cStringIO, which is a faster version of the StringIO class.

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1 Comment

cStringIO was gone away since python 3.0. Please use io.StringIO instead
9

since i use this quite a lot to stream results asynchronously from sanic back to the user as csv data i wrote the following snippet for Python 3.

The snippet lets you reuse the same StringIo buffer over and over again.

import csv
from io import StringIO


class ArgsToCsv:
    def __init__(self, seperator=","):
        self.seperator = seperator
        self.buffer = StringIO()
        self.writer = csv.writer(self.buffer)

    def stringify(self, *args):
        self.writer.writerow(args)
        value = self.buffer.getvalue().strip("\r\n")
        self.buffer.seek(0)
        self.buffer.truncate(0)
        return value + "\n"

example:

csv_formatter = ArgsToCsv()

output += csv_formatter.stringify(
    10,
    """
    lol i have some pretty
    "freaky"
    strings right here \' yo!
    """,
    [10, 20, 30],
)

Check out further usage at the github gist: source and test

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Comments

7

I found the answers, all in all, a bit confusing. For Python 2, this usage worked for me:

import csv, io

def csv2string(data):
    si = io.BytesIO()
    cw = csv.writer(si)
    cw.writerow(data)
    return si.getvalue().strip('\r\n')

data=[1,2,'a','He said "what do you mean?"']
print csv2string(data)
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Comments

1

i adopt the answer of @user2099484 above, with a small amendment for unicode

import csv, io

def csv2string(data):
    si = io.BytesIO()
    cw = csv.writer(si)
    data = [f.encode('utf-8') if type(f) == unicode else f for f in data]
    cw.writerow(data)
    return si.getvalue().strip('\r\n')

data=[1,2,'a','He said "what do you mean?"']
print csv2string(data)
data2=[1,2,u'שלום','He said "what do you mean?"']
print csv2string(data2)
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Comments

0

Here's the version that works for utf-8. csvline2string for just one line, without linebreaks at the end, csv2string for many lines, with linebreaks:

import csv, io

def csvline2string(one_line_of_data):
    si = BytesIO.StringIO()
    cw = csv.writer(si)
    cw.writerow(one_line_of_data)
    return si.getvalue().strip('\r\n')

def csv2string(data):
    si = BytesIO.StringIO()
    cw = csv.writer(si)
    for one_line_of_data in data:
        cw.writerow(one_line_of_data)
    return si.getvalue()
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Comments

-2 
import csv
from StringIO import StringIO
with open('file.csv') as file:
    file = file.read()

stream = StringIO(file)

csv_file = csv.DictReader(stream)
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1 Comment

Code-only answers are discouraged, you should add some clarification to your answer

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