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Writing a Python list of lists to a csv file

I have a long list of lists of the following form ---

a = [[1.2,'abc',3],[1.2,'werew',4],........,[1.4,'qew',2]]

i.e. the values in the list are of different types -- float,int, strings.How do I write it into a csv file so that my output csv file looks like

1.2,abc,3
1.2,werew,4
.
.
.
1.4,qew,2

b
bucky

Python's built-in CSV module can handle this easily:

import csv

with open("output.csv", "wb") as f:
    writer = csv.writer(f)
    writer.writerows(a)

This assumes your list is defined as a, as it is in your question. You can tweak the exact format of the output CSV via the various optional parameters to csv.writer() as documented in the library reference page linked above.

Update for Python 3

import csv

with open("out.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(a)

For Python 3 compatibility, remove the "b" from "wb".
With Python 3 - open('output.csv', 'w', newline=''). I get an extra line if I omit the newline parameter. docs.python.org/3/library/csv.html#csv.writer
In python3 I had to use open('output.csv', 'w', newline="")
WOW that python 3 error is very unhelpful. Thanks @vladV (a bytes-like object is required, not 'str'). It kinda makes sense in hindsight, but not informative of where to look at all.
@tlalco that probably means you're using Python 2, in which case you should use the first code block rather than the second. (It also means you should consider switching to Python 3.)
A
Akavall

You could use pandas:

In [1]: import pandas as pd

In [2]: a = [[1.2,'abc',3],[1.2,'werew',4],[1.4,'qew',2]]

In [3]: my_df = pd.DataFrame(a)

In [4]: my_df.to_csv('my_csv.csv', index=False, header=False)

I don't think should use pandas if built-in library csv can do it.
i like pandas because its powerful
pandas is powerful, sure, but I'm not going to use a McLaren to drive to the corner store next door.
Note: For cases, where the list of lists is very large, sequential csv write operations can make the code significantly slower. In such cases using pandas would be wiser.
M
Mike H-R
import csv
with open(file_path, 'a') as outcsv:   
    #configure writer to write standard csv file
    writer = csv.writer(outcsv, delimiter=',', quotechar='|', quoting=csv.QUOTE_MINIMAL, lineterminator='\n')
    writer.writerow(['number', 'text', 'number'])
    for item in list:
        #Write item to outcsv
        writer.writerow([item[0], item[1], item[2]])

official docs: http://docs.python.org/2/library/csv.html


This would get my +1 if you could explain your answer with some comments.
writerow does not take multiple arguments.
>>> w.writerow("a", "b", "c") Traceback (most recent call last): File "", line 1, in TypeError: writerow() takes exactly one argument (3 given)
@Amber what version of python do you use?
@Amber excuse me. i missed []
D
Dinidiniz

Using csv.writer in my very large list took quite a time. I decided to use pandas, it was faster and more easy to control and understand:

 import pandas

 yourlist = [[...],...,[...]]
 pd = pandas.DataFrame(yourlist)
 pd.to_csv("mylist.csv")

The good part you can change somethings to make a better csv file:

 yourlist = [[...],...,[...]]
 columns = ["abcd","bcde","cdef"] #a csv with 3 columns
 index = [i[0] for i in yourlist] #first element of every list in yourlist
 not_index_list = [i[1:] for i in yourlist]
 pd = pandas.DataFrame(not_index_list, columns = columns, index = index)

 #Now you have a csv with columns and index:
 pd.to_csv("mylist.csv")

t
tegan

If for whatever reason you wanted to do it manually (without using a module like csv,pandas,numpy etc.):

with open('myfile.csv','w') as f:
    for sublist in mylist:
        for item in sublist:
            f.write(item + ',')
        f.write('\n')

Of course, rolling your own version can be error-prone and inefficient ... that's usually why there's a module for that. But sometimes writing your own can help you understand how they work, and sometimes it's just easier.


f
fabda01

If you don't want to import csv module for that, you can write a list of lists to a csv file using only Python built-ins

with open("output.csv", "w") as f:
    for row in a:
        f.write("%s\n" % ','.join(str(col) for col in row))

S
Semjon Mössinger

Ambers's solution also works well for numpy arrays:

from pylab import *
import csv

array_=arange(0,10,1)
list_=[array_,array_*2,array_*3]
with open("output.csv", "wb") as f:
    writer = csv.writer(f)
    writer.writerows(list_)

G
Good Will

How about dumping the list of list into pickle and restoring it with pickle module? It's quite convenient.

>>> import pickle
>>> 
>>> mylist = [1, 'foo', 'bar', {1, 2, 3}, [ [1,4,2,6], [3,6,0,10]]]
>>> with open('mylist', 'wb') as f:
...     pickle.dump(mylist, f) 


>>> with open('mylist', 'rb') as f:
...      mylist = pickle.load(f)
>>> mylist
[1, 'foo', 'bar', {1, 2, 3}, [[1, 4, 2, 6], [3, 6, 0, 10]]]
>>> 

J
Jonas Czech

Make sure to indicate lineterinator='\n' when create the writer; otherwise, an extra empty line might be written into file after each data line when data sources are from other csv file...

Here is my solution:

with open('csvfile', 'a') as csvfile:
    spamwriter = csv.writer(csvfile, delimiter='    ',quotechar='|', quoting=csv.QUOTE_MINIMAL, lineterminator='\n')
for i in range(0, len(data)):
    spamwriter.writerow(data[i])

J
Jie

I got an error message when following the examples with a newline parameter in the csv.writer function. The following code worked for me.

 with open(strFileName, "w") as f:
    writer = csv.writer(f, delimiter=',',  quoting=csv.QUOTE_MINIMAL)
    writer.writerows(result)

I
IAmBotmaker

In case of exporting lll list of lists of lists to .csv, this will work in Python3:

import csv
with open("output.csv", "w") as f:
    writer = csv.writer(f)
    for element in lll:
        writer.writerows(element)

P
PanDe

Didn't see even a single answer on this page that includes how to include header as well to create the file. Here is a method incorporating that as well. Method works great with python 3

csv_filename = 'abc.csv'
fieldnames = ['Col_1','Col_2','Col_3','Col_4']
with open(csv_filename, mode='w') as csv_file:
    writer = csv.DictWriter(csv_file, fieldnames=fieldnames)
    writer.writeheader()
    for i in my_list:
        writer.writerow({fieldnames[0]: i[0], fieldnames[1]: i[1], fieldnames[2]: i[2],fieldnames[3]: i[3]})