Write a dictionary of lists to csv in Python

Question

I have a dictionary like {a: [1, 2, 3], b: [4, 5, 6], c: [7, 3, 2]} and I would like to write this to a csv file in Python. I was the csv file as:

col1,col2,col3,col4
a,1,2,3
b,4,5,6
c,7,3,2

I tried using CSV Dict writer but I could not get as value is a list here.

Any help is appreciated!

do you mean `{'a':[1,2,3],...}` ? or is `a` really unquoted? — Joran Beasley, Mar 23 '16 at 19:02
Well, a ***.csv** file is just a text file. You can just try iterating through a dictionary and building a string. Later on, just write this string to the file using `open()` function. Your string should look like: `col1;col2\n 1;4\n 2;5\n 3;6\n` — Laszlowaty, Mar 23 '16 at 19:03
try `pandas` `DataFrame.from_csv`: http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.from_csv.html — n1c9, Mar 23 '16 at 19:04
@JoranBeasley yes it is a string. it should be like {'a':[1,2,3]}.But I want this key and list of values to be in a single row in different columns — Cool Geek, Mar 23 '16 at 19:05

Martin Evans · Answer 1 · 2021-07-19T08:52:03.233

You can use Python's csv library for doing this as follows (for Python 3.x see below):

import csv

my_dict = {"a" : [1, 2, 3], "b" : [4, 5, 6], "c" : [7, 3, 2]}

with open("output.csv", "wb") as f_output:
    csv_output = csv.writer(f_output)
    csv_output.writerow(['col1', 'col2', 'col3', 'col4'])
    
    for key in sorted(my_dict.keys()):
        csv_output.writerow([key] + my_dict[key])

Giving you:

col1,col2,col3,col4
a,1,2,3
b,4,5,6
c,7,3,2

The .writerow() function takes a list of items and writes them correctly formatted to your output CSV file. As you want the key as the first item, it passes the key concatenated with them items found when looking the key up in the my_dict dictionary, thus resulting in the output that you wanted.

Note: If Python 3.x is used, the file open line would need to be modified, and sorting would not be required:

with open("output.csv", "w", newline="") as f_output:

score 0 · Answer 2 · answered Mar 23 '16 at 19:07

with open("out.csv","wb") as csv_out:
    # first the header
    csv_out.write(",".join("Col%d"%i for i in range(len(a_dict.values()[0]))+"\n")
    # then each row .... make sure its sorted by the keys ...
    for key,a_list in sorted(a_dict.items(),key=lambda row:row[0])
        x = [key,]
        x.extend(a_list)
        csv_out.write(",".join(str(i) for i in x)

score 0 · Answer 3 · answered Mar 23 '16 at 19:07

0

Hope following code can help:

fo = open("example.csv", "wr")
fo.write("col1,col2,col3,col4\n")
fo.write("a,1,2,3\n")
fo.write("b,4,5,6\n")
fo.write("c,7,8,9\n")
fo.close()

answered Mar 23 '16 at 19:07

Yi Chen

1
4

2

what is `"wr"` im pretty sure thats not legal ... – Joran Beasley Mar 23 '16 at 19:08

score 0 · Answer 4 · answered Mar 23 '16 at 19:08

>>> mydict
{'b': [4, 5, 6], 'c': [7, 3, 2], 'a': [1, 2, 3]}
>>> result ="\n".join(["{0} {1}".format(key, " ".join([str(x) for x in list_value])) for key, list_value in mydict.iteritems()])
>>> print result
b 4 5 6
c 7 3 2
a 1 2 3

Writes result into a csv file.

If the order matters, you can use OrderedDict

score 0 · Answer 5 · answered Mar 23 '16 at 19:31

d={'a': [1, 2, 3],'b': [4, 5, 6], 'c': [7, 3, 2]}
with open("result.txt","w+") as f:
    for k,v in d.items():
            alist=[k]+[str(r) for r in v]#alist=['a','1','2','3']
            line=' '.join(alist)+"\n"#line="a  1 2 3\n"
            f.write(line)

score 0 · Answer 6 · answered Mar 23 '16 at 19:35

Just for practice, here's another version;

mydict = {'a': [1, 2, 3], 'c': [7, 3, 2], 'b': [4, 5, 6]}

with open('test.csv','w') as f:
  for k,v in mydict.iteritems():
    f.write(str.join(',', [k]+[str(i) for i in v])+'\n')

It just loops though the dictionary items, generating a row for each.

Each row is generated by creating an array with the key as first element with the values converted to strings following. It then just joins the elements of the array using a comma separator.

Write a dictionary of lists to csv in Python

6 Answers6