write multiple lists to csv (one is a nested list)

Question

I have say three lists, and one of them is nested like so:

a = [1, 2, 3, 4]
b = ["abc", "def", "dec", "erf"]
c = [[5, 6, 7], [8, 9, 10], [11, 12, 13.3], [14, 15, 16]]

I would like a CSV file output of it which looks like this:

1,abc,5,6,7
2,def,8,9,10
3,erf,11,12,13.3
...

I tried zipping them and writing to a CSV file like so:

with open('filer.csv', "w") as f:
    writer = csv.writer(f)
    for row in zip(a, b, c):
        writer.writerow(row)

but the output has these stupid brackets like so:

1,abc,"[5,6,7]"
2,def,"[8,9,10]"
3,erf,"[11,12,13.3]"
...

but I want them without brackets and without the quotes like so:

1,abc,5,6,7
2,def,8,9,10
3,erf,11,12,13.3
...

:(

Perhaps c should use ','.join(c). So unpack the zip into individual varaibles. — s3dev
– s3dev, Commented Oct 20, 2021 at 14:31

Dani Mesejo · Accepted Answer · 2021-10-20 14:53:22Z

5

Use:

with open('filer.csv', "w") as f:
    writer = csv.writer(f)
    for ai, bi, ci in zip(a, b, c):
        writer.writerow([ai, bi, *ci])

Output

1,abc,5,6,7
2,def,8,9,10
3,dec,11,12,13.3
4,erf,14,15,16

As you said you have nested list, so you need to flatten the list, the expression [ai, bi, *ci] flattens it.

The reason that you get the brackets is that writerow is (probably) applying the function str for each element of the iterable passed to it. And the string representation of a list includes the brackets, for example:

In[3]: str([1, 2, 3])
Out[3]: '[1, 2, 3]'

edited Oct 20, 2021 at 14:53

answered Oct 20, 2021 at 14:32

Dani Mesejo

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2 Comments

JohnJ Over a year ago

thank you for solving this for me - I just spent 2 hours on this - I should really quit being a programmer! Thank you again.

Dani Mesejo Over a year ago

@JohnJ Glad I could help!

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