Write to CSV file Column by Column from For loop output

Question

I want to create a csv file as follows with 2 column headers as "Y1" and "Y2"

Y1 Y2 
i1 z1
i2 z2
i3 z3

The code I developed is as follows:

with open(r"./file.csv", "a") as f1:
    writer = csv.writer(f1, delimiter="\t", lineterminator="\n")
    row = []
    row_1 = []
    for i in test_data["data"][1]:
        row.append(i)

    for z in test_results:
        row_1.append(z)

    writer.writerow([row, row_1])

I am only getting rows and not the i and z in a different columns. How can write to the csv and get the i and z next to each other in 2 different columns

So, are test_data['data'][1] and test_results the same size? If so, you can just zip them. I'll show in an answer. — Tim Roberts
– Tim Roberts, Commented Sep 14, 2022 at 17:21

Tim Roberts · Accepted Answer · 2022-09-14 17:23:09Z

2

The "zip" here combines the two lists, so that row is a 2-tuple with one column from each. That's exactly what writerow wants.

with open(r'./file.csv','a') as f1:
    writer=csv.writer(f1, delimiter='\t',lineterminator='\n',)
    for row in zip(test_data['data'][1], test_results):
        writer.writerow(row)

You might even be able to do it in one shot:

with open(r'./file.csv','a') as f1:
    writer=csv.writer(f1, delimiter='\t',lineterminator='\n',)
    writer.writerows( zip(test_data['data'][1], test_results) )

Note "writerows" instead of "writerow".

answered Sep 14, 2022 at 17:23

Tim Roberts

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Comments

Rob Wells · Accepted Answer · 2022-09-14 17:28:20Z

0

And please make sure that you adhere to the RFC-4180 format for CSV files.

It can save you a whole lot of heartache for many edge cases for data containing apostrophes, commas, and quotation marks.

answered Sep 14, 2022 at 17:28

Rob Wells

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2 Comments

Zach Young Over a year ago

How do you see OP not adhering to RFC-4180... they're using the csv module which is compliant, what's missing?

Rob Wells Over a year ago

If its compliant then all good. I was just highlighting to ensure they are aware. Thanks for the update @ZachYoung

Zach Young · Accepted Answer · 2022-09-14 18:39:33Z

It's hard to tell without the actual input data, or the actual output, what's going on.

I mocked up some input data like this to satisfy for i in test_data["data"][1]:

test_data = {
    "data": [
        "foo",
        ["Y1", "i1", "i2", "i3"],
    ]
}

and for z in test_results:

test_results = ["Y2", "z1", "z2", "z3"]

Now, when I run your code as-is, I get an output CSV that looks like:

['Y1', 'i1', 'i2', 'i3']    ['Y2', 'z1', 'z2', 'z3']

(I assume your header isn't in the actual input, but I included it just to make your small example code work.)

Following @Tim Roberts' suggestion, using zip() is probably the most correct way to join your two columns:

rows = zip(test_data["data"][1], test_results)

print(list(rows))

and we can see that the values from data and results have been properly joined:

[
    ("Y1", "Y2"),
    ("i1", "z1"),
    ("i2", "z2"),
    ("i3", "z3"),
]

From here, using the csv writer is just a few lines of code:

with open("output.csv", "w", newline="") as f_out:
    writer = csv.writer(f_out, delimiter="\t", lineterminator="\n")
    writer.writerows(rows)

and I get:

Y1  Y2
i1  z1
i2  z2
i3  z3

To me, the most important concept to take away is get your data set up first (like we can see in that print(list(rows)) statement), then do the CSV writing stuff.

Here's the complete program:

import csv

test_data = {
    "data": [
        "foo",
        ["Y1", "i1", "i2", "i3"],
    ]
}

test_results = ["Y2", "z1", "z2", "z3"]

rows = zip(test_data["data"][1], test_results)

with open("output.csv", "w", newline="") as f_out:
    writer = csv.writer(f_out, delimiter="\t", lineterminator="\n")
    # writer.writerow(["Y1", "Y2"])  # if you need to write your header manually
    writer.writerows(rows)

I also don't see a need to append: get your data composed and sorted, then just "write" to the CSV file, once.

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