Python: CSV write by column rather than row

Question

I have a python script that generates a bunch of data in a while loop. I need to write this data to a CSV file, so it writes by column rather than row.

For example in

Answer 1

wr.writerow(item)  #column by column
wr.writerows(item) #row by row

This is quite simple if your goal is just to write the output column by column.

If your item is a list:

yourList = []

with open('yourNewFileName.csv', 'w', ) as myfile:
    wr = csv.writer(myfile, quoting=csv.QUOTE_ALL)
    for word in yourList:
        wr.writerow([word])

Answer 2

Updating lines in place in a file is not supported on most file system (a line in a file is just some data that ends with newline, the next line start just after that).

As I see it you have two options:

1. Have your data generating loops be generators, this way they won't consume a lot of memory - you'll get data for each row "just in time"
2. Use a database (sqlite?) and update the rows there. When you're done - export to CSV

Small example for the first method:

from itertools import islice, izip, count
print list(islice(izip(count(1), count(2), count(3)), 10))

This will print

[(1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 6), (5, 6, 7), (6, 7, 8), (7, 8, 9), (8, 9, 10), (9, 10, 11), (10, 11, 12)]

even though count generate an infinite sequence of numbers

Answer 3

As an alternate streaming approach:

• dump each col into a file
• use python or unix paste command to rejoin on tab, csv, whatever.

Both steps should handle steaming just fine.

Pitfalls:

• if you have 1000s of columns, you might run into the unix file handle limit!

Answer 4

what about Result_* there also are generated in the loop (because i don't think it's possible to add to the csv file)

i will go like this ; generate all the data at one rotate the matrix write in the file:

A = []

A.append(range(1, 5))  # an Example of you first loop

A.append(range(5, 9))  # an Example of you second loop

data_to_write = zip(*A)

# then you can write now row by row

Answer 5

The reason csv doesn't support that is because variable-length lines are not really supported on most filesystems. What you should do instead is collect all the data in lists, then call zip() on them to transpose them after.

>>> l = [('Result_1', 'Result_2', 'Result_3', 'Result_4'), (1, 2, 3, 4), (5, 6, 7, 8)]
>>> zip(*l)
[('Result_1', 1, 5), ('Result_2', 2, 6), ('Result_3', 3, 7), ('Result_4', 4, 8)]

Answer 6

Read it in by row and then transpose it in the command line. If you're using Unix, install csvtool and follow the directions in: https://unix.stackexchange.com/a/314482/186237