Python: Write nested list objects to csv file

Question

I'm trying to write data from a list of lists to a csv file. This is a simplified version of what I have

class Point(object): 
    def __init__(self, weight, height):
        self.weight = weight
        self.height = height
    def get_BMI(self):
        return (self.weight * self.height) / 42  # this is not how you calculate BMI but let's say

myList = [[Point(30, 183)],[Point(63, 153)]]

Because of the way the data is set up, I store the points in a nested loop. If I wanted to access the first point object’s BMI, I would type

myList[0][0].get_BMI()

I want to write each point's BMI to a CSV (delimited by a comma). How do I do that?

Here's how I thought but it isn't exactly straight forward:

import csv
with open('output.csv', 'w') as csvfile:
    writer = csv.writer(csvfile)
    writer.writerows(myList)

It doesn't return any error however it doesn't actually create the CSV file either. Also I want to write the values in myList[i][j].get_BMI() to file. I don't have a problem with permissions because I use Spyder (python IDE) as root. Right now I'm just running the script through the Spyder console but it should still work and output the CSV file.

Francis Colas · Accepted Answer · 2015-04-04 15:55:22Z

1

writerows expects a list of list of strings or numbers. You should start by creating a list with the BMI values so that they can get written into your csv file:

import csv
BMIs = [[point.get_BMI() for point in points] for points in myList]
with open('output.csv', 'w') as csvfile:
    writer = csv.writer(csvfile)
    writer.writerows(BMIs)

edited Apr 4, 2015 at 15:55

answered Apr 4, 2015 at 15:50

Francis Colas

3,6672 gold badges30 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

inspectorG4dget · Accepted Answer · 2015-04-04 16:03:49Z

You can do this with writerows, as it expects a list of rows - each row should be formatted as per the dialect parameter to csv.writer, which can be ignored in this case without fear of any backfiring.

So writerows can take a structure that looks like myList. The problem is that you need to access all the points and grab their BMIs (this can be done in a list comprehension)

To illustrate how writerows can be used (and to add a number to each point, so that all your rows don't have just one entry (which would be frustrating), I added the call to enumerate.

Therefore, you no longer need complex loops or any such. Enjoy:

myList = [[Point(30, 183)],[Point(63, 153)]]

with open('output.csv', 'w') as outfile:
    writer = csv.writer(outfile)
    writer.writerows(enumerate(p.get_BMI() for p in itertools.chain.from_iterable(myList)))

Glorfindel · Accepted Answer · 2021-04-20 05:50:23Z

1

There are three issues:

The nested lists must be flattened. To accomplish this, use itertools.chain.from_iterable.
The row data for the CSV must be customized. To accomplish this, use list comprehensions.
output.csv is not being created. I suspect that the output.csv is being created but being placed in an unexpected location. You could try hardcoding a full path for testing to see if this is the case.

Here is code that demonstrates #1 and #2:

import csv
from itertools import chain

with open('output.csv', 'w') as csvfile:
    writer = csv.writer(csvfile)
    flattenedList = chain.from_iterable(myList)

    writer.writerows((pt.weight, pt.height, pt.get_BMI()) for pt in flattenedList)

edited Apr 20, 2021 at 5:50

Glorfindel

22.8k13 gold badges97 silver badges124 bronze badges

answered Apr 4, 2015 at 15:56

David Anderson

8,6463 gold badges30 silver badges27 bronze badges

2 Comments

Francis Colas Over a year ago

For point 3, I believe that csv is silently ignoring object that are neither a string nor a number and then decided to not write anything.

David Anderson Over a year ago

@FrancisColas The open call just by itself will create an empty file and the objects will have a string representation such as <__main__.Point object at 0x022E02D0> that csv will use by default (via the equivalent of the str operator).

crax · Accepted Answer · 2015-04-04 16:08:26Z

-1

myList = [[Point(30, 183)],[Point(63, 153)]]

with open('output.csv', 'w') as outfile:
    writer = csv.writer(outfile,delimiter=',')
    for i in myList:
         writer.writerow([i])

edited Apr 4, 2015 at 16:08

answered Apr 4, 2015 at 16:02

crax

5662 gold badges9 silver badges20 bronze badges

Collectives™ on Stack Overflow

Python: Write nested list objects to csv file

4 Answers 4

Comments

Comments

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related