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.

Answer 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)

Answer 2

There are three issues:

1. The nested lists must be flattened. To accomplish this, use itertools.chain.from_iterable .

2. The row data for the CSV must be customized. To accomplish this, use list comprehensions .

3. 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)

Answer 3

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)))

Answer 4

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])