Is it possible to keep the column order using csv.DictReader?

Question

For example, my csv has columns as below:

ID, ID2, Date, Job No, Code

I need to write the columns back in the same order. The dict jumbles the order immediately, so I believe it's more of a problem with the reader.

Answer 1

Make an OrderedDict from each row dict sorted by DictReader.fieldnames.

import csv
from collections import OrderedDict

reader = csv.DictReader(open("file.csv"))
for row in reader:
    sorted_row = OrderedDict(sorted(row.items(),
          key=lambda item: reader.fieldnames.index(item[0])))

Answer 2

from csv import DictReader, DictWriter

with open("input.csv", 'r') as input_file:
    reader = DictReader(f=input_file)
    with open("output.csv", 'w') as output_file:
        writer = DictWriter(f=output_file, fieldnames=reader.fieldnames)
        for row in reader:
            writer.writerow(row)

Answer 3

I know this question is old...but if you use DictReader, you can pass it an ordered list with the fieldnames to the fieldnames param

Answer 4

Edit: as of python 3.6 dicts are ordered by insertion order, essentially making all dicts in python OrderedDicts by default. That being said the docs say dont rely on this behaviour because it may change. I will challenge that, lets see if it ever changes back :)

---

Unfortunatley the default DictReader does not allow for overriding the dict class, a custom DictReader would do the trick though

import csv

class DictReader(csv.DictReader):
    def __init__(self, *args, **kwargs):
        self.dict_class = kwargs.pop(dict_class, dict)
        super(DictReader, self).__init__(*args, **kwargs)

    def __next__(self):
        ''' copied from python source '''
        if self.line_num == 0:
            # Used only for its side effect.
            self.fieldnames
        row = next(self.reader)
        self.line_num = self.reader.line_num

        # unlike the basic reader, we prefer not to return blanks,
        # because we will typically wind up with a dict full of None
        # values
        while row == []:
            row = next(self.reader)
        # using the customized dict_class
        d = self.dict_class(zip(self.fieldnames, row))
        lf = len(self.fieldnames)
        lr = len(row)
        if lf < lr:
            d[self.restkey] = row[lf:]
        elif lf > lr:
            for key in self.fieldnames[lr:]:
                d[key] = self.restval
        return d

use it like so

import collections

csv_reader = DictReader(f, dict_class=collections.OrderedDict)
# ...

Answer 5

I wrote a little tool to sort the order of CSV columns: I don't claim that it's great I know little of Python, but it does the job:

import csv
import sys

with open(sys.argv[1], 'r') as infile:
    csvReader = csv.DictReader(infile)
    sorted_fieldnames = sorted(csvReader.fieldnames)
    writer = csv.DictWriter(sys.stdout, fieldnames=sorted_fieldnames)
    # reorder the header first
    writer.writeheader()
    for row in csvReader:
        # writes the reordered rows to the new file
        writer.writerow(row)