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

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

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 4

Python's dicts do NOT maintain order prior to 3.6 (but, regardless, in that version the csv.DictReader class was modified to return OrderedDicts).

However, the instance of csv.DictReader that you're using (after you've read the first row!-) does have a .fieldnames list of strings, which IS in order.

So,

for rowdict in myReader:
  print ['%s:%s' % (f, rowdict[f]) for f in myReader.fieldnames]

will show you that the order is indeed maintained (in .fieldnames of course, NEVER in the dict -- that's intrinsically impossible in Python!-).

So, suppose you want to read a.csv and write b.csv with the same column order. Using plain reader and writer is too easy, so you want to use the Dict varieties instead;-). Well, one way is...:

import csv

a = open('a.csv', 'r')
b = open('b.csv', 'w')
ra = csv.DictReader(a)
wb = csv.DictWriter(b, None)

for d in ra:

  if wb.fieldnames is None:
    # initialize and write b's headers
    dh = dict((h, h) for h in ra.fieldnames)
    wb.fieldnames = ra.fieldnames
    wb.writerow(dh)

  wb.writerow(d)

b.close()
a.close()

assuming you have headers in a.csv (otherewise you can't use a DictReader on it) and want just the same headers in b.csv.

Answer 5

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