CODEWITHSUNDEEP
pythoncsv
Alex
Alex

Reputation: 1370

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

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.

Upvotes: 42

Views: 32624

Answers (6)

renoX
renoX

Reputation: 11

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)

Upvotes: 0

Pykler
Pykler

Reputation: 14865

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)
# ...

Upvotes: 2

Alex Martelli
Alex Martelli

Reputation: 882281

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.

Upvotes: 66

xvan
xvan

Reputation: 4855

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])))

Upvotes: 10

Pablo K
Pablo K

Reputation: 125

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

Upvotes: 2

khatchad
khatchad

Reputation: 3177

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)

Upvotes: 9

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