replace blank values in column in csv with python

I think you're pretty close. You need to pass the fieldnames to the writer and then you can edit the row directly, because it's simply a dictionary. For example:

with open(inputFileName, "rb") as infile, open(outputFileName, "wb") as outfile:
    r = csv.DictReader(infile)
    w = csv.DictWriter(outfile, r.fieldnames)
    w.writeheader()
    for row in r:
        if not row["Author"].strip():
            row["Author"] = "DMD"
        w.writerow(row)

turns

a,b,c,d,e,Author,g,h
1,2,3,4,5,Smith,6,7
8,9,10,11,12,Jones,13,14
13,14,15,16,17,,18,19

into

a,b,c,d,e,Author,g,h
1,2,3,4,5,Smith,6,7
8,9,10,11,12,Jones,13,14
13,14,15,16,17,DMD,18,19

I like using if not somestring.strip(): because that way it won't matter if there are no spaces, or one, or seventeen and a tab. I also prefer DictReader to the standard reader because this way you don't have to remember which column Author is living in.

[PS: The above assumes Python 2, not 3.]

Dictionaries don't need the replace method because simple assignment does this for you:

for row in r:
    if row[header-6] == "":
        row[header-6] = "DMD"
    w.writerow(row)

Where header-6 is the name of your sixth column

Also note that your call to DictReader appears to have the wrong fields attribute. That argument should be a list (or other sequence) containing all the headers of your new CSV, in order.

For your purposes, it appears to be simpler to use the vanilla reader:

import csv
inputFileName = "C:\Author.csv"
outputFileName = os.path.splitext(inputFileName)[0] + "_edited.csv"

with open(inputFileName) as infile, open(outputFileName, "w") as outfile:
    r = csv.reader(infile)
    w = csv.writer(outfile)
    w.writerow(next(r))  # Writes the header unchanged
    for row in r:
        if row[5] == "":
            row[5] = "DMD"
        w.writerow(row)

(1) to use os.path.splitest, you need to add an import os

(2) Dicts don't have a replace method; dicts aren't strings. If you're trying to alter a string that's the value of a dict entry, you need to reference that dict entry by key, e.g. row['Author']. If row['Author'] is a string (should be in your case), you can do a replace on that. Sounds like you need an intro to Python dictionaries, see for example http://www.sthurlow.com/python/lesson06/ .

(3) A way to do this, that also deals with multiple spaces, no spaces etc. in the field, would look like this:

field = 'Author'
marker = 'DMD'
....

## longhand version
candidate = str(row[field]).strip()
if candidate:
    row[field] = candidate
else:
    row[field] = marker

or

## shorthand version
row[field] = str(row[field]).strip() and str(row[field]) or marker

Cheers