Python: How to add values obtained from an operation in a new column of given CSV?

Question

i have a sample CSV given as follows:

Name,Birthdate,Age,Address,Zone
ABC,3-10-2016 11:00:00AM,21,XYZ Street 21, zone
BCD,3-11-2016 15:54:00PM,22,WXY Street 21/A, S zone
CDW,4-11-2015 21:09:00PM,22,ZYX Street 21Avenue, North Zone

i have used the following code to obtain the weekday from a given date in the data:

import csv
from datetime import datetime


with open(filename, 'rb') as infile:
    reader = csv.DictReader(infile)
    for row in reader:
         birthdate = row['Birthdate']  # keys are named in the first row of your CSV
         birthdate = datetime.strptime(birthdate, '%m-%d-%Y %H:%M:%S%p')
         print birthdate.strftime('%A')

The output was:

Thursday
Friday
Saturday

i want to append this output into the same CSV with a new column name as weekday .

NOTE: the above code and data are all sampled and original data is way different.

See Add columns to CSV while writing the CSV for how you could do in-place CSV updating. — Martijn Pieters
– Martijn Pieters, Commented Mar 11, 2016 at 11:22
@MartijnPieters The solution didn't helped me much though. But i am in a seek of some base script which can write the obtained values into a new column. — desmond.carros
– desmond.carros, Commented Mar 11, 2016 at 12:40

Tim007 · Accepted Answer · 2016-03-11 11:48:59Z

2

Just for fun

import re
from datetime import datetime as dt

def changedate(birthdate):
    weekday = dt.strptime(birthdate, '%m-%d-%Y %H:%M:%S%p').strftime('%A')
    return birthdate + ',' + weekday

str = open(filename).read()
str = re.sub(r'Name,Birthdate,Age,Address','Name,Birthdate,Weekday,Age,Address', str)
str = re.sub(r'(\d+-\d+-\d+\s\d+:\d+:\d+\w{2})',lambda m: changedate(m.group()), str)

with open(filename, 'w') as f:
    f.write(str)

answered Mar 11, 2016 at 11:48

Tim007

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6 Comments

desmond.carros Over a year ago

what if we don't want to use lambda? any generic and simple way to do it?

Tim007 Over a year ago

Hundred ways to do that stackoverflow.com/search?q=Add+columns+to+CSV+python choose whatever you like.

desmond.carros Over a year ago

i used your approach for better results. thanks much. but when i re run this script then a new weekday column is also created. that as if i run this script again and again then weekday will be added as many times as i run the script. ANy way to control this?

Tim007 Over a year ago

p = re.compile(ur'(\d+-\d+-\d+\s\d{2}:\d{2}:\d{2}\w{2})(?=(?:,[^,]+){3}$)', re.MULTILINE)  str = re.sub(p, lambda m: changedate(m.group()), str)

add (?:,[^,]+){3} for counting number of columns after date column

desmond.carros Over a year ago

please update the answer for better understanding. thanks. :)

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