Python: add column to CSV file based on existing column

Question

I already have written what I need for identifying and parsing the value I am seeking, I need help writing a column to the csv file (or a new csv file) with the parsed value. Here's some pseudocode / somewhat realistic Python code for what I am trying to do:

# Given a CSV file, this function creates a new CSV file with all values parsed
def handleCSVfile(csvfile):
  with open(csvfile, 'rb') as file:
    reader = csv.reader(file, delimiter=',', lineterminator='\n')
    for row in reader:
        for field in row:
          if isWhatIWant(field):
            parsedValue = parse(field)
            # write new column to row containing parsed value

I've already written the isWhatIWant and parse functions. If I need to write a completely new csv file, then I am not sure how to have both open simultaneously and read and write from one into the other.

Answer 1

I'd do it like this. I'm guessing that isWhatIWant() is something that is supposed to replace a field in-place.

import csv

def handleCSVfile(infilename, outfilename):
    with open(infilename, 'rb') as infile:
        with open(outfilename, 'wb') as outfile:
            reader = csv.reader(infile, lineterminator='\n')
            writer = csv.writer(outfile, lineterminator='\n')

            for row in reader:
                for field_index, field in enumerate(row):
                    if isWhatIWant(field):
                        row[field_index] = parse(field)
                writer.writerow(row)

This sort of pattern occurs a lot and results in really long lines. It can sometimes be helpful to break out the logic from opening and files into a different function, like this:

import csv

def load_save_csvfile(infilename, outfilename):
    with open(infilename, 'rb') as infile:
        with open(outfilename, 'wb') as outfile:
            reader = csv.reader(infile, lineterminator='\n')
            writer = csv.writer(outfile, lineterminator='\n')

            read_write_csvfile(reader, writer)

def read_write_csvfile(reader, writer)
    for row in reader:
        for field_index, field in enumerate(row):
            if isWhatIWant(field):
                row[field_index] = parse(field)
        writer.writerow(row)

This modularizes the code, making it easier for you to change the way the files and formats are handled from the logic independently from each other.

Additional hints:

• Don't name variables file as that is a built-in function. Shadowing those names will bite you when you least expect it.
• delimiter=',' is the default so you don't need to specify it explicitly.