Deleting Rows from CSV file using DictWriter

Question

Here is a very basic sample of what I'm looking to do I want to delete a row (or set of rows, using a list) with the DictWriter library but I don't know the syntax for row deletion This is what I have so far

import csv

filename = "sampletable.csv"
with open(filename, 'r') as csvfile: 
    csvreader = csv.DictReader(csvfile)
    
    with open("samplerowdeletion.csv", "w") as newfile:
        fieldnames = ["high","low","precipitation"]
        csvwriter = csv.DictWriter(newfile, fieldnames=fieldnames)
        csvwriter.writeheader() 
        
        for row in csvreader:
            del(row[1])

Answer 1

Reading from one csv and writing some of the rows to another csv is an example of filtering . The easiest way to do this is by skipping the rows that you don't want:

for row in reader:
    if row['high'] == '10':
        # go back to the top of the loop
        continue
    writer.writerow(row)

Instead of hardcoding the check, you could write a function that will return True if a row should be kept and False otherwise (this kind of function is known as a predicate ):

def is_ok(row):
    if row['high'] in list_of_bad_values:
        return False
    return True

for row in reader:
    if not is_ok(row):
        # go back to the top of the loop
        continue
    writer.writerow(row)

You avoid the explicit for loop by using Python's filter built-in function, or a list comprehension :

def is_ok(row):
    # Shorter version
    return not (row['high'] in list_of_bad_values)
 
# Using filter
good_rows = filter(is_ok, reader)
writer.writerows(list(good_rows))

# Using a list comprehension
good_rows = [row for row in reader if is_ok(row)]
writer.writerows(rows)

If you want to filter by row number, you can use the enumerate built-in function. enumerate will generate a row number for each row, starting at zero - you can adjust this by passing a start argument:

enumerate(some_iterable, start=1)

import csv

ROWS_TO_DELETE = [3, 4]

def is_not_ok(row_number):
    return row_number in ROWS_TO_DELETE
    

filename = "sampletable.csv"
with open(filename, 'r', newline=") as csvfile: 
    csvreader = csv.DictReader(csvfile)
    
    with open("samplerowdeletion.csv", "w", newline=") as newfile:
        fieldnames = ["high","low","precipitation"]
        csvwriter = csv.DictWriter(newfile, fieldnames=fieldnames)
        csvwriter.writeheader() 
        
        for row_number, row in enumerate(csvreader):
            if is_not_ok(row_number):
                continue
            writer.writerow(row)

Alternatively, you could make a row counter variable and manually increment it

filename = "sampletable.csv"
with open(filename, 'r', newline=") as csvfile: 
    csvreader = csv.DictReader(csvfile)
    
    with open("samplerowdeletion.csv", "w", newline=") as newfile:
        fieldnames = ["high","low","precipitation"]
        csvwriter = csv.DictWriter(newfile, fieldnames=fieldnames)
        csvwriter.writeheader() 
        
        row_number = 0
        for row in csvreader:
            if row_number in ROWS_TO_DELETE:
                continue
            writer.writerow(row)
            row_number += 1