Writing to CSV files

• Write using lists or dictionaries

writer

• creates a writer object for writing to the CSV
• writerow - method on a writer to write a row to the CSV

Name,Country,Height (in cm)
Ryu,Japan,175
Ken,USA,175
Chun-Li,China,165
Guile,USA,182

from csv import writer
 
with open('fighters.csv', "w") as file:
	csv_reader = writer(file) 
	csv_reader.writerow(["Test1","Test2"])

Test1,Test2

• writerow completely overwrites because the flag of open() is "w"
• If the flag is "a"

Name,Country,Height (in cm)
Ryu,Japan,175
Ken,USA,175
Chun-Li,China,165
Guile,USA,182
Test1,Test2

Another example

"""
we're getting all data from file1, and transfer the data but UPPERCASED
"""
import csv reader, writer
 
with open('file1.csv') as file:
	csv_reader = reader(file)
	data = [[data.upper() for data in row] for row in csv_reader]
 
with open('file2.csv', "w") as file:
	csv_writer = writer(file)
	for uppercase_data in data:
		csv_writer.writerow(uppercase_data)
 

DictWriter

• DictWriter - creates a writer object for writing using dictionaries
• fieldnames - kwarg for the DictWriter specifying headers
• writeheader - method on a writer to write header row
• writerow - method on a writer to write a row based on a dictionary

from csv import DictWriter
 
with open('example.csv', 'w') as file:
	headers = ["Character", "Age"]
	csv_writer = DictWriter(file, fieldnames = headers)
	csv_writer.writeheader()
	csv_writer.writerow({
		"Character": "Kirby",
		"Age": 100
	})
	csv_writer.writerow({
		"Character": "Yoshi",
		"Age": 100
	})
 
	# if you want to print out the new file data:
	csv_reader = DictReader(file)
	file.seek(0)
	for row in csv_reader:
		print(row)

• This then creates a new file example.csv because of the w flag (and example.csv doesn’t exist:

Character,Age
Kirby,100
Yoshi,100

Examples

For this exercise, you’ll be working with a file called users.csv . Each row of data consists of two columns: a user’s first name, and a user’s last name.

Implement the following function:

update_users : Takes in an old first name, an old last name, a new first name, and a new last name. Updates the users.csv file so that any user whose first and last names match the old first and last names are updated to the new first and last names. The function should return a count of how many users were updated.

'''
update_users("Grace", "Hopper", "Hello", "World") # Users updated: 1.
update_users("Colt", "Steele", "Boba", "Fett") # Users updated: 2.
update_users("Not", "Here", "Still not", "Here") # Users updated: 0.
'''
 
from csv import reader, writer
 
def update_users(old_first, old_last, new_first, new_last):
    with open('users.csv') as f:
        csv_reader = reader(f)
        data = list(csv_reader)
        
    count = 0
    with open('users.csv', 'w') as f:
        csv_writer = writer(f)
        for user in data:
            if (user[0] == old_first) and (user[1] == old_last):
                csv_writer.writerow([new_first,new_last])
                count += 1
            else:
                csv_writer.writerow(user)
                
    return f"Users updated: {count}."

delete_users : Takes in a first name and a last name. Updates the users.csv file so that any user whose first and last names match the inputs are removed. The function should return a count of how many users were removed.

'''
delete_users("Grace", "Hopper") # Users deleted: 1.
delete_users("Colt", "Steele") # Users deleted: 2.
delete_users("Not", "Here") # Users deleted: 0.
'''
 
from csv import reader, writer
 
def delete_users(first, last):
    with open('users.csv') as f:
        csv_reader = reader(f)
        data = list(csv_reader)
        
    count = 0
    with open('users.csv', 'w') as f:
        csv_writer = writer(f)
        for user in data:
            if (user[0] == first) and (user[1] == last):
                count += 1
            else:
                csv_writer.writerow(user)
                
    return f"Users deleted: {count}."