import csv

account_password = ""

with open("accounts.csv") as csvfile:
    reader = csv.reader(csvfile)

    while True:
        username = input("nEnter username: ")
        # Checks if username exists
        for row in reader:
            if row[0] == username:
                account_password = row[1] # Get user's password
                break

        password = input("Enter password: ")

        # Check if password is valid
        if password == account_password:
            break
        else:
            print("Username/password is incorrect. Try again.")


print("nSuccessfully logged in!")

Tim,myPassword
John,monkey32
Fred,WooHoo!