Making a function to save a list to a .txt file

Question

I am trying to write a function which saves a list created by reading a .txt file back in its original format.

Code which generates the list in this format:

(datetime.datetime(2014, 2, 28, 0, 0), 'tutorial signons')

From the format of the .txt file:

"tutorial signons,28/02/2014"

DATE_FORMAT = '%d/%m/%Y'
import datetime
def as_datetime(date_string):
    try:
        return datetime.datetime.strptime(date_string, DATE_FORMAT)
    except ValueError:
        # The date string was invalid
        return None

def load_list(filename):
    new_list = []
    with open(filename, 'rU') as f:
        for line in f:
            task, date = line.rsplit(',', 1)
            try:
                # strip to remove any extra whitespace or new lines
                date = datetime.datetime.strptime(date.strip(), DATE_FORMAT)
            except ValueError:
                continue #go to next line
            new_list.append((date,task))
    return new_list

Function attempting to save "todolist" as a new "filename":

def save_list(todolist, filename):
    with open('todo.txt', 'w') as f:
        print>>f.write("\n".join(todolist))

Using the desired input:

>>>save_list(load_list('todo.txt'), 'todo2.txt')

Returns this error:

Traceback (most recent call last):
  File "<pyshell#127>", line 1, in <module>
    save_list(load_list('todo.txt'), 'todo2.txt')
  File "testing.py", line 32, in save_list
    print>>f.write("\n".join(todolist))
TypeError: sequence item 0: expected string, tuple found

It is expecting a string, how would I change this to print the tuple in the original format?

Answer 1

The exception clearly identifies your problem:

  File "C:\University\CSSE1001\assignment 1\testing.py", line 21, in load_list
    task, date = line.rsplit(',', 1)
ValueError: need more than 1 value to unpack

The data you are reading doesn't contain a comma. Hence, when you split it by a comma, you receive only one value and this can be assigned to a tuple.

Please examine your input data. You can also add following line:

try:
  task, data = line.rsplit(',', 1)
except ValueError as e:
  print "Coultn't parse:", line
  raise e

to display what line is a problem.

Answer 2

Working code calling two functions:

def as_datetime(date_string):
    try:
        return datetime.datetime.strptime(date_string, DATE_FORMAT)
    except ValueError:
        # The date string was invalid
        return None

def as_date_string(date):
    return date.strftime(DATE_FORMAT)


def load_list(filename):
    new_list = []
    with open(filename, 'rU') as f:
        for line in f:
            task, date = line.rsplit(',', 1)
            try:
                # strip to remove any extra whitespace or new lines
                dates = as_datetime(date.strip())
            except ValueError:
                continue #go to next line
            new_list.append((dates,task))
    return new_list

def save_list(todolist, filename):
    with open(filename, 'w') as f:
        for date, task in todolist:
            dates = as_date_string(date)
            f.write("%s,%s\n" % (task, dates))

'date', 'task' were back to front in the for loop. Used 'dates' as a new variable.