Saving list from python to text file in format

Question

Say I have a txt file,

fantasy,12/03/2014
sci-fi,13/04/2014
history,03/04/2014

And I use the following code to convert it into a python list

def load_list(filename):
    my_file = open(filename, "rU")
    my_list = []
    for line in my_file:
        a,b = line.rstrip("\n").split(",")
        my_list.append((as_datetime(b),a))
    my_file.close()
    return my_list

which outputs [(datetime.datetime(2014, 3, 12, 0, 0), 'fantasy'), (datetime.datetime(2014, 4, 3, 0,
0), 'history'), (datetime.datetime(2014, 4, 12, 0, 0), 'sci-fi')].

I can assume that the list will be further modified (appended and removed) while being used. My question is, how can I implement a function to export the list as a txt file that followed the formatting of the original file?

alecxe · Accepted Answer · 2014-04-06 14:30:20Z

1

You can use csv module for reading and writing.

In order to follow the date format, use strptime() and strftime() (docs) from the datetime module.

Here's a complete example:

import csv
from datetime import datetime

FORMAT = '%d/%m/%Y'

def load_list(filename):
    my_list = []
    with open(filename, "rU") as f:
        reader = csv.reader(f)
        for line in reader:
            my_list.append([line[0], datetime.strptime(line[1], FORMAT)])
    return my_list


def save_list(filename, my_list):
    with open(filename, "w") as f:
        writer = csv.writer(f)
        for item in my_list:
            writer.writerow([item[0], item[1].strftime(FORMAT)])


my_list = load_list('input.txt')
save_list('output.txt', my_list)

It reads the data from input.txt and writes it to output.txt. After running the script, output.txt contains the same data as input.txt:

fantasy,12/03/2014
sci-fi,13/04/2014
history,03/04/2014

Hope that helps.

answered Apr 6, 2014 at 14:30

alecxe

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6 Comments

Chrystael Over a year ago

One more question - instead of the from datetime import datetime that you are using, I have been given seperate functions to convert dates to and from datetime. the function that converts the string into datetime works fine ( my_list.append([as_datetime(line[1]), line[0]]), but converting the datetime into a string in the save_list function is giving an error. The function for converting datetime to string is def as_date_string(date): return date.strftime(DATE_FORMAT) is not working when I call it like this writer.writerow([item[0], as_date_string(item[1])])

alecxe Over a year ago

@Chrystael ok, what error are you getting? What is the value of a datetime at the point? What is DATE_FORMAT?

Chrystael Over a year ago

This is the entire error:

Traceback (most recent call last):   File "<pyshell#12>", line 1, in <module>     save_list("outputtest.txt", my_list)   File "C:\Users\anon\Desktop\Python 1\assign1test.py", line 53, in save_list     writer.writerow([item[0], as_date_string(item[1])])   File "C:\Users\anon\Desktop\Python 1\assign1test.py", line 30, in as_date_string     return date.strftime(DATE_FORMAT) AttributeError: 'str' object has no attribute 'strftime'

DATE_FORMAT is simply FORMAT from your solution.

alecxe Over a year ago

@Chrystael the error says that date is a string at the point, not datetime. Cannot say more without seeing the code you are using.

Chrystael Over a year ago

The difference being that instead of using from datetime import datetime, I am using import datetime and the seperate functions def as_datetime(date_string): try: return datetime.datetime.strptime(date_string, DATE_FORMAT) and def as_date_string(date): return date.strftime(DATE_FORMAT) to change the date between a string and a datetime.

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