todo.py

#!/usr/bin/env python
import sys
from datetime import datetime
HELP_MESSAGE = '''Usage :-
$ ./todo add "todo item"  # Add a new todo
$ ./todo ls               # Show remaining todos
$ ./todo del NUMBER       # Delete a todo
$ ./todo done NUMBER      # Complete a todo
$ ./todo help             # Show usage
$ ./todo report           # Statistics'''

TODO_FILE = "todo.txt"
DONE_FILE = "done.txt"
now = datetime.now()

def add(task):
    # Add task to todo.txt file

    if(task == ""):
        print("Error: Missing todo string. Nothing added!")
        return

    with open(TODO_FILE, 'a') as fd:
        fd.write(task)
        fd.write("\n")
    print(f'Added todo: "{task}"')

def ls(_):
    NO_PENDING_TASKS_MESSAGE = "There are no pending todos!"
    try:
        with open(TODO_FILE) as fd:
            todo_list = fd.readlines() #reads the todo.txt file and saves the tasks in a list

            list_size = len(todo_list)

            if(list_size==0):
                print(NO_PENDING_TASKS_MESSAGE)

            for index, todo_list in enumerate(reversed(todo_list)):
                x = list_size-index
                x = str(x)
                string_var = '[' + x + ']' + " " + todo_list.strip() + "\n"
                sys.stdout.buffer.write(string_var.encode('utf8')) 
                #print(f"[{list_size - index}] {todo_list.strip()}")
    except FileNotFoundError:
        print(NO_PENDING_TASKS_MESSAGE)


def delete(task):
    if task =="":
        print("Error: Missing NUMBER for deleting todo.")
        return
    task = int(task)
    with open(TODO_FILE) as fd:
        todo_list = fd.readlines()
        if len(todo_list) < task or task<=0:
          print(f"Error: todo #{task} does not exist. Nothing deleted.") 
          return
        task = task-1
    with open(TODO_FILE, 'w') as fd:
        for pos, todo_list in enumerate(todo_list): 
          if pos != task:
            fd.write(todo_list)
    print(f"Deleted todo #{task+1}")


def done(task):
   if task =="":
        print("Error: Missing NUMBER for marking todo as done.")
        return
   with open(TODO_FILE) as fd:
        todo_list = fd.readlines()
        task = int(task)
        if len(todo_list) < task or task<=0:
          print(f"Error: todo #{task} does not exist.")
          return
        
        task = task-1
   with open(DONE_FILE, 'a') as fd:
        fd.write("x ")
        fd.write(now.strftime("%Y-%m-%d "))
        fd.write(todo_list[task])
   with open(TODO_FILE, 'w') as fd:
        for pos, todo_list in enumerate(todo_list): 
          if pos != task:
            fd.write(todo_list)

   print(f"Marked todo #{task+1} as done.")

def report(_):
    with open(TODO_FILE, 'r') as fd:
        todo_list = fd.readlines()

        todo_size = len(todo_list)
    with open (DONE_FILE, 'r') as fd:
        done_list = fd.readlines()

        done_size = len(done_list)
    print(f"{now.strftime('%Y-%m-%d')} Pending : {todo_size} Completed : {done_size}")

def help(_):
    sys.stdout.buffer.write(HELP_MESSAGE.encode('utf8'))

def  controller(command, task):
    operator = {
        "help": help,
        "add" : add,
        "ls" : ls,
        "del": delete,
        "done":done,
        "report" : report,
    }. get(command, help) 
    return operator(task)



def main():
    if len(sys.argv)<2:
        sys.stdout.buffer.write(HELP_MESSAGE.encode('utf8'))
        sys.exit()

    _,command,*task = sys.argv
    task = " ".join(task)

    controller(command, task)

if __name__ == "__main__":
    main()