[Tutor] __init__ doesn't seem to be running

[Hugo Arts]

On Fri, Mar 15, 2013 at 9:21 PM, Cameron Macleod <cmacleod170 at googlemail.com
> wrote:

> Hello everyone, I'm using Python 3.3 and am trying to write a simple to-do
> list program. I have a class which runs pretty much everything called todo
> and the __init__ method doesn't seem to be running.
>
> class todo():
>     def __init__(self):
>         tasks = []
>
>     def writeTask(todoList):
>         name = input("Please enter the task > ")
>         desc = input("Please enter a short description (optional) > ")
>         while True:
>             try:
>                 imp = int(input("How important is this task? 1-100 > "))
>                 break
>             except TypeError:
>                 imp = int(input("How important is this task? 1-100 > "))
>         if imp > 100:
>             imp = 100
>         elif imp < 1:
>             imp = 1
>         todoList.write("\"" + name + "\",\"" + desc + "\",\"" + str(imp) +
> "\"")
>         print("Task written!")
>     try:
>         todoList = open('todo.txt', 'r+')
>     except IOError:
>         todoList = open('todo.txt', 'w')
>     for line in todoList:
>         tasks.append(line.strip())
>     writeTask(todoList)
>     todoList.close()
>
> main = todo()
>
> Whenever I run this, I get the error:
>
> Traceback (most recent call last):
>   File "C:\Python33\todo.py", line 8, in <module>
>     class todo():
>   File "C:\Python33\todo.py", line 34, in todo
>     tasks.append(line.strip())
> NameError: name 'tasks' is not defined
>
> Indicating that __init__ hasn't run since that is the initialization for
> tasks. I had always understood that __init__ was the equivalent of a
> constructor, and should be run at the instantiation of any class.
>

That's not what it indicates, you should assume less and debug more. If you
add a print statement in __init__, I can guarantee that you will see it
executed. Go, try it now, don't assume I'm right ;)

Now, the only thing that this error actually indicates is that "tasks"
doesn't exist when writeTask is being executed. In both __init__ and
writeTask, the "tasks" variable is function-local: it exists only for the
duration of the function. What you want is to attach tasks to your object
instance, which you do through "self". self is the reference to the current
instance, much like the this pointer in Java or C++. So if you do:

class Todo:
    def __init__(self):
        self.tasks = []

    def write_task(self, task):
        # other code around here
        self.tasks.append(line.strip())

everything will be fine.

HTH,
Hugo
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