[Tutor] Deleting an instance of an object
kent_johnson at skillsoft.com
Thu Sep 23 14:17:08 CEST 2004
First, I'll try to shed a little light on some misconceptions you seem to have.
The name that you have stored in the collect_results list is not really
associated with the Article any more, it is just a reference to a string.
That is why you get the error 'str' object has no attribute '__del__'.
You shouldn't be calling __del__ yourself. __del__ is an internal method
that implements the del statement.
Objects are deleted from a container by calling del on the container. In
other words, you don't ask an object to delete itself from a container, you
ask the container to delete the object.
OK, so how should you accomplish what you want? There are two ways to
delete from a list, by index and by reference (see
http://docs.python.org/lib/typesseq-mutable.html). For example, here is a
simple Article class that has a name and shows the name when printed:
>>> class Article:
... def __init__(self, name):
... self.name = name
... def __repr__(self):
... return self.name
Make a few articles and put them in a list:
>>> a=Article('Learning Python')
>>> b=Article('Python Networking')
>>> c=Article('Using Twisted')
>>> articles = [a, b, c]
[Learning Python, Python Networking, Using Twisted]
Delete the first article by index:
>>> del articles
[Python Networking, Using Twisted]
Delete article b by reference:
Notice that in each case I do something with the list, and in each case I
need to know something about the article object besides its name - either
its index in the list, or a reference to the actual article object.
So, what does this mean for you? I suggest that you save more information
about an article in the collect_results list. You could save the
articlesheld list, and the article object. You don't need the name as it is
in the article. A simple way to do this is to store a tuple in the list :
result = (issue_object.articlesheld, article_object)
To print the list, you need to unpack the tuple and get the article name back:
for article_list, article in collect_results:
(You can read a little more about tuples here:
To delete an entry, get the list and article from the entry and remove the
article from the list:
article_list, article = collect_results[del_choice_int]
At 10:19 AM 9/23/2004 +0100, Adam Cripps wrote:
>I've designed an OOP program, where I keep a catalogue of
>publications. I want to be able to delete an instance of Article
>(Article is a subclass of Issue, which is a subclass of Title, which
>is a subclass of MagazineCollection). Each instance of Article has an
>attribute name (article.name).
>I've created a loop which searches through the article.name(s) and
>found the one I want. The user has specified which one they want to
>delete and I've collected that.
>The problem I'm having is finding out the instantiation of self.name -
>is there a hard and fast way of identifying a particular instance of
>I'm getting the error:
>AttributeError: 'str' object has no attribute '__del__'
>which is strange, as the Deleted is coming from a __del__ function
>that I placed in the Article class.
>Tutor maillist - Tutor at python.org
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