Python language problem

[ripleyfu]

>>> class A:
.... pass
....

a = A()
b = a
del b
a

<__main__.A instance at 0x00B91BC0>
I want to delete 'a' through 'b', why It does't?
How can I do that?

[Boris Borcic]

ri******@gmail.com wrote:

class A: ... pass
... a = A()
b = a
del b
a

<__main__.A instance at 0x00B91BC0>
I want to delete 'a' through 'b', why It does't?
How can I do that?

del a,b

[Laszlo Nagy]

ri******@gmail.com i'rta:

class A:
... pass
...
a = A()
b = a
del b
a

<__main__.A instance at 0x00B91BC0>
I want to delete 'a' through 'b', why It does't?
How can I do that?

You must undestand that 'a' and 'b' are names. You can only delete
names, not objects. Objects are freed by the garbage collector,
automatically. Probably you used to write programs in C or Pascal or
other languages with pointers. In Python, there are no pointers, just
references and you cannot free an object. You can only delete the
references to it. The good question is: why would you like to free an
object manually? The garbage collector will do it for you automatically,
when the object has no more references. (Well, cyclic references are
also garbage collected but not immediately.)

If you need to handle resources, you can still use the try-finally
statement. Like in:

f = file('example.txt','r')
try:
s = f.read()
finally:
f.close() # The resource is freed. But the object that was used to
access the resource, may not be freed here....

Regards,

Laszlo

[ripley]

Boris Borcic wrote:

ri******@gmail.com wrote:

> class A:

... pass
...

> a = A()
> b = a
> del b
> a

<__main__.A instance at 0x00B91BC0>
I want to delete 'a' through 'b', why It does't?
How can I do that?

del a,b

But 'b' is also deleted, i want use 'b' to delete 'a', 'b' is exists.

[ripley]

Laszlo Nagy wrote:

You must undestand that 'a' and 'b' are names. You can only delete
names, not objects. Objects are freed by the garbage collector,
automatically. Probably you used to write programs in C or Pascal or
other languages with pointers. In Python, there are no pointers, just
references and you cannot free an object. You can only delete the
references to it. The good question is: why would you like to free an
object manually? The garbage collector will do it for you automatically,
when the object has no more references. (Well, cyclic references are
also garbage collected but not immediately.)

If you need to handle resources, you can still use the try-finally
statement. Like in:

f = file('example.txt','r')
try:
s = f.read()
finally:
f.close() # The resource is freed. But the object that was used to
access the resource, may not be freed here....

Regards,

Laszlo
Thanks for your so detailed explain, I think I know you, But my Engish
is not enough
to explain.

I create same object in Tree, I want to update Tree, when I need to
delete subtree.
If where no references, I can't do that. some thing like blow:
for i in list:
del iFrom that code, I can't update list anymore.

[Diez B. Roggisch]

>

Thanks for your so detailed explain, I think I know you, But my Engish
is not enough
to explain.

I create same object in Tree, I want to update Tree, when I need to
delete subtree.
If where no references, I can't do that. some thing like blow:
for i in list:
del i

This makes no sense. In python, objects are garbage collected when there are
no references to the anymore. What you do is

- create a reference to an object by letting i point to it

- remove i, such that the reference is removed.

This has no effect at all -- 1 - 1 = 0

If you want to remove items from a list, do something like this:

l[:] = [e for e in l if predicate(e)]

This will alter the list l such that only the elements of it that fullfill
the predicate are retained.

In other words:

your code above should work when written like this:

l[:] = []

which effectively removes all elements from the list.

This does NOT mean that the objects referenced to by the list are deleted!
If they are referenced from anywhere else, they are kept!!

Diez

[Steve Holden]

ripley wrote:

Boris Borcic wrote:

ri******@gmail.com wrote:

>>class A:

... pass
...

>>a = A()
>>b = a
>>del b
>>a

<__main__.A instance at 0x00B91BC0>
I want to delete 'a' through 'b', why It does't?
How can I do that?

del a,b

But 'b' is also deleted, i want use 'b' to delete 'a', 'b' is exists.

You can't do what you want to do.

Python names are independent references to objects. The "del" statement
deletes a name from a namespace (or an item from a structure), and
cannot be used to delete all references to a given object. In general
there's no way to delete a referenced object - we normally rely on the
implementation (in CPython reference counting plus garbage collection,
in other implementations just plain garbage collection) to perform the
deletion when no live references to an object remain.

Perhaps you'd like to explain *why* you find a need to do this (in other
words, what's your use case)?

Weak references are one possibility that might help you, but without
knowing your real requirements it's difficult to be more helpful.

regards
Steve
--
Steve Holden [phone] [phone]
Holden Web LLC/Ltd http://www.holdenweb.com
Love me, love my blog http://holdenweb.blogspot.com
Recent Ramblings http://del.icio.us/steve.holden

[baiju]

May be you are looking for weakref module:

http://www.python.org/doc/current/li...e-weakref.html

--
Baiju M

[ripley]

Thanks all, you all are nice man.

Dennis Lee Bieber 写�：

while len(lst): #list is a built-in function, don't use as a name
del lst[0] #delete the first item IN the lst

I found Dennis 's code is usefull that is a way to solve my problem.