如果无法使用附加的变量来查找其索引，如何从列表中删除特定的对象？

Question

I have this function that creates a Ball object and appends it to a list:

list_of_balls = []

def create_new_ball():
    newball = Ball()
    list_of_balls.append(newball)

I need to be able to remove specific instances of Ball from that list. I have no idea how to do this. I cant use list_of_balls.index() because all the instances are attached to the private variable newball. When I get the list something like this is returned:

[< main .Foo instance at 0x22149c>, < main .Foo instance at 0x22111c>, < main .Foo instance at 0x2209b4>, < main .Foo instance at 0x22129c>]

So I have no idea of how to aim specific instances.

Is there a way to remove specific instances of Ball from that list? Each Ball instance needs to be able to remove itself from the list, but how do the instance knows who it is on the list and/or what is its index?

Edit: What I need to do is to identify them on that list (or dictionary, or other alternative). I know how to remove items from a list, what I can't figure out is how to do it specifically.

I want to do exactly what aList.remove() does, but I don't have a variable to use as argument, so I need an alternative.

Answer 1

list.remove seems to work Ok:

>>> class Ball(object): pass
... 
>>> balls = [ Ball() for _ in range(10) ]
>>> balls
[<__main__.Ball object at 0xe5350>, <__main__.Ball object at 0xe52b0>, <__main__.Ball object at 0xe5250>, <__main__.Ball object at 0xe5370>, <__main__.Ball object at 0xe5390>, <__main__.Ball object at 0xe53b0>, <__main__.Ball object at 0xe53d0>, <__main__.Ball object at 0xe5410>, <__main__.Ball object at 0xe5430>, <__main__.Ball object at 0xe5450>]
>>> balls.remove(balls[4])
>>> balls
[<__main__.Ball object at 0xe5350>, <__main__.Ball object at 0xe52b0>, <__main__.Ball object at 0xe5250>, <__main__.Ball object at 0xe5370>, <__main__.Ball object at 0xe53b0>, <__main__.Ball object at 0xe53d0>, <__main__.Ball object at 0xe5410>, <__main__.Ball object at 0xe5430>, <__main__.Ball object at 0xe5450>]
>>> len(balls)
9

The real question is how do you know which Ball you want to remove?

Based on the comments, it looks like you want all of the balls which have bounced N times to be removed:

to_remove = [x for x in balls if x.times_bounced > N]
for x in to_remove:
    balls.remove(x)

Alternatively, if you don't need to do this in place, you can create a new list with balls which haven't bounced N times:

balls = [ x for x in balls if x.times_bounced <= N ]

Finally, you can use this idiom and slice assignment to do the whole thing in place if you need to:

balls[:] = [ x for x in balls if x.times_bounced <= N ]

Answer 2

So I have no idea of how to aim specific instances.

You need to have some idea of how to identify the instances; and in that case, you can simply carry over that into an actual working way. For example give them a name, or an id. You could then use a dictionary or something else. By default the instances are only identifiable by their exact identity.

Each Ball instance needs to be able to remove itself from the list

>>> class Ball ():
        pass
>>> list_of_balls = []
>>> def create_new_ball():
        list_of_balls.append(Ball())
>>> def remove_yourself(ball):
        list_of_balls.remove(ball)
>>> for i in range(10): create_new_ball()
>>> list_of_balls
[<__main__.Ball object at 0x0000000002EB5E48>, <__main__.Ball object at 0x0000000002EBD908>, <__main__.Ball object at 0x0000000002EBDEB8>, <__main__.Ball object at 0x0000000002EBDE80>, <__main__.Ball object at 0x0000000002EBD978>, <__main__.Ball object at 0x0000000002ED41D0>, <__main__.Ball object at 0x0000000002ED4208>, <__main__.Ball object at 0x0000000002ED4240>, <__main__.Ball object at 0x0000000002ED4278>, <__main__.Ball object at 0x0000000002ED42B0>]
>>> one_ball = list_of_balls[4] # random ball, chosen by fair dice roll
>>> one_ball in list_of_balls
True
>>> remove_yourself(one_ball)
>>> one_ball in list_of_balls
False