Remove class from one list without removing it from others

Question

I have a piece of code which is doing a different thing to what I feel it should. This is a module I built to test this problem in a larger piece of code:

class plant:
    def __init__ (self, param):
        self.param = param
plants = []
for i in range(10):
    plants.append(plant(10))
plant1 = plants[0]
plants1 = plants
print(len(plants))
for i in plants:
    print("Item removed")
    plants.remove(i)
print(plant1)
print(len(plants))
print(len(plants1))

This produces the output as

10

Item removed

Item removed

Item removed

Item removed

Item removed

<__main__.plant object at 0x02780F30>

<__main__.plant object at 0x02780F30>

5
5

Why does it remove the items from plants1 as well but not from plant1? And how do I stop it doing that?

@PadraicCunningham definitely similar answers but though it's a question about duplicates it's not necessarily a duplicate question! — deau, Sep 20 '14 at 09:43
Thank you. What does my code there do which makes it behave that way? — Sacasco, Sep 20 '14 at 09:43
@Sacasco, the answer to how to change it is in the first comment I made and the reason why is in the link. — Padraic Cunningham, Sep 20 '14 at 09:48
@dsclose: it certainly is a dupe; the other post *answers* this one. — Martijn Pieters, Sep 20 '14 at 09:57
SO member Ned Batchelder has a nice articles (with diagrams) that explains what's going on: [Facts and myths about Python names and values](http://nedbatchelder.com/text/names.html) — PM 2Ring, Sep 20 '14 at 12:17

score 0 · Accepted Answer · edited May 23 '17 at 12:12

Therelevant part of your code is here:

plant1 = plants[0]
plants1 = plants

plant1 now refers to tthe object at the first index of plants and this will still point to that same object even if it's removed from plants. You still only have one object, rather than two duplicates.

Similarly, plants and plants1 refer to the same list. You have not duplicated your list by assigning one to another. Thus anything you do to plants1 you are also tacitly doing to plants.

Really, you need to look at this question on duplicating a list.

Remove class from one list without removing it from others

1 Answers1