[x3d-public] x3d.py problem

[Andreas Plesch]

Thanks for recognizing the issue, and suggesting a solution. All nodes with
children, or other mutable valued parameters, are likely affected.

Andreas

---on the phone---

On Tue, May 12, 2020, 9:26 PM vmarchetti at kshell.com <vmarchetti at kshell.com>
wrote:

> This is an unfortunate and common bug that occurs in Python.
>
> In the definition of the class Scene (around line 8795 of x3d.py) is this
> definition of the __init__ function for Scene:
>
>     def __init__(self, children=list()):
>         self.children = children
>
> This code is read once when the x3d.py module is loaded, and it creates a
> class-level default value for the children argument.
> If you ever use the default value by calling Scene() {with no parameter},
> this class-level instance of list is modified, and this SAME INSTANCE of
> list
> if used for any further calls of Scene()
>
> The way to avoid this is to define the __init__ function this way:
>
> def __init__(self, children=None):
> if children is None:
> self.children = list() # creates a new empty list for each Scene instance
> else:
> self.children = children
>
>
> Vince Marchetti
>
>
> On May 12, 2020, at 5:59 PM, Andreas Plesch <andreasplesch at gmail.com>
> wrote:
>
> I discovered a simple workaround. Always pass a children parameter, even
> if empty.
>
> On Tue, May 12, 2020 at 5:16 PM Andreas Plesch <andreasplesch at gmail.com>
> wrote:
>
>> Hi,
>>
>> after some debugging I discovered another issue with x3d.py. It can be
>> boiled down t o a few lines of code:
>>
>> import x3d.x3d as x
>>
>> my_scene = x.Scene()
>> group = x.Group()
>> my_scene.children.append(group)
>> print(my_scene.XML())
>>
>> => output
>> => <Scene>
>> =>  <Group/>
>> => </Scene>
>>
>> my_other_scene = x.Scene()
>> print(my_other_scene.XML())
>>
>> => output
>> => <Scene>
>> =>  <Group/>
>> => </Scene>
>>
>> I think my_other_scene should be empty but remembers the content of the
>> first scene.
>> Can you reproduce that ?
>> On the other hand this works (in a new session):
>>
>> my_scene = x.Scene( children = [x.Group()] )
>> print(my_scene.XML())
>>
>> => output
>> => <Scene>
>> =>  <Group/>
>> => </Scene>
>>
>> my_other_scene = x.Scene()
>> print(my_other_scene.XML())
>>
>> => output
>> => <Scene>
>> => </Scene>
>>
>> --
>> Andreas Plesch
>> Waltham, MA 02453
>>
>
>
> --
> Andreas Plesch
> Waltham, MA 02453
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>
>
>
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