Force creation of new memory object python

Question

How do I forcefully create a new object (in this case a dictionary) in python? The following should explain my problem:

>>> class A():
...   def __init__(self, a={}):
...     self.a=a
... 
>>> id(A().a) == id(A().a)
True

>>> class A():
...   def __init__(self, a=dict()):
...     self.a=a
... 
>>> id(A().a) == id(A().a)
True

>>> class A():
...   def __init__(self, a={}):
...     self.a=copy.deepcopy(a)
... 
>>> id(A().a) == id(A().a)
False

As you can see, the first two methods give me a pointer to the same empty dictionary, which means if I modify the dictionary in one object, it gets modified in the other. The third method seems so "unclean" to me. Is there an alternative? I know I could also use copy.copy, but I prefer deepcopy, for exactly these reasons.

Related reading: [“Least Astonishment” in Python: The Mutable Default Argument](http://stackoverflow.com/q/1132941/953482) — Kevin, Aug 18 '16 at 13:40
By the way, `id(A().a) == id(A().a)` is better written as `A().a is A().a` — mgilson, Aug 18 '16 at 13:42
I was debugging this visually with id(), that's why I used it. But you're right, it's more pythonic indeed. Will keep that in mind next time I post, thanks — pidgey, Aug 18 '16 at 13:46

score 2 · Accepted Answer · answered Aug 18 '16 at 13:38

2

Here is a common idiom:

def __init__(self, a=None):
  if a is None:
    a = {}
  ...

answered Aug 18 '16 at 13:38

NPE

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That quickly makes the constructor a lot bigger... But if there is no alternative, I guess I'll have to deal with it :p Thanks! – pidgey Aug 18 '16 at 13:42
class A(dict):pass A().update(key='val') – vadim vaduxa Aug 18 '16 at 13:45

Force creation of new memory object python

1 Answers1