How to avoid arg kwargs to be non mutable?

Question

I have a class A

class A:
  __dict__ = {"x": 0}

  def __init__(self, **kwargs):
    self.__dict__.update(kwargs.copy())

When I create A objects:

>>> a = A(x=2)
>>> b = A(x=3)
>>> print(a.__dict__)
{x: 3}
>>> print(b.__dict__)
{x: 3}

When I preset dict without init, it becomes mutable, how can I avoid that?

maybe add a more elaborate example, showing an expected an actual outputs? — Adam.Er8, May 21 '20 at 13:13
You do not create any reference to the object, so it garbage collected and the same memory is used for the new object, resulting in the same ID. — Klaus D., May 21 '20 at 13:39
To make it something like default argument, because I want `A` to be inherited — Just_Me, May 21 '20 at 19:05

Adam.Er8 · Answer 1 · 2020-05-23T10:47:46.457

0

you can use dict.copy() to get a new (shallow!) copy of kwargs, like this:

class A:
  def __init__(self, **kwargs):
    self.local_copy = kwargs.copy()
    print(id(self.local_copy))

a2 = A(x=2)
a3 = A(x=3)
print(a2.local_copy)
print(a3.local_copy)

example output:

15067008
15031824
{'x': 2}
{'x': 3}

EDIT:
as @chepner pointed out in the comments, the copy is unnecessary.
I assume the reason you get the same id for the 2 different invocations is that kwargs is unreferenced once the function returns, so the object may be garbage collected, and its address is reused.

even without copy, I get the same results: (2 different IDs, and distinct A objects)

edited May 23 '20 at 10:47

answered May 21 '20 at 13:08

Adam.Er8

12,675
3
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I tried this but when I update object's dict object with that copy, it doesn't copy – Just_Me May 21 '20 at 13:09
It's a shallow copy – Eric Jin May 21 '20 at 13:11
You don't need a copy of `kwargs`; it is already a brand new `dict` created from the given keyword arguments. – chepner May 21 '20 at 19:00
@chepner you are very much correct! so I guess when it is not saved anywhere, the same memory is just reused for the next call to `__init__`? – Adam.Er8 May 23 '20 at 10:44
The same memory may or may not be reused; it doesn't matter one way or the other. – chepner May 23 '20 at 12:41

How to avoid arg kwargs to be non mutable?

1 Answers1