James Smith wrote:
> Say an object like this exists:
> class test:
> a = ""
> b = ""
>
> You pickle it.
>
> You change the object definition to have a new field:
> class test
> a = ""
> b = ""
> c = ""
>
> You read the pickled object.
> Will it load but ignore the new field?
> That is what I want.
Classes are pickled by name. You are free to replace them with whatever you
want:
>>> class Test:
... a = 1
... b = 2
...
>>> import pickle
>>> p = pickle.dumps(Test)
>>> class Test:
... a = 10
... b = 20
... c = 30
...
>>> P = pickle.loads(p)
>>> P.c
30
>>> P.a
10
>>> P is Test
True
If you introduce a new *instance* attribute that will not magically appear
on unpickling:
>>> class T:
... def __init__(self, a, b):
... self.a = a
... self.b = b
... def __str__(self): return "T(a={0.a}, b={0.b})".format(self)
...
>>> t = T(1, 2)
>>> print(t)
T(a=1, b=2)
>>> p = pickle.dumps(t)
>>> class T:
... def __init__(self, a, b, c):
... self.a = a
... self.b = b
... self.c = c
... def __str__(self): return "T(a={0.a}, b={0.b},
c={0.c})".format(self)
...
>>> print(T(10, 20, 30))
T(a=10, b=20, c=30)
>>> v = pickle.loads(p)
No problem so far as the initializer is not called so that the missing c is
not a problem. But then:
>>> print(v)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in __str__
AttributeError: 'T' object has no attribute 'c'
One easy fix is to use a class attribute as the fallback:
>>> T.c = "class attribute"
>>> print(v)
T(a=1, b=2, c=class attribute)
But remember not to try this with mutable attributes. When `c` is a list
v.c.append(42)
will affect all instances with a missing `c` instance attribute.
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