[issue29597] __new__ / __init__ calls during unpickling not documented correctly

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

This is a duplicate of issue27635.

--
nosy: +serhiy.storchaka
resolution:  -> duplicate
stage:  -> resolved
status: open -> closed
superseder:  -> pickle documentation says that unpickling may not call __new__

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[issue29597] __new__ / __init__ calls during unpickling not documented correctly

[Max]

New submission from Max:

According to the 
[docs](https://docs.python.org/3/library/pickle.html#pickling-class-instances):

> Note: At unpickling time, some methods like `__getattr__()`, 
> `__getattribute__()`, or `__setattr__()` may be called upon the instance. In 
> case those methods rely on some internal invariant being true, the type 
> should implement `__getnewargs__()` or `__getnewargs_ex__()` to establish 
> such an invariant; otherwise, neither `__new__()` nor `__init__()` will be 
> called.

It seems, however, that this note is incorrect. First, `__new__` is called even 
if `__getnewargs__` isn't implemented. Second, `__init__` is not called even if 
it is (while the note didn't say that `__init__` would be called when 
`__getnewargs__` is defined, the wording does seem to imply it).


class A:
def __new__(cls, *args):
print('__new__ called with', args)
return object.__new__(cls)

def __init__(self, *args):
print('__init__ called with', args)
self.args = args

def __getnewargs__(self):
print('called')
return ()

a = A(1)
s = pickle.dumps(a)
a = pickle.loads(s) # __new__ called, not __init__
delattr(A, '__getnewargs__') 
a = A(1)
s = pickle.dumps(a)
a = pickle.loads(s) # __new__ called, not __init__

--
assignee: docs@python
components: Documentation
messages: 288088
nosy: docs@python, max
priority: normal
severity: normal
status: open
title: __new__ / __init__ calls during unpickling not documented correctly
versions: Python 3.6

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