[ZODB-Dev] Re: false write conflicts

[Tim Peters]

[Casey Duncan]
>>> Augmented assignment causes a setattr on the containing object. This
>>> is the way Python works. These examples are semantically
>>> equivilant::
>>>
>>>   self.x += 1
>>>   self.x = self.x + 1
>>>   setattr(self, 'x', self.x + 1)
>>>
>>> They all cause self to be modified through __setattr__. If self is a
>>> persistent instance then it will be marked changed in all three
>>> cases above.

[John Belmonte]
>> To be more accurate, it seems like
>>
>>     foo.bar += 1
>>
>> becomes the equivalent of
>>
>>     x = foo.bar; x += 1; foo.bar = x

[Casey]
> Ok, I'm not sure how that's more accurate ;^) My example don't create
> a local variable 'x'.

That's true, and if we assume foo.bar was originally bound to an int,
they're all the same (module the 'x' business).  Else John's

    x += 1

covers a subtlety not covered in yours, that x.__add__(1) and x.__iadd__(1)
may do entirely different things, so that any spelling containing "self.x +
1" is open to exposing such a difference.  You even tell this from the byte
code <wink>:

  2           0 LOAD_FAST                0 (foo)
              3 DUP_TOP
              4 LOAD_ATTR                1 (bar)
              7 LOAD_CONST               1 (1)
             10 INPLACE_ADD
             11 ROT_TWO
             12 STORE_ATTR               1 (bar)

Without *some* instance of "+=" in the putative workalike sequence, you
won't get the INPLACE_ADD opcode.

...

>> This wrapper for Persistent.__setattr__ looks like it will suit my
>> needs:
>>
>>  class MyPersistent(Persistent):
>>    def __setattr__(self, key, val):
>>      if not (self.__dict__.has_key(key) and self.__dict__[key] is val):
>>         Persistent.__setattr__(self, key, val)

> Ugh. This is bound to bite you somewhere down the line. I'd say its
> simpler to avoid augmented assignment when you don't want the
> container marked as changed. But that's me.

Also me.  When the distinction between "mutate in place or don't" is
crucial, augmented assignment notation doesn't help, and may mislead.