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Tue Nov 21 03:53:04 CET 2006 · https://mail.python.org/pipermail/python-dev/2006-November/069989.html

On Mon, 20 Nov 2006 23:55:42 +0100, "\"Martin v. Löwis\"" <martin at v.loewis.de> wrote:
>Fredrik Lundh schrieb:
>> the FAQ contains a list of "atomic" operation, and someone recently
>> asked whether += belongs to this group.
>
>In general, += isn't atomic: it may invoke __add__ or __iadd__ on the
>left-hand side, or __radd__ on the right-hand side.
>
>>From your list, I agree with Josiah Carlson's observation that the
>examples you give involve separate name lookups (e.g. L.append(x)
>loads L, then fetches L.append, then loads x, then calls apped,
>each in a single opcode); the actual operation is atomic.
>
>If you only look at the actual operation, the these aren't atomic:
>
>x.field = y # may invoke __setattr__, may also be a property
>D[x] = y    # may invoke x.__hash__, and x.__eq__
>
>I'm uncertain whether D1.update(D2) will invoke callbacks (it
>probably will).

Quite so:

    >>> class X:
    ...     def __del__(self):
    ...             print 'X.__del__'
    ... 
    >>> a = {1: X()}
    >>> b = {1: 2}
    >>> a.update(b)
    X.__del__
    >>> 

Jean-Paul

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[Python-Dev] PyFAQ: thread-safe interpreter operations