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

"Martin v. Löwis" martin at v.loewis.de
Mon Nov 20 23:55:42 CET 2006

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).


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