On 2/20/06, Ian Bicking <[EMAIL PROTECTED]> wrote:
> Also, is default_factory=list threadsafe in the same way .setdefault is?
> That is, you can safely do this from multiple threads:
>
> d.setdefault(key, []).append(value)
>
> I believe this is safe with very few caveats -- setdefault itself is
> atomic (or else I'm writing some bad code ;).
Only if the key is a string and all values in the dict are also
strings (or other builtins). And I don't think that Jython or
IronPython promise anything here.
Here's a sketch of a situation that isn't thread-safe:
class C:
def __eq__(self, other):
return False
def __hash__(self):
return hash("abc")
d = {C(): 42}
print d["abc"]
Because "abc" and C() have the same hash value, the lookup will
compare "abc" to C() which will invoke C.__eq__().
Why are you so keen on using a dictionary to share data between
threads that may both modify it? IMO this is asking for trouble --
the advice about sharing data between threads is always to use the
Queue module.
[...]
> Note that multidict -- among other possible concrete collection patterns
> (like Bag, OrderedDict, or others) -- can be readily implemented with
> threading guarantees.
I don't believe that this is as easy as you think.
--
--Guido van Rossum (home page: http://www.python.org/~guido/)
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