Hello all,
I'm starting to learn how to use weakrefs. I'm curious
about the function named 'mapping'. It is implemented as:
> def mapping(dict=None,weakkeys=0):
> if weakkeys:
> return WeakKeyDictionary(dict)
> else:
> return WeakValueDictionary(dict)
Why is this a useful function? Shouldn't people just call
WeakKeyDictionary and WeakValueDictionary directly instead
of calling mapping with a parameter to specify which class
to construct?
If anything, this function is very confusing. Take the
associated documentation as a case in point:
> mapping([dict[, weakkeys=0]])
> Return a weak dictionary. If dict is given and not None,
> the new dictionary will contain the items contained in dict.
> The values from dict must be weakly referencable; if any
> values which would be inserted into the new mapping are not
> weakly referencable, TypeError will be raised and the new
> mapping will be empty.
>
> If the weakkeys argument is not given or zero, the values in
> the dictionary are weak. That means the entries in the
> dictionary will be discarded when no strong reference to the
> value exists anymore.
>
> If the weakkeys argument is nonzero, the keys in the
> dictionary are weak, i.e. the entry in the dictionary is
> discarded when the last strong reference to the key is
> discarded.
As far as I can tell, this documentation is wrong, or at
the very least confusing. For example, it says:
> The values from dict must be weakly referencable
but when the weakkeys argument is nonzero,
> the keys in the dictionary are weak
So must both keys and values be weak? Or only the keys?
I hope the latter since there are cases I can think of
where I want the keys to be weak and the values be types,
hence non-weakreferencable.
Wouldn't it be better to remove the 'mapping' function and
only have the WeakKeyDictionary and WeakValueDictionary.
In which case the documentation becomes:
> WeakValueDictionary([dict])
> Return a weak dictionary. If dict is given and not None,
> the new dictionary will contain the items contained in dict.
> The values from dict must be weakly referencable; if any
> values which would be inserted into the new mapping are not
> weakly referencable, TypeError will be raised and the new
> mapping will be empty.
>
> The values in
> the dictionary are weak. That means the entries in the
> dictionary will be discarded when no strong reference to the
> value exists anymore.
> WeakKeyDictionary([dict])
> Return a weak dictionary. If dict is given and not None,
> the new dictionary will contain the items contained in dict.
> The keys from dict must be weakly referencable; if any
> keys which would be inserted into the new mapping are not
> weakly referencable, TypeError will be raised and the new
> mapping will be empty.
>
> The keys in
> the dictionary are weak. That means the entries in the
> dictionary will be discarded when no strong reference to the
> key exists anymore.
Easier to read and to see the parallels between the two
styles, IMHO of course.
I am not on this list though I will try to read the
archives online for the next couple of days. Please
CC me about any resolution to this topic.
Sincerely,
Andrew
dalke@acm.org
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