Skip Montanaro wrote:
>
> What stimulated this thread about making mutable objects (temporarily)
> immutable? Can someone give me an example where this is actually useful and
> can't be handled through some existing mechanism? I'm definitely with
> Fredrik on this one. Sounds like madness to me.
This thread is an offspring of the "for something in dict:" thread.
The problem we face when iterating over mutable objects is that
the underlying objects can change. By marking them read-only we can
safely iterate over their contents.
Another advantage of being able to mark mutable as read-only is
that they may become usable as dictionary keys. Optimizations such
as self-reorganizing read-only dictionaries would also become
possible (e.g. attribute dictionaries which are read-only could
calculate a second hash value to make the hashing perfect).
> I'm just guessing here, but since the most common need for immutable objects
> is a dictionary keys, I can envision having to test the lock state of a list
> or dict that someone wants to use as a key everywhere you would normally
> call has_key:
>
> if l.islocked() and d.has_key(l):
> ...
>
> If you want immutable dicts or lists in order to use them as dictionary
> keys, just serialize them first:
>
> survey_says = {"spam": 14, "eggs": 42}
> sl = marshal.dumps(survey_says)
> dict[sl] = "spam"
Sure and that's what .items(), .keys() and .values() do. The idea
was to avoid the extra step of creating lists or tuples first.
> Here's another pitfall I can envision.
>
> survey_says = {"spam": 14, "eggs": 42}
> survey_says.lock()
> dict[survey_says] = "Richard Dawson"
> survey_says.unlock()
>
> At this point can I safely iterate over the keys in the dictionary or not?
Tim already pointed out that we will need two different read-only
states:
a) temporary
b) permanent
For dictionaries to become usable as keys in another dictionary,
they'd have to marked permanently read-only.
--
Marc-Andre Lemburg
______________________________________________________________________
Company: http://www.egenix.com/
Consulting: http://www.lemburg.com/
Python Pages: http://www.lemburg.com/python/
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