Mailing List Archive: Python: Dev

One obvious way to do interning [Was: Retrieve an arbitrary element from a set without removing it]

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alexander.belopolsky at gmail

Oct 26, 2009, 9:43 AM

Post #1 of 3 (238 views)
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One obvious way to do interning [Was: Retrieve an arbitrary element from a set without removing it]

Changing the subject to reflect branched discussion and forwarding to
python-ideas where it probably belongs.

On Mon, Oct 26, 2009 at 12:02 PM, Terry Reedy <tjreedy [at] udel> wrote:
> Alexander Belopolsky wrote:
>
>> Here is an alternative idea on how storing interned objects in a set
>> can be supported. Currently set.add method returns None and has no
>> effect when set already has an object equal to the one being added. I
>> propose to consider changing that behavior to make set.add return the
>> added object or the set member that is equal to the object being
>> added. It is unlikely that many programs rely on the return value
>> being None (with doctests being a probable exception), so adding this
>> feature is unlikely to cause much grief.
>
> I had exactly the same idea, but did not post because it violates the
> general rule that mutators return None.

Is there such a rule? What about set/dict pop?

> On the other hand, the returned
> value here would not be the mutated collection, so no chaining is possible.

I assume you refer to chaining as in s.add(1).add(2) which would be
enabled if s.add(..) returned s. My proposal would enable a different
type of "chaining" which I would find useful, but ready to hear
objections:

v = compute_value()
s.add(v)
# use v

can, with my proposal, be rewritten as v = s.add(compute_value()) with
an added benefit that v that is used is the "canonical" value.

> And 'add' is clearly intended to change something.
>
Is this an argument for or against the proposal?

> On the other hand, frozensets do not have an add method.

However, PySet_Add "works with frozenset instances (like
PyTuple_SetItem() it can be used to fill-in the values of brand new
frozensets before they are exposed to other code). "
<http://docs.python.org/3.1/c-api/set.html#PySet_Add>

I will experiment with changing PySet_Add to see how much it would
break and whether it will be helpful in implementing set-based
interning of python strings.
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tjreedy at udel

Oct 26, 2009, 12:20 PM

Post #2 of 3 (225 views)
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Re: One obvious way to do interning [Was: Retrieve an arbitrary element from a set without removing it] [In reply to]

Alexander Belopolsky wrote:
> Changing the subject to reflect branched discussion and forwarding to
> python-ideas where it probably belongs.
>
> On Mon, Oct 26, 2009 at 12:02 PM, Terry Reedy <tjreedy [at] udel> wrote:
>> Alexander Belopolsky wrote:
>>
>>> Here is an alternative idea on how storing interned objects in a set
>>> can be supported. Currently set.add method returns None and has no
>>> effect when set already has an object equal to the one being added. I
>>> propose to consider changing that behavior to make set.add return the
>>> added object or the set member that is equal to the object being
>>> added. It is unlikely that many programs rely on the return value
>>> being None (with doctests being a probable exception), so adding this
>>> feature is unlikely to cause much grief.
>> I had exactly the same idea, but did not post because it violates the
>> general rule that mutators return None.
>
> Is there such a rule? What about set/dict pop?

The rule perhaps should be restated as 'Collection mutators return None
or possible an item but not the collection.'

>> On the other hand, the returned
>> value here would not be the mutated collection, so no chaining is possible.

>> And 'add' is clearly intended to change something.
>>
> Is this an argument for or against the proposal?

Mildly for.

>> On the other hand, frozensets do not have an add method.
>
> However, PySet_Add "works with frozenset instances (like
> PyTuple_SetItem() it can be used to fill-in the values of brand new
> frozensets before they are exposed to other code). "
> <http://docs.python.org/3.1/c-api/set.html#PySet_Add>
>
> I will experiment with changing PySet_Add to see how much it would
> break and whether it will be helpful in implementing set-based
> interning of python strings.

Terry Jan Reedy

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ncoghlan at gmail

Oct 26, 2009, 2:50 PM

Post #3 of 3 (222 views)
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Re: One obvious way to do interning [Was: Retrieve an arbitrary element from a set without removing it] [In reply to]

Terry Reedy wrote:
> Alexander Belopolsky wrote:
>> Terry Reedy wrote:
>>> I had exactly the same idea, but did not post because it violates the
>>> general rule that mutators return None.
>>
>> Is there such a rule? What about set/dict pop?
>
> The rule perhaps should be restated as 'Collection mutators return None
> or possible an item but not the collection.'

And to clarify the rationale for that guideline: it is to make it clear
that the mutator is changing the container in place and *not* creating a
new container object.

myset.pop() # No new container, returns popped object
mylist.sort() # No new container, returns None
sorted(mylist) # New container, so return it
mystr.lower() # Creates new string, so return it

Cheers,
Nick.

--
Nick Coghlan | ncoghlan [at] gmail | Brisbane, Australia
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