Proposal for adding symbols within Python

[Pierre Barbier de Reuille]

Ben Finney a écrit :
> Pierre Barbier de Reuille <pierre.barbier at cirad.fr> wrote:
> 
>>Mike Meyer a écrit :
>>
>>>Hmm. You know, $symbol$ doesn't seem nearly as bad as $symbol. It
>>>tickles TeX, not P***. I could live with that.
>>
>>Yep, I like this $symbol$ notation !
> 
> 
> Gets a big -1 here.
> 
> I've yet to see a convincing argument against simply assigning values
> to names, then using those names.
> 

I can see three interests :
1 - ensure values are unique (i.e. a bit like using instances of object)
2 - values are meaningful (i.e. with introspection on the values you get
a human-readable value, unlike with instances of object)
3 - getting an *easy* access to those two properties

1 and 2 require a new type, 3 a new syntax (IMO).

Here's a try for the symbol class :

class symbol(object):
  def __init__(self, value):
    self._value = value
  def _get_value(self):
    return self._value
  value = property(_get_value)
  def __eq__(self, other):
    return self.value == other.value
  def __str__(self):
    return str(self.value)
  def __repr__(self):
    return "symbol(%s)" % (repr(self.value),)

One thing to do would be to return the same object for symbols with the
same value (when possible ...).

For example, if we limit symbol to hashable types, we can implement
something which can be tested with "is" instead of "==":

class symbol(object):
  _cache = {}
  def __new__(cls, value):
    if value in symbol._cache:
      return symbol._cache[value]
    self = object.__new__(cls)
    self._value = value
    symbol._cache[value] = self
    return self
  def _get_value(self):
    return self._value
  value = property(_get_value)
  def __eq__(self, other):
    return self.value == other.value
  def __str__(self):
    return str(self.value)
  def __repr__(self):
    return "symbol(%s)" % (repr(self.value),)

Then, as I suggested, you can do something like :

a = symbol((file, "opened"))

But it's less readable than $file.opened$ (or something similar).

Pierre