Changing self: if self is a tree how to set to a different self
Bart Kastermans
kasterma at bart-kastermanss-macbook.local
Sat Jul 12 13:18:32 CEST 2008
Terry Reedy <tjreedy at udel.edu> writes:
> Bart Kastermans wrote:
>> I am playing with some trees. In one of the procedures I wrote
>> for this I am trying to change self to a different tree. A tree
>> here has four members (val/type/left/right). I found that self = SS
>> does not work; I have to write self.val = SS.val and the same for
>> the other members (as shown below). Is there a better way to do this?
>>
>> In the below self is part of a parse tree, F is the parse tree of a
>> function f with argument x. If a node in the parse tree is labelled
>> f, we should replace it by the parse tree for the function f, F, with
>> the remainder of the tree substituted for the input variable for the
>> function f, here x.
>>
>> def elimF (self):
>> if self.val == "f":
>> SS = F.copy ()
>> SS.subst ('x', self.left)
>> self.val = SS.val # from here: set self to be SS
>> self.type = SS.type
>> self.left = SS.left
>> self.right = SS.right # completed: set self to be SS
>
> If you examine nodes from their parent, I believe you can do the
> substitution in one step. Something like:
>
> for slot,child in ( ('left',self.left), ('right',self.right) ):
> if child is not None:
> if child.val == 'f':
> setattr(self, slot, F.copy().subst('x', child.left))
> child.elimF
>
> where .subst returns the modified tree.
That worked very well. The option that looks even better to me was
just to build a copy (the exceptional case of the root of the tree
that is needed bugged me a bit). Then self does not have to be
changed at all. Your use of setattr made me find getattr, and this
allowed for a quite smooth (I think) implementation.
def elimF (self):
""" do the substitution of all nodes labelled f.
"""
if self.val == "f":
ret_val = F.subst ('x', self.left)
ret_val.left = NoneOr (ret_val.left, 'elimF')
ret_val.right = NoneOr (ret_val.right, 'elimF')
return ret_val
else:
return Tree (self.val, self.type, \
NoneOr (self.left, 'elimF'), \
NoneOr (self.right, 'elimF') )
This uses the function:
def NoneOr (tree, mem_function, *arguments):
""" if tree is not None then tree.mem_function (arguments). """
if tree == None:
return None
else:
return getattr (tree, mem_function) (*arguments)
Bart
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