Changing self: if self is a tree how to set to a different self

Paul McGuire ptmcg at austin.rr.com
Sat Jul 12 14:46:03 CEST 2008


On Jul 12, 6:18 am, Bart Kastermans <kaste... at bart-kastermanss-
macbook.local> wrote:
> 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

<persnickety>
This code reads wrongly to me on a couple of levels.  First, I think
the general computing consensus is that if-then-else is more readable/
logical if you assert the positive condition for the then-part, and
put the alternative condition in the else-part.  My impression is that
the non-None-ness of tree is actually the positive assertion, as in:

    if tree != None:
        return getattr(tree, mem_function)(*arguments)
    else:
        return None

Next, the more Pythonic test for None-ness is most clearly written as:

    if tree is not None:

as there is only one None, and the identity "is not" check is simpler/
faster for Python to execute (and possibly - and more importantly -
also simpler for readers to follow, as this reads more like a
continuous sentence instead of a mixture of prose and mathematical
notations).

One might even suggest you could further abbreviate this test to:

    if tree:

and get the same behavior.  I would quibble with that, however, that
this merely exploits a side-effect of Python, in which None values are
always False, and *most* non-None values are True.  But Python also
interprets many non-None values as False, such as 0, or empty
containers, such as lists, tuples, dicts, and strings.  In fact, your
tree class sounds like a structured container to me, and it would be
reasonable to assume that you might implement __nonzero__ (pre-Python
2.6) or __bool__ (Python 2.6 and later) in your class to return False
for an empty tree, which would still be a valid and not-None tree.
You should be able to invoke methods on an empty tree just as one can
call "".upper().  So for this case, I would stick with the more
explicit "if tree is not None".

Another Pythonicity is that methods will *always* return a value, even
if you do not write a return statement - and that value is None.  So
if you assert the tree-not-None as the if condition, you don't even
need the else part.  You could just write:

    def NoneOr (tree, mem_function, *arguments):
        """ if tree is not None then tree.mem_function (arguments).
"""
        if tree is not None:
            return getattr(tree, mem_function)(*arguments)

Surprisingly, this actually reads almost verbatim from your doc
string!  So I would guess that this implementation is probably closest
to your original intent for this method.  Still, for explicitness'-
sake, you might want to keep the else-part, just to make your intent
clear and spelled-out.  (Complaining about the presence or absence of
this bit of code goes beyond "persnickety"...)
</persnickety>

Note that the original code is perfectly valid Python, and will run
just as efficiently as any of my alternative suggestions, which is why
I enclosed my comments in 'persnickety' (http://
dictionary.reference.com/browse/persnickety) tags.

-- Paul



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