On Jul 12, 6:18 am, Bart Kastermans <[EMAIL PROTECTED]
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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