Something or Nothing^H^H^H^H^H^H^HEmpty

[Magnus Lie Hetland]

In article <3cb3fbc0$0$78808$edfadb0f at dspool01.news.tele.dk>, Anders
J. Munch wrote:
>Pythons truth values are not "Something or Nothing".  The phrase seems
>to have caught on following Laura's post, but I believe it is
>misleading.

I don't.

>[] and {} are objects,

Yes, as are None and 0.

> they are objects with identity, and they are
>mutable objects.  They are empty, but they are by no means "nothing".

Would you call them false? Would you call 0 empty? It's an intuitive
way of explaining the concept, not a precise one. Nothing is nothing,
and as long as you try to represent it with a value (which must, by
necessity, be something, you will fail).

>Example:
>
>def show(foo, history=None):
>    """prints foo, optionally storing what was printed in a history list."""
>    print foo
>    if history:
>        history.append(foo)

Very odd function. "If there is a history" (i.e. the history list
contains something) "append something else"...

I don't think this is less clear if one considers the empty list as
"nothing". If your history is represented as a history, I'd say you
had no history if the list were empty.

The fact that you're using None as a placeholder means that you
shouldn't use a plain boolean check for the history here -- you should
use the standard idiom:

def show(foo, history=None):
    print foo
    if history is not None:
        history.append(foo)

or perhaps:

def show(foo, history=None):
    print foo
    try: history.append(foo)
    except TypeError: pass

(might be more efficient but will hide the error of passing a
non-appendable object as history.)

>This works as intended:
[snip]
>But this doesn't:
>
>   history = []
>   show("hello world", history) # strange..history doesn't change here?

IMO that should be obvious from your function definition. I don't see
what it is you feel this example proves. That seeing the empty list as
"a list-shaped nothing" (in Laura's words) would make people think
that only None qualifies as false (as opposed to an empty list)? I
really don't understand your interpretation here.

>It doesn't work, because show() tests for emptyness with "if
>show_history:".

Of course.

> What show() should have done is test for nothingness
>using "if show_history is None:".

No -- it should have tested whether history was the default value you
provided. This is not a test of nothingness -- it's not even a test of
None-ness (using the "ness" ending to indicate structural polymorphic
equivalence, which is normal in Python); it's a test of whether or not
the object is identical to None.

--
Magnus Lie Hetland                                  The Anygui Project
http://hetland.org                                  http://anygui.org