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

[Anders J. Munch]

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.

[] and {} are objects, they are objects with identity, and they are
mutable objects.  They are empty, but they are by no means "nothing".

Example:

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

This works as intended:

    # Omit default argument, don't remember history
    show("hello world")

This works as intended:

    history = ["in the beginning was nothing"]
    show("hello world", history) # adds "hello world" to history list

But this doesn't:

   history = []
   show("hello world", history) # strange..history doesn't change here?

It doesn't work, because show() tests for emptyness with "if
show_history:".  What show() should have done is test for nothingness
using "if show_history is None:".

--
Anders Munch. Software Engineer, Dancontrol A/S, Haderslev, Denmark
Still confused but at a higher level.