[Tutor] Question about the use of None

[Danny Yoo]

On Fri, 11 Jun 2004, [iso-8859-1] Wilfred Y wrote:

> Hi,I came across this keyword None. The document I read says "None is
> python's way of representing nothing. It evaluates to False when treated
> as a condition"


Hi Wilfred,

Which document are you referring to?  We can also take a look and see if
the wording is weird.  What you're describing right now actually sounds
subtly off.  *grin*


The 'None' value is a single value that's used to represent the idea of
nothing.  It can be used in comparisons:

###
>>> None == 1
False
>>> None == 0
False
>>> None == ""
False
>>> None == None
True
###

and the only 'value' that None compares favorably to is itself the None
value.



> So I assume, if I do start=None, then it's as good as saying start="" or
> start=0Now, the code to represent it's use is pretty confusing. It goes
> like this:start=Nonewhile start != ""  start=raw_input("Press enter to
> exit, or key in a string: ")The confusing part is here, since start =
> "", then it should never be able to go into the while loop.


[Small note: use indentation to set off your code from your explanation;
it'll make it easier for us to test and see how your code works.]


Let's look at the code again:

###
start = None
while start != "":
    start = raw_input("Press enter to exit, or key in a string: " )
###


> So it seems None does have a value, which explains why it managed to
> enter the while loop.

Yes.  None is a perfectly legitimate value.



What the author of the material you were reading was trying to say,
though, is that None is one of the values that Python considers as a
'false' value.  If we are working with an 'if' statement, like:

###
if foo:
    ...
###


then the '...' part gets executed only if 'foo' is a true-ish value.
Every value in Python is considered true, except for a few special cases.



You may find the following useful:

###
>>> def testTruth(value):
...     """Prints true or false, depending if the value that we get is
...        a true or false value."""
...     if value:
...         print "True!"
...     else:
...         print "False!"
...
>>> testTruth('hello')
True!
>>> testTruth(42)
True!
>>> testTruth(False)
False!
###

The function above will say "True!" or "False!", depending if the value we
pass is considered True or False to the 'if' statement.

Try testTruth() on these values:

    1
    42 - 2 * 21
    0 == 0
    0
    None
    "hello"
    "hello"[0:0]

If you can predict when it says True! or False! with good accuracy, then
you probably understand the author's point.  *grin*


Python is actually quite loose when it comes to truth and falsehood.  In
fact, there are other computer languages that are really strict about this
issue.