[Tutor] assert - was: Calculating a math formula

[Michael Janssen]

On Tue, 14 Jan 2003, Jens Kubieziel wrote:

> On Mon, Jan 13, 2003 at 03:22:07AM -0800, Danny Yoo wrote:
> > def factorial(z):
> >     assert z >= 0, "Invalid input: z must be nonnegative."
>       ^^^^^^
> Where can I find some information what this exactly does? I searched my
> docs and find something on $PYTHONDOC/ref/assert.html. Are there some
> other examples?

another bit is in lib/module-exceptions.html#l2h-233:

   exception AssertionError
           Raised when an assert statement fails.

assert tests if expression is true and raise "AssertionError" if not.

In "long form":
if not z >= 0:
   raise AssertionError, "Invalid input: z must be nonnegative."


Why to use this? As ref/assert.html states:
"Assert statements are a convenient way to insert debugging assertions
into a program" - it's shorter and more meaningful than an if-raise
statement.

Michael

>
> Now this function _always_ raises an exception. And I'm a bit perplex ...

sure? not on my system:

>>> def factorial(z):
...     assert z >= 0, "Invalid input: z must be nonnegative."
...     if z == 1 or z == 0:
...         return 1
...     else:
...         return z*factorial(z-1)
...
>>> for n in range(5,-2,-1):
...   print n, ":", factorial(n)
...
5 : 120
4 : 24
3 : 6
2 : 2
1 : 1
0 : 1
-1 :


> --
> Jens Kubieziel                                  mailto:jens@kubieziel.de
> So, how's your love life?  Still holding your own?
>
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