[Tutor] Dots-And-Boxes

[Kalle Svensson]

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[Pijus Virketis]
> Reading through Danny's code, I found a frequently used keyword
> "assert", like so:
>  
> assert 2 <= self.width and 2 <= self.height,\
>                "Game can't be played on this board's dimension."
>  
> I've never seen if before, so I checked the Reference, and found a
> description here:
> http://www.python.org/doc/current/ref/assert.html. However, the
> syntax given there is different:
>  
> if __debug__:
>    if not expression: raise AssertionError

This is the eqivalent replacement syntax for

assert expression

> Also, the keyword itself is not really mentioned. So, my question is
> this: what does "assert" do, and where can I find out more about its
> use the way Danny employs it?

The assert statement evaluates an expression, and if the result is
false (0, None, [], () or "", for example) raises an AssertionError.
This only happens if the built-in variable __debug__ is true.

Thus:

assert a == b

and 

if __debug__:
    if not a == b:
        raise AssertionError

do the same thing.

__debug__ is always true if you haven't started python with the -O
option.  For example:

: kalle@chloe [~]$ ; python -O -c "assert 0; print 'Hi'"
Hi
: kalle@chloe [~]$ ; python -c "assert 0; print 'Hi'"
Traceback (most recent call last):
  File "<string>", line 1, in ?
AssertionError

Peace,
  Kalle
- -- 
Kalle Svensson, http://www.juckapan.org/~kalle/
Student, root and saint in the Church of Emacs.
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