How can I make my assertions smarter?

Tim Peters tim_one at
Wed Nov 3 06:41:17 CET 1999

[Stuart Hungerford, desperately hoping that design by contract will
 be easier in Python if he calls it by some other name ...]

> I'm using some simple functions to implement crude precondition,
> postcondition and invariant checking in Python classes.
> (Tim: please be gentle ;-)

I am always gentle with you, Stuart!  Your heart is in the right place --
although it may as well be firmly stuck up your ass for all the good it's
doing you <wink>.

> Is there some neat way to wrap these tests up so I can print any
> code expression that fails, e.g:
>      Precondition failed line 42: spoon.shape == 'bent'
> which would appear at line 42 of as
>      pre(spoon.shape == 'bent')

As Michael Hudson amply demonstrated, you're fighting the language.  If you
can't live with

    assert spoon.shape == 'bent', "precondition failed"

then, if I were you, I'd write a little preprocessor to transform your
pre/post/invariant(...) thingies into the desired code.  That's fighting the
language too, of course, but in a way that's easier to extend along with
your ambitions.  DBC runs deep!  I don't think it can be tacked on

generously-leaving-eiffel-a-reason-to-exist<wink>-ly y'rs  - tim

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