Assertion in list comprehension

[Chris Mellon]

On 01 Aug 2007 16:55:53 GMT, Stargaming <stargaming at gmail.com> wrote:
> On Wed, 01 Aug 2007 11:28:48 -0500, Chris Mellon wrote:
>
> > On 8/1/07, beginner <zyzhu2000 at gmail.com> wrote:
> >> Hi,
> >>
> >> Does anyone know how to put an assertion in list comprehension? I have
> >> the following list comprehension, but I want to use an assertion to
> >> check the contents of rec_stdl. I ended up using another loop which
> >> essentially duplicates the functions of list comprehension. It just
> >> look like a waste of coding and computer time to me.
> >>
> >> I just wish I could put the assertions into list comprehensions.
> >>
> >>         x=[(rec_stdl[0].st/10000.0,
> >>             rec_stdl[0].cl,
> >>             rec_stdl[0].bb,
> >>             rec_stdl[0].bo,
> >>             rec_stdl[1].bb,
> >>             rec_stdl[1].bo,
> >>             rec_stdl[0].ex
> >>            )
> >>            for rec_stdl in rec_by_ex if len(rec_stdl)==2
> >>         ]
> >>
> >>         #duplicated loop
> >>         if __debug__:
> >>             for rec_stdl in rec_by_ex:
> >>                 l=len(rec_stdl)
> >>                 assert(l<=2 and l>0)
> >>                 if l==2:
> >>                     assert(rec_stdl[0].c=="C" and rec_stdl[1].c=="P")
> >>                     assert(rec_stdl[0].ex==rec_stdl[1].ex)
> >>                     assert(rec_stdl[0].st==rec_stdl[1].st)
> >>                     assert(rec_stdl[0].cp==rec_stdl[1].cp)
> >
> > First: All your asserts are wrong. Assert is a statement, not a
> > function. These specific ones will behave as expected, but it's easy to
> > accidentally write ones that always pass this way.
>
> Could you come up with an example? I can only think of accidentally
> injecting a comma, what would create a (true, in a boolean context) tuple.
>
> And, well, if you're only using () for readabilty, this might sometimes
> look messy when calling assert with the extended syntax::
>
>   assert(False), "error text"
>

It's very easy to write this as assert(False, "error text") if you're
in the habit of thinking that assert is a function.

> Where one could expect the construction of a tuple.
>
> > Secondly: This is a waste of code, because if __debug__ is not defined
> > asserts will be skipped by the compiler. You could use the same loop
> > block for both branches.
>
> Well, the `assert` isn't there for no reason, but if you're serious about
> it, `raise` could be better.
>
> > Thirdly: This sort of testing is precisely what unit tests and/or
> > doctests are for.
>
> Huh? What beginner is doing there seems more like input validation than
> testing. Unit or doctests are meant for testing (and in case of doctests,
> showing) whether a function works as expected.

Not in a big __debug__ block it isn't.

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