Assertion in list comprehension

[Chris Mellon]

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.

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.

Thirdly: This sort of testing is precisely what unit tests and/or
doctests are for.