Assertion in list comprehension

[Chris Mellon]

On 8/1/07, beginner <zyzhu2000 at gmail.com> wrote:
> On Aug 1, 11:28 am, "Chris Mellon" <arka... at gmail.com> wrote:
> > On 8/1/07, beginner <zyzhu2... 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.
>
>
> Do you mean I should not use the parentheses?
>

Yes.

>
> > 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.
>
> I know. My original question was how. Dan suggested to write a checker
> function.
>

Use the second block in all cases. In any situation where "if
__debug__" is False, the asserts are a noop and in fact won't even be
present in the bytecode.

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