Assertion in list comprehension

danmcleran at yahoo.com danmcleran at yahoo.com
Wed Aug 1 18:09:00 CEST 2007


On Aug 1, 9:37 am, 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)
>
> Thanks,
> Geoffrey

Can't you just call a function from within your list comprehension and
do whatever you want for each item? Something like this (not tested):

def checker(item):
    assert(len(item) <= 2 and len(item) > 0)
    if len(item) == 2:
        assert(item[0].c == "C" and item[1].c == "P"

    return len(item) == 2

x = [whatever for item in all_items if checker(item = item)]




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