On Wed, 01 Aug 2007 11:28:48 -0500, Chris Mellon wrote: > On 8/1/07, beginner <[EMAIL PROTECTED]> 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" 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. -- http://mail.python.org/mailman/listinfo/python-list
