Chris Angelico wrote: > On Wed, Oct 22, 2014 at 12:44 PM, Steven D'Aprano > <steve+comp.lang.pyt...@pearwood.info> wrote: >> def do_something(instance_or_id): >> instance = Model.get(instance_or_id) >> assert isinstance(instance, Model) >> # Code that assumes that instance is an object of type Model >> >> >> That means that the logic for what is acceptable as a Model is all in one >> place, namely the Model.get method, and callers don't need to care about >> the pre-condition "argument is a Model or an integer ID", they only need >> to care about the post-condition "result of get() is a Model". > > And at that point, the assertion is redundant, on par with: > > a = len(seq) > assert isinstance(a, int) > > because you shouldn't have to assert what's part of a function's > guarantee.
That depends on how well you trust the function, how paranoid you are, and whether you wish to include a checked comment, among other considerations. I'd prefer to write the above than: a = len(seq) # At this point, a is an int. because comments inside code that aren't checked are technically known as "lies" <http://import-that.dreamwidth.org/956.html>. Ha ha only serious. I wouldn't write such a comment for len() since I would expect anyone reading the code to know what len() does, but the same doesn't necessarily apply for every function call I make. Checking the post-condition of a built-in like len() is too paranoid for my tastes, as len() enforces the rule that __len__() methods return a non-negative integer, and there are millions of lines of Python code around the world calling len(). Somebody surely have noticed by now if len() violated that post-condition. But for a function from my own code base, where I might only have dozens of users (oh to have that many!) and fewer unit tests than perhaps I ought to, I might not be quite so confident that the assertion was redundant. To err is human, and so there are occasions when it is appropriate to trust but verify. As I wrote: This is why assert can be divisive. Since we can vary in our confidence about the correctness of code, one person's useful assertion may be another person's useless runtime test. http://import-that.dreamwidth.org/676.html -- Steven -- https://mail.python.org/mailman/listinfo/python-list
