When to use assert

Steven D'Aprano steve+comp.lang.python at pearwood.info
Wed Oct 22 17:49:31 CEST 2014

Chris Angelico wrote:

> On Wed, Oct 22, 2014 at 12:44 PM, Steven D'Aprano
> <steve+comp.lang.python at 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.



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