> I wonder, could this be simplified a bit, on the assumption that a
> well-written assertion shouldn't have a problem with being executed
> twice?
While I agree as an engineering principle an assert should not have side
effects
and hence re-evaluation should be fine in most cases, it is not universal.
It is possible
for assertions to not have side effects but yet change value between
evaluations if they
interact with a shared resource such as the file system..
For example consider the following assertion:
assert os.path.isdir("config") and os.path.isfile("config/setup.yml")
It is completely possible for the value of this expression to change
between evaluations. Granted this would
like mean their is some more significant issue with my code, however, I
would like the interpreter to give me
accurate information about why my assertion failed. Bad information is
worse than no information. Like imagine
that on the first evaluation the directory config does not exist but on the
second it has been created by another process.
A naive revaluation strategy would likely result in it pointing at the
second clause and saying the assertion failed their
when it really failed on the first clause. This would send me down a rabbit
hole of debugging why setup.yml was not
constructed properly instead of debugging why the config directory didn’t
exist.
Further while it is bad engineering practices to have side effects in an
assert it is completely possible.
For example consider the following pathological example:
class PathologicalFoo:
def __init__(self):
self._val = 0
def get(self):
old_val = self._val
self._val = 1
return old_val
foo = PathologicalFoo()assert foo.get() == 1
Or worse it is possible for revaluation to cause errors
class UniquePtr:
def __init__(self, obj):
self.set(obj)
def get(self):
if self._valid:
self._valid = False
obj = self._obj
self._obj = None
return obj
else:
raise ValueError()
def set(self, obj):
self._obj = obj
self._valid = True
x = UniquePtr(1)assert x.get() == 0
x.set(0)
How would the interpreter handle this?
On Sun, Sep 12, 2021 at 11:39 AM Chris Angelico <[email protected]> wrote:
> On Mon, Sep 13, 2021 at 1:37 AM Serhiy Storchaka <[email protected]>
> wrote:
> >
> > 12.09.21 17:28, Guido van Rossum пише:
> > > This is cool.
> > >
> > > AFAIK pytest does something like this. How does your implementation
> differ?
> >
> > What pytest does is awesome. I though about implementing it in the
> > standard compiler since seen it the first time.
> >
> > > What is your argument for making this part of the language? Why not a
> > > 3rd party library?
> >
> > It needs a support in the compiler. The condition expression should be
> > compiled to keep all immediate results of subexpressions on the stack.
> > If the final result is true, immediate results are dropped. If it is
> > false, the second argument of assert is evaluated and its value together
> > with all immediate results of the first expression, together with
> > references to corresponding subexpressions (as strings, ranges or AST
> > nodes) are passed to the special handler. That handler can be
> > implemented in a third-party library, because formatting and outputting
> > a report is a complex task. The default handler can just raise an
> > AttributeError.
> >
>
> I wonder, could this be simplified a bit, on the assumption that a
> well-written assertion shouldn't have a problem with being executed
> twice? Instead of keeping all the subexpressions around (a run-time
> cost), keep the AST of the expression itself (a compile-time cost).
> Then, when the exception is about to be printed to the console,
> re-evaluate it and do the display.
>
> ChrisA
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