Why assert is not a function?

[Stestagg]

There is also the, I think seldom used, feature that calling python with
'-O' removes all assert statements at parse/compile time (I believe for
performance reasons)

So for example:

$ python -c 'assert False'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AssertionError

But:

$ python -O -c 'assert False'
[no output]
$

Ignoring the question about this feature being particularly useful, it
would be much harder to implement if assert were a standard function (and
re-bindable), as the 'arguments' in the assert construct are not even
executed when '-O' is used (this is the performance gain):

$ python -O -c 'assert DoesNotExist'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
NameError: name 'DoesNotExist' is not defined

vs.

$ python -O -c 'assert DoesNotExist'
[no output]
$

so the compiler would have to do a lot of work to identify assert calls and
remove them.

Steve



On Tue, Mar 2, 2021 at 9:04 PM Chris Angelico <rosuav at gmail.com> wrote:

> On Wed, Mar 3, 2021 at 7:53 AM Marco Sulla <Marco.Sulla.Python at gmail.com>
> wrote:
> >
> > I have a curiosity. Python, as many languages, has assert as a
> > keyword. Can't it be implemented as a function? Is there an advantage
> > to have it as a keyword?
>
> It could, but it would need some magic. A lot of test frameworks have
> their own set of assertions and you have to pick the one you want -
> for instance:
>
> assertEqual(x, y)
>
> instead of
>
> assert x == y
>
> The trouble is that, if you write a function for this, it will just
> get True or False, without any explanation of the cause - making it
> somewhat unhelpful when something goes wrong. To compensate, function
> equivalents inevitably have to have myriad types of assertions, where
> the language just needs one.
>
> ChrisA
> --
> https://mail.python.org/mailman/listinfo/python-list
>