On Tue, Feb 11, 2020 at 07:10:18PM -0000, [email protected] wrote:
> I have been taught that assertions must not produce side effects at all.
That's good practice, but saying that they "must not" is too strong.
The risk here is not the side-effect, but relying on that side-effect
(which may be disabled) outside of the assertion. Some side-effects are
perfectly fine:
assert log(message) or condition
> This is a problem in languages such as C. But not in Python (until
> 3.8) since it does not allow assignment expressions.
Assignment is not the only possible side-effect. These have been possible
back to Python 2.4 and probably older:
assert mylist.pop()
assert globals().update({'flag': expression}) or flag
assert delattr(mymodule, 'spam') or condition
In Python 3, we can also do things like this:
assert print(message) or condition
assert exec("value = expression") or condition
Since assertions can call functions (and methods), and functions can
have side-effects, Python has *always* been able to have side-effects
inside assertions.
> But now, for best or worst, we have the walrus operator
Oh, definitely for the best. This now allows us to write complex
assertions much more easily and efficiently:
assert log(flag := complex_expression) or flag
Previously we would have to write this:
if __debug__:
flag = complex_expression
assert flag
which is much less convenient, but is another way to get side-effects in
an assertion.
> However, if assertions are disabled:
>
> `python3.8 -Oc "assert (always := True); print(f'{always=}')"`
>
> it raises an exception:
Indeed. So does this:
[steve@ando ~]$ python -c "if False:
always = 999
print(always)"
Traceback (most recent call last):
File "<string>", line 3, in <module>
NameError: name 'always' is not defined
and for roughly the same reason.
[...]
> Do not you think that disabling walrus operators in assertions would
> be a good idea?
No.
--
Steven
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