Assertions

[Steve D'Aprano]

On Fri, 22 Sep 2017 02:59 am, Ned Batchelder wrote:

> On 9/21/17 12:29 PM, Tobiah wrote:
>> Are these completely equivalent?
[... assert, versus test and raise AssertionError ...]

> Let's see:
[...]
> Yes, they are completely equivalent, compiling to precisely the same
> bytecode.


That is definitely version-dependent, because I've just tried it and got
different byte-code in Python 2.7.

py> import dis
py> def test1():
...     assert foo, "bar baz"
...
py> def test2():
...     if not foo: raise AssertionError("bar baz")
...
py>
py> dis.dis(test1)
  2           0 LOAD_GLOBAL              0 (foo)
              3 POP_JUMP_IF_TRUE        15
              6 LOAD_GLOBAL              1 (AssertionError)
              9 LOAD_CONST               1 ('bar baz')
             12 RAISE_VARARGS            2
        >>   15 LOAD_CONST               0 (None)
             18 RETURN_VALUE
py> dis.dis(test2)
  2           0 LOAD_GLOBAL              0 (foo)
              3 POP_JUMP_IF_TRUE        21
              6 LOAD_GLOBAL              1 (AssertionError)
              9 LOAD_CONST               1 ('bar baz')
             12 CALL_FUNCTION            1
             15 RAISE_VARARGS            1
             18 JUMP_FORWARD             0 (to 21)
        >>   21 LOAD_CONST               0 (None)
             24 RETURN_VALUE



So I do not believe that we can say that they will always compile to the same
byte-code. There were differences in past versions, there could be differences
again in the future.


I was going to be cheeky and run the same test with -0, giving:

py> dis.dis(test1)
  2           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE

which of course is a big difference between the two, but it turned out that
there can be other byte-code differences.

So no, I don't believe we can say they are *completely* equivalent, since they
could compile to slightly different byte-code with slightly-different
performance characteristics. But I think we can say that *semantically* the two
ought to be the same.




-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.