Assertions

[Steve D'Aprano]

On Fri, 22 Sep 2017 03:31 am, Chris Angelico wrote:

> On Fri, Sep 22, 2017 at 3:23 AM, Steve D'Aprano
> <steve+python at pearwood.info> wrote:
>> 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
> 
> Impressive. That means that, in 2.7, it's actually equivalent to:
> 
>>>> def test3():
> ...     if not foo: raise AssertionError, "bar baz"

That's nothing. In 1.5 (yes, *one* point five) it's equivalent to something more
or less like this:

def test4():
    if __debug__:
        if foo:
            return
        raise AssertionError('bar baz')


> Although in the 2.7 that I have, the assert statement does actually
> *call* AssertionError (ie it constructs an instance and raises it).
> What version are you running? Here's mine:
> 
> $ python2
> Python 2.7.13 (default, Jan 19 2017, 14:48:08)
> [GCC 6.3.0 20170118] on linux2

Now that you've showed me yours, I suppose I have to show you mine.

Python 2.7.2 (default, May 18 2012, 18:25:10)

So definitely version dependent.


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