Python Assert Quiz


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At Europython conference, in my presentation, I talked about re-formulations of Python into simpler Python. It is my intention to turn this into a series of Python quiz questions that you will hopefully enjoy.

Update

Due to comments feedback, I made it more clear that "-O" affects of course both cases, and due to work on getting the recent CPython2.7 test suite to work, I noticed, how the re-formulation for Quiz Question 2 needed a version dependent solution.

And I thought this one was easy. :-)

Quiz Question 1

Say you have the following code:

assert x == y

How can you achieve the same thing, without using the assert statement at all. The behavior is required to be absolutely the same.

The answer is in the next paragraph, so stop reading if you want to find out yourself.

Solution 1

The correct answer is that assertions are the same as a raise exception in a conditional statement.

if not x == y:
    raise AssertionError

The thing where this makes a difference, is "-O", which will discard assertions, but I consider it rarely used. To be really compatible with that, it should be:

if __debug__ and not x == y:
    raise AssertionError

Quiz Question 2

But wait, there is slightly more to it. Say you have the following code:

assert x == y, arg

How can you achieve the same thing, without using the assert statement at all. The behavior is required to be absolutely the same.

The answer is in the next paragraph, so stop reading if you want to find out yourself.

Solution 2

This is actually version dependent, due to recent optimizations of CPython.

  • For version 2.6 it is as follows:

    The extra value to assert, simply becomes an extra value to raise, which indicates, delayed creation of the AssertionError exception.

    if not x == y:
        raise AssertionError, arg
    
  • For version 2.7 and higher it is as follows:

    The extra value to assert, simply becomes the argument to creating the AssertionError exception.

    if not x == y:
        raise AssertionError( arg )
    

So, even in the more complex case, you end up with a conditional raise.

The only thing where this makes a difference, is "-O", which will discard assertions, but I consider it rarely used. To be really compatible with that, it should be:

if __debug__ and not x == y:
   raise AssertionError ....

Surprised? Well, yes, there really is nothing to assert statements. I am using this for my Python compiler Nuitka which benefits from having not to deal with assert as anything special at all. See also the respective section in the Developer Manual which explains this and other things.

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