asserts in python may not work as expected

While getting the phone suite “paroli” (http://paroli-project.org) working on OpenWrt, I got some thrown exceptions in the OpenWrt-environment which I didn’t get in the official OM-builds.
I tried to figure out what exactly is causing the exception on the OpenWrt-target and created a minimal program which raises it:

You don’t have to know what’s <tichy> and its <Item> class, nor what <issubclass> is doing exactly – just mind the return values.

root@OpenWrt:/# DISPLAY=:0 python
Python 2.6.1 (r261:67515, JunĀ  8 2009, 09:18:20)
[GCC 4.1.2] on linux2
Type “help”, “copyright”, “credits” or “license” for more information.
>>> from tichy.item import Item
>>> issubclass(bool, Item)
False
>>> assert issubclass(bool, Item)
Traceback (most recent call last):
File “<stdin>”, line 1, in <module>
AssertionError

Ah, ok. On the OpenWrt-target the assert throws an exception, because <issubclass> returns <False>

So, logically, within the Openmoko-environment, the <issubclass>-statement must return <True> to not get assert raising an exception – but that’s not the case:

root@om-gta02:~# DISPLAY=:0 python
Python 2.6.2 (r262:71600, May 31 2009, 00:18:54)
[GCC 4.1.2] on linux2
Type “help”, “copyright”, “credits” or “license” for more information.
>>> from tichy.item import Item
>>> issubclass(bool, Item)
False
>>> assert issubclass(bool, Item)
>>>

Huh – ok, let’s take a closer look here:

<issubclass (bool, Item)> is returning False, but <assert issubclass(bool, Item)> is not throwing an exception – that’s weird and caused me to do test the assert-statement in a very simple way:

root@om-gta02:~# DISPLAY=:0 python
Python 2.6.2 (r262:71600, May 31 2009, 00:18:54)
[GCC 4.1.2] on linux2
Type “help”, “copyright”, “credits” or “license” for more information.
>>> assert False
>>>

That’s definitely not the expected result of assert.

So, why is that happening? Why python’s assert is just doing nothing?

Some research about that got me into this statement:

“[..]The current code generator emits no code for an assert statement when optimization is requested at compile time. Note that it is unnecessary to include the source code for the expression that failed in the error message; it will be displayed as part of the stack trace.[..]” – http://docs.python.org/reference/simple_stmts.html#grammar-token-assert_stmt

Let’s check that with a regular python-installation:

$ python
Python 2.5.4 (r254:67916, Feb 18 2009, 03:00:47)
[GCC 4.3.3] on linux2
Type “help”, “copyright”, “credits” or “license” for more information.
>>> assert False
Traceback (most recent call last):
File “<stdin>”, line 1, in <module>
AssertionError
>>>
$ python -O # enable optimizations
Python 2.5.4 (r254:67916, Feb 18 2009, 03:00:47)
[GCC 4.3.3] on linux2
Type “help”, “copyright”, “credits” or “license” for more information.
>>> assert False
>>>

Conclusion:
– some distributions are shipping their python-package configured for executing python-scripts with optimizations enabled
– asserts may not act as expected, means, they may don’t do anything when above is true
– i’m not using asserts anymore when my app is relying on asserts and I don’t know for sure which target/distribution my scripts are running on