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In certain circumstances, "if __debug__" seems to be evaluating its "else" clause even when -O is not specified. This can cause very surprising behavior.
a.zip contains a single file named a/__init__.pyo. It is the compiled form of the following code:
 if __debug__:
 print 'DEBUG'
 else:
 print 'RELEASE'
 if __debug__ == True:
 raise Exception("this is impossible")
pythonbug.py evaluates this script with the following:
 import sys
 sys.path = ['a.zip']
 import a
When using Windows Python 2.6.2 and 2.6.5, running pythonbug.py produces this output:
 RELEASE
 Traceback (most recent call last):
 File "pythonbug.py", line 3, in <module>
 import a
 File "__init__.py", line 8, in <module>
 raise Exception("this is impossible")
 Exception: this is impossible
When -O is passed, the exception is not raised.
My best guess is that the Python bytecode is optimizing the "if __debug__" test away in the produced .pyo, but does not actually affect the value of __debug__ or check more complex expressions involving __debug__.