[Python-Dev] #12982: Should -O be required to *read* .pyo files?

[Nick Coghlan]

On Thu, Jun 14, 2012 at 6:06 AM, Terry Reedy <tjreedy at udel.edu> wrote:
> On 6/13/2012 2:46 PM, Antoine Pitrou wrote:
>
>> Not only docstrings, but also asserts. I think running a pyo without -O
>> would be a bug.
>
>
> That cat is already out of the bag ;-)
> People are doing that now by renaming x.pyo to x.pyc.
> Brett claims that it is also easy to do in 3.3 with a custom importer.

Right, but by resorting to either of those approaches, people are
clearly doing something that isn't formally supported by the core.
Yes, you can do it, and most of the time it will work out OK, but any
weird glitches that result are officially *not our problem*.

The main reason this matters is that the "__debug__" flag is
*supposed* to be process global - if you check it in one place, the
answer should be correct for all Python code loaded in the process. If
you load a .pyo file into a process running without -O (or a .pyc file
into a process running *with* -O), then you have broken that
assumption. Because the compiler understands __debug__, and is
explicitly free to make optimisations based on the value of that flag
at compile time (such as throwing away unreachable branches in if
statements or applying constant folding operations), the following
code will do different things if loaded from a .pyo file instead of
.pyc:

    print("__debug__ is not a builtin, it is checked at compile time")
    if __debug__:
        print("A .pyc file always has __debug__ == True")
    else:
        print("A .pyo file always has __debug__ == False")

$ ./python -c "import foo"
__debug__ is not a builtin, it is checked at compile time
A .pyc file always has __debug__ == True
$ ./python -O -c "import foo"
__debug__ is not a builtin, it is checked at compile time
A .pyo file always has __debug__ == False
$ ./python __pycache__/foo.cpython-33.pyo
__debug__ is not a builtin, it is checked at compile time
A .pyo file always has __debug__ == False
$ ./python -O __pycache__/foo.cpython-33.pyc
__debug__ is not a builtin, it is checked at compile time
A .pyc file always has __debug__ == True

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia