python assert fires with -O -

i trying make sure assert not executed python when using -o. test program indicates executed. use -o on command line , used -o when ran setup.py both build , install. before submit bug report wanted make sure did not rookie mistakes...

so need else or different assert not executed?

my simple script:

print __debug__  if __debug__:     print "true branch" else:     print "false branch"  assert(false) 

works when run standalone. prints:

false false branch 

when copy snippet in main program (that cannot include here...) get:

false true branch assertionerror 

i utterly confused how can come about. python 2.7.6 on mac. (sorry mac, have use work.)

you can achieve effect describe running .pyc file directly -o flag. that's abuse of way things supposed work. want either:

1. run .py file or without -o flag (the usual approach), or
2. run .pyc file without -o flag, or
3. run .pyo file with -o flag.

if run .pyc file -o flag, or .pyo file without it, you're going surprises one.

what's happening @ compile time peephole optimiser has optimised away if __debug__ branches, .pyc or .pyo files execute appropriate branch unconditionally. when run wrong -o specification, you'll running value of __debug__ doesn't match optimisation applied @ compile time.

there similar issue reported on python issue tracker while back, though opposite situation: running .pyo file without using -o flag.

a quick example: suppose i've got file yours named "debug_example.py" sitting in current directory:

noether:desktop mdickinson$ cat debug_example.py def main():     print "__debug__ {}".format(__debug__)     if __debug__:         print "__debug__ true"     else:         print "__debug__ false"  if __name__ == '__main__':     main() 

if execute file directly, or without -o flag, see expected results:

noether:desktop mdickinson$ python2 debug_example.py __debug__ true __debug__ true noether:desktop mdickinson$ python2 -o debug_example.py __debug__ false __debug__ false 

now let's compile file "debug_example.pyc" file using handy py_compile module. (in case compilation being performed part of setup.py installation.):

noether:desktop mdickinson$ python2 -m py_compile debug_example.py noether:desktop mdickinson$ ls -l debug_example.pyc -rw-r--r--  1 mdickinson  staff  350 24 mar 21:41 debug_example.pyc 

now execute debug_example.pyc file, (wrongly) using -o flag, , python gets confused:

noether:desktop mdickinson$ python2 -o debug_example.pyc __debug__ false __debug__ true 

we can use python's dis module see bytecode inside module:

python 2.7.6 (default, nov 18 2013, 15:12:51)  [gcc 4.2.1 compatible apple llvm 5.0 (clang-500.2.79)] on darwin type "help", "copyright", "credits" or "license" more information. >>> import debug_example >>> import dis >>> dis.dis(debug_example) disassembly of main:   2           0 load_const               1 ('__debug__ {}')               3 load_attr                0 (format)               6 load_global              1 (__debug__)               9 call_function            1              12 print_item                        13 print_newline           4          14 load_const               2 ('__debug__ true')              17 print_item                        18 print_newline                     19 load_const               0 (none)              22 return_value         

note there's no bytecode corresponding if statement there @ all: see unconditional printing of '__debug__ true'.

solution: don't execute .pyc or .pyo files directly: execute .py file , let python figure out whether use .pyc or .pyo appropriate.