Conditional __debug__ statement not executed though __debug__ is True

Question

Short Version

I have a section of code I'm debugging that checks the value of __debug__ and executes some code if it is True.

if __debug__:
  <stuff happens>

The problem is that "stuff" never happens, even though __debug__ appears to be True.

Long Version / Details

To check this, I am printing out the values of several variables, most notably __debug__, to a file as the function executes, using the following pattern. (I am using os.open because open is already defined in this module.)

try:
  myfile = os.open("test.txt", os.O_RDWR|os.O_CREAT|os.O_APPEND)
  # work + some print statements to check the value of __DEBUG__
finally:
  os.close(myfile)

The piece of code I'm most confused by looks like this:

os.write(myfile, "LINE %s | LDAP FUNCTION __DEBUG__: %s \n" %(sys._getframe(0).f_lineno, __debug__))
os.write(myfile, "LINE %s | LDAP FUNCTION __DEBUG__: %s \n" %(sys._getframe(0).f_lineno, type(__debug__)))
os.write(myfile, "LINE %s | LDAP FUNCTION __DEBUG__: %s \n" %(sys._getframe(0).f_lineno, bool(__debug__)))
if __debug__:
  os.write(myfile, "LINE %s | LDAP FUNCTION __DEBUG__: %s \n" %(sys._getframe(0).f_lineno, __debug__))
if bool(__debug__):
  os.write(myfile, "LINE %s | LDAP FUNCTION __DEBUG__: %s \n" %(sys._getframe(0).f_lineno, __debug__))
if True:
  os.write(myfile, "LINE %s | LDAP FUNCTION __DEBUG__: %s \n" %(sys._getframe(0).f_lineno, __debug__))
if __debug__:
  os.write(myfile, "LINE %s | LDAP FUNCTION __DEBUG__: %s \n" %(sys._getframe(0).f_lineno, __debug__))

And the output file looks like this:

LINE 82 | LDAP FUNCTION __DEBUG__: True 
LINE 83 | LDAP FUNCTION __DEBUG__: <type 'bool'> 
LINE 84 | LDAP FUNCTION __DEBUG__: True 
LINE 88 | LDAP FUNCTION __DEBUG__: True 
LINE 90 | LDAP FUNCTION __DEBUG__: True 

The first 3 statements (lines 82-84) are every way I could think of checking if __debug__ is "truthy", and all 3 imply that __debug__ is True. Similarly, casting __debug__ as a boolean and then evaluating if (line 88) works as expected too. Line 90 is a silly sanity check.

Is there anything I'm missing in the way __debug__ works that may be causing this?

Note: I found this while I was working through an error I am getting in the _ldap_function_call function in the python-ldap module. I only get this error when using IIS - everything works fine with Django's development server.

Answer 1

If you rebind __debug__, it can cause symptoms exactly like this.

This is because __debug__ is somewhat magical. During module compilation, the same code that handles literals also handles the magic constants ..., None, True, False, and __debug__. (See, for example, expr_constant.)

If you run dis on your code to dump out the bytecode, you'll see that if __debug__: statements are either removed entirely, or use LOAD_CONST to load the compile-time debug constant, while if bool(__debug__): statements use LOAD_GLOBAL to load the value of __debug__.

Of course these are guaranteed to be the same… unless you rebind __debug__. Somewhere around 2.3, it became illegal to just write __debug__ = False. In 2.7 and 3.0, it became illegal to bind any attribute named __debug__, which means you can no longer do things like sys.modules[__name__].__debug__ = False. But you can still do, e.g., globals()['__debug__'] = False.

And either way, you get the same effect:

if __debug__:
    print "debug"
if bool(__debug__):
    print "bool"

import sys
sys.modules[__name__].__debug__ = False

if __debug__:
    print "debug2"
if bool(__debug__):
    print "bool2"

This prints out:

debug
bool
debug2

And likewise for code that sets it to True when run with python -O.