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.
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
.
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