Python equivalent for C module

[Bruno Desthuilliers]

Derek Martin a écrit :
> I'd like to know if it's possible to code something in Python which
> would be equivalent to the following C:
> 
> [Assume bool is typedef'd to int, and TRUE and FALSE are #defined to 1
> and 0, respectively]
> 
> ---- debug.c ----
> #include <stdio.h>
> 
> bool DEBUG;
> 
> void dprint(char *msg)
> {
> 	if (DEBUG){
> 		printf("DEBUG: %s", msg);
> 	}
> }


This should have been:

     fprintf(STDERR, "DEBUG: %s", msg);

STDOUT is for *normal* program outputs. Debug informations, warnings, 
and all verbosity should go to STDERR.


> ---- end of debug.c ----
> 
> The idea being that all modules of the program would "import" this
> code via the header file:
> 
> ---- debug.h ----
> extern bool DEBUG;
> void dprint(char *msg);
> ---- end of debug.h ----
> 
> I'm specifically trying to avoid having to create a debug object and
> pass it around... All modules should have visibility into the state of
> whether DEBUG is turned on or off, and be able to use dprint().  Can
> Python do this?

Yes, indeed.

> I tried creating debug.py as such:
> 
> ---- debug.py ----
> DEBUG = True
> def dprint(msg):
>     if DEBUG:
>         print("DEBUG: %s" % msg)
> ---- end ----

Same remark : please use sys.stderr


> Then in the modules that wanted to use it, I did:
> 
> from debug import DEBUG, dprint
> But I got some weird behavior.  The imported copy

It's not a copy.

> of DEBUG is
> read-only;

It's not read-only.

> if you update it, the name DEBUG points to a different
> object which the other modules can't see.

Indeed : the way you imported it explicitely made it a (module) local name.

>  After doing some reading of
> the docs, this behavior is explained and understood (though obviously
> not what I want).  It just occured to me that I might be able to get
> around that by using a setter function in the module itself...

Just use a fully qualified name, so you dont make DEBUG local:

import debug
print debug.DEBUG
debug.DEBUG = True
print debug.DEBUG

Now note that ALL_UPPER names are - by convention - considered 
'constants'. If this is supposed to be altered, don't write it ALL_UPPER.

> The other weird behavior was, once I changed the value of DEBUG,
> dprint() started to behave oddly.  No matter what I passed as an
> argument (and no matter what I set the value of DEBUG to be), it
> started printing the exact literal string:
> 
> DEBUG: %s
 >
> whenever it was called.  It was as if the function couldn't see the
> parameter msg, which was passed via the call.  Most unexpected, and
> definitely undesirable.

I just fail to see how the code you posted could expose such a 
behaviour. Please post actual code.


Also and FWIW, Python has a logging module in it's stdlib. Please use it 
instead of any half-backed squared-wheel homegrown solution.

My 2 cents