Using debug print routine inside assert

[Edvard Majakari]

Hello, fellow Pythonistas!

Today I shortly discussed the problems in using print statements for
debugging problems in Python. Often in coding, in addition to asserts it
is nice to have a debug routine like

import time
global DEBUG_LVL = 0
...
def debug(msg, threshold=1):
    if threshold > DEBUG_LVL:
       return

    print "%s %s\n" %(time.strftime("%d%m%y %H:%M:%S"), msg)

# somewhere in the program
debug("subfrobnicate(%s, %s) returned %s" %(p1, p2, subfrobnicate(p1, p2)))

The idea here is to have consistent debug print messages. However,
parameters or expressions inside the debug statement might be very
time-consuming to run. Assume for a moment that calling subfrobnicate()
takes 0.02 seconds, and it is called 10 000 times in your test
program. During the debug stage, over three minutes of waiting is not
necessarily bad, but if you have lots of that kind of debug statements, it
really pays to take them away from the final product. But this could be
tedious if you have lots of code. 

The problem is, if DEBUG_LVL is less than threshold (like in the example
above), parameters to method call are evaluated but results are thrown
away, rendering parameter evaluation (and potential calculations)
useless. You spent extra time in evaluating debug parameter just to throw
results away. If you could skip the evaluation of debug(), given low
enough DEBUG_LVL, you would be fine.

One solution - though not elegant and rather ugly - is to use Python's
ability to remove assert statements from byte-compiled programs, and
modify debug routine so that it prints given argument and returns true:

def debug(msg, threshold=1):
    if threshold > DEBUG_LVL:
       return

    print "%s %s\n" %(time.strftime("%d%m%y %H:%M:%S"), msg)
    return True   # return true so that assert always succeeds

# use in the program:

assert(debug("subfrobnicate(%s, %s) returned %s" \
             %(p1, p2, subfrobnicate(p1, p2))))


In C, you could use macros to implement assert-wrapped debug function, but
due to their hard-to-detect nature I wouldn't resort to macros, and
nevertheless, in Python, you don't have such thing as macros(?).

What do you think, what is the best solution for this kind of problem? I
for one wouldn't like to see myself writing assert(debug(...)) every now
and then :)

-- 
# Edvard Majakari		Software Engineer
# PGP PUBLIC KEY available    	Soli Deo Gloria!

$_ = '456476617264204d616a616b6172692c20612043687269737469616e20'; print
join('',map{chr hex}(split/(\w{2})/)),uc substr(crypt(60281449,'es'),2,4),"\n";