Python Cookbook 2Nd Edition Jun 1002005 [Electronic resources] نسخه متنی

Recipe 8.1. Disabling Execution of Some Conditionals and Loops

Credit: Chris McDonough, Srinivas B, Dinu Gherman

Problem

While developing or
debugging, you want certain conditional or looping sections of code
to be temporarily omitted from execution.

Solution

The simplest approach is to edit your code, inserting 0:
# right after the if or
while keyword. Since 0 evaluates as false, that
section of code will not execute. For example:

if i < 1:
doSomething( )
while j < k:
j = fleep(j, k)

into:

if 0: # i < 1:
doSomething( )
while 0: # j < k:
j = fleep(j, k)

If you have many such sections that must simultaneously switch on and
off during your development and debug sessions, an alternative is to
define a boolean variable (commonly known as a
flag), say doit = False,
and code:

if doit and i < 1:
doSomething( )
while doit and j < k:
j = fleep(j, k)

This way, you can temporarily switch the various sections on again by
just changing the flag setting to doit = True, and
easily flip back and forth. You can even have multiple such flags. Do
remember to remove the doit and parts once
you're done developing and debugging, since at that
point all they would do is slow things down.

Discussion

Of course, you have other alternatives, too. Most good editors have
commands to insert or remove comment markers from the start of each
line in a marked section, like Alt-3 and
Alt-4 in the editor of the IDLE
IDE (Integrated Development Environment) that comes with Python; a
common convention in such editors is to start such temporarily
commented-out lines with two comment markers,
##, to distinguish them from
"normal" comments.

One Python-specific technique you can use is the _ _debug_
_ read-only global boolean variable. _ _debug_
_ is true when Python is running without
the -O (optimize) command-line option,
False when Python is running with that option.
Moreover, the Python compiler knows about _ _debug_
_ and can completely remove any block guarded by
if _ _debug_ _ when Python is running with the
command-line optimization option, thus saving memory as well as
execution time.

See Also

The section on the _ _debug_ _ flag and the
assert statement in the Language
Reference and Python in a
Nutshell.