This file describes some special Python build types enabled via compile-time

preprocessor directives.

IMPORTANT: if you want to build a debug-enabled Python, it is recommended that

you use ``./configure --with-pydebug``, rather than the options listed here.

However, if you wish to define some of these options individually, it is best

to define them in the EXTRA_CFLAGS make variable;

``make EXTRA_CFLAGS="-DPy_REF_DEBUG"``.

Py_REF_DEBUG

------------

Turn on aggregate reference counting. This arranges that extern _Py_RefTotal

hold a count of all references, the sum of ob_refcnt across all objects.

Passing ``-X showrefcount`` on the command line causes the interactive

interpreter to print the reference count total as well the number of memory

blocks allocated after each statement:

>>> 23

23

[8288 refs, 14332 blocks]

>>>

Note that if this count increases when you're not storing away new objects,

there's probably a leak. Remember, though, that in interactive mode the special

name "_" holds a reference to the last result displayed!

Py_REF_DEBUG also checks after every decref to verify that the refcount hasn't

gone negative, and causes an immediate fatal error if it has.

Py_DEBUG implies Py_REF_DEBUG.

Special gimmicks:

sys.gettotalrefcount()

Return current total of all refcounts.

Py_TRACE_REFS

-------------

Build option: ``./configure --with-trace-refs``.

Turn on heavy reference debugging. This is major surgery. All live

heap-allocated objects are traced in a hash table. Most built-in type objects

are not in this list, as they're statically allocated.

Special gimmicks:

sys.getobjects(max[, type])

Return list of the (no more than) max most-recently allocated objects, most

recently allocated first in the list, least-recently allocated last in the

list. max=0 means no limit on list length. If an optional type object is

passed, the list is also restricted to objects of that type. The return

list itself, and some temp objects created just to call sys.getobjects(),

are excluded from the return list. Note that the list returned is just

another object, though, so may appear in the return list the next time you

call getobjects(); note that every object in the list is kept alive too,

simply by virtue of being in the list.

envvar PYTHONDUMPREFS

If this envvar exists, Py_FinalizeEx() arranges to print a list of all

still-live heap objects. This is printed twice, in different formats,

before and after Py_FinalizeEx has cleaned up everything it can clean up. The

first output block produces the repr() of each object so is more

informative; however, a lot of stuff destined to die is still alive then.

The second output block is much harder to work with (repr() can't be invoked

anymore -- the interpreter has been torn down too far), but doesn't list any

objects that will die. The tool script combinerefs.py can be run over this

to combine the info from both output blocks. The second output block, and

combinerefs.py, were new in Python 2.3b1.

Py_DEBUG

--------

This is what is generally meant by "a debug build" of Python.

Py_DEBUG implies Py_REF_DEBUG. In addition, C assert()s are enabled

(via the C way: by not defining NDEBUG), and some routines do additional sanity

checks inside "#ifdef Py_DEBUG" blocks.

Also, compile in support for "lltrace" (Low Level TRACE-ing) of the main

interpreter loop. Before _PyEval_EvalFrameDefault executes a frame's code, it

checks the frame's global namespace for a variable "__lltrace__" (as well as for

the environment variable PYTHON_LLTRACE"). If such a variable is found, mounds

of information about what the interpreter is doing are sprayed to stdout, such

as every opcode and opcode argument and values pushed onto and popped off the

value stack. Higher integer values for the environment variable result in more

and more detail being printed (the global __lltrace__ always enables the maximum

output). Not useful very often, but *very* useful when needed.