1.Command line and environment

The CPython interpreter scans the command line and the environment for varioussettings.

CPython implementation detail: Other implementations’ command line schemes may differ. SeeAlternate Implementations for further resources.

1.1.Command line

When invoking Python, you may specify any of these options:

python[-bBdEhiIOqsSuvVWx?][-ccommand|-mmodule-name|script|-][args]

The most common use case is, of course, a simple invocation of a script:

pythonmyscript.py

1.1.1.Interface options

The interpreter interface resembles that of the UNIX shell, but provides someadditional methods of invocation:

  • When called with standard input connected to a tty device, it prompts forcommands and executes them until an EOF (an end-of-file character, you canproduce that withCtrl-D on UNIX orCtrl-Z, Enter on Windows) is read.

  • When called with a file name argument or with a file as standard input, itreads and executes a script from that file.

  • When called with a directory name argument, it reads and executes anappropriately named script from that directory.

  • When called with-ccommand, it executes the Python statement(s) given ascommand. Herecommand may contain multiple statements separated bynewlines. Leading whitespace is significant in Python statements!

  • When called with-mmodule-name, the given module is located on thePython module path and executed as a script.

In non-interactive mode, the entire input is parsed before it is executed.

An interface option terminates the list of options consumed by the interpreter,all consecutive arguments will end up insys.argv – note that the firstelement, subscript zero (sys.argv[0]), is a string reflecting the program’ssource.

-c <command>

Execute the Python code incommand.command can be one or morestatements separated by newlines, with significant leading whitespace as innormal module code.

If this option is given, the first element ofsys.argv will be"-c" and the current directory will be added to the start ofsys.path (allowing modules in that directory to be imported as toplevel modules).

Raises anauditing eventcpython.run_command with argumentcommand.

-m <module-name>

Searchsys.path for the named module and execute its contents asthe__main__ module.

Since the argument is amodule name, you must not give a file extension(.py). The module name should be a valid absolute Python module name, butthe implementation may not always enforce this (e.g. it may allow you touse a name that includes a hyphen).

Package names (including namespace packages) are also permitted. When apackage name is supplied insteadof a normal module, the interpreter will execute<pkg>.__main__ asthe main module. This behaviour is deliberately similar to the handlingof directories and zipfiles that are passed to the interpreter as thescript argument.

Note

This option cannot be used with built-in modules and extension moduleswritten in C, since they do not have Python module files. However, itcan still be used for precompiled modules, even if the original sourcefile is not available.

If this option is given, the first element ofsys.argv will be thefull path to the module file (while the module file is being located, thefirst element will be set to"-m"). As with the-c option,the current directory will be added to the start ofsys.path.

-I option can be used to run the script in isolated mode wheresys.path contains neither the current directory nor the user’ssite-packages directory. AllPYTHON* environment variables areignored, too.

Many standard library modules contain code that is invoked on their executionas a script. An example is thetimeit module:

python-mtimeit-s'setup here''benchmarked code here'python-mtimeit-h# for details

Raises anauditing eventcpython.run_module with argumentmodule-name.

See also

runpy.run_module()

Equivalent functionality directly available to Python code

PEP 338 – Executing modules as scripts

Changed in version 3.1:Supply the package name to run a__main__ submodule.

Changed in version 3.4:namespace packages are also supported

-

Read commands from standard input (sys.stdin). If standard input isa terminal,-i is implied.

If this option is given, the first element ofsys.argv will be"-" and the current directory will be added to the start ofsys.path.

Raises anauditing eventcpython.run_stdin with no arguments.

<script>

Execute the Python code contained inscript, which must be a filesystempath (absolute or relative) referring to either a Python file, a directorycontaining a__main__.py file, or a zipfile containing a__main__.py file.

If this option is given, the first element ofsys.argv will be thescript name as given on the command line.

If the script name refers directly to a Python file, the directorycontaining that file is added to the start ofsys.path, and thefile is executed as the__main__ module.

If the script name refers to a directory or zipfile, the script name isadded to the start ofsys.path and the__main__.py file inthat location is executed as the__main__ module.

-I option can be used to run the script in isolated mode wheresys.path contains neither the script’s directory nor the user’ssite-packages directory. AllPYTHON* environment variables areignored, too.

Raises anauditing eventcpython.run_file with argumentfilename.

See also

runpy.run_path()

Equivalent functionality directly available to Python code

If no interface option is given,-i is implied,sys.argv[0] isan empty string ("") and the current directory will be added to thestart ofsys.path. Also, tab-completion and history editing isautomatically enabled, if available on your platform (seeReadline configuration).

Changed in version 3.4:Automatic enabling of tab-completion and history editing.

1.1.2.Generic options

-?
-h
--help

Print a short description of all command line options.

-V
--version

Print the Python version number and exit. Example output could be:

Python 3.8.0b2+

When given twice, print more information about the build, like:

Python 3.8.0b2+ (3.8:0c076caaa8, Apr 20 2019, 21:55:00)[GCC 6.2.0 20161005]

New in version 3.6:The-VV option.

1.1.3.Miscellaneous options

-b

Issue a warning when comparingbytes orbytearray withstr orbytes withint. Issue an error when theoption is given twice (-bb).

Changed in version 3.5:Affects comparisons ofbytes withint.

-B

If given, Python won’t try to write.pyc files on theimport of source modules. See alsoPYTHONDONTWRITEBYTECODE.

--check-hash-based-pycs default|always|never

Control the validation behavior of hash-based.pyc files. SeeCached bytecode invalidation. When set todefault, checked and uncheckedhash-based bytecode cache files are validated according to their defaultsemantics. When set toalways, all hash-based.pyc files, whetherchecked or unchecked, are validated against their corresponding sourcefile. When set tonever, hash-based.pyc files are not validatedagainst their corresponding source files.

The semantics of timestamp-based.pyc files are unaffected by thisoption.

-d

Turn on parser debugging output (for expert only, depending on compilationoptions). See alsoPYTHONDEBUG.

-E

Ignore allPYTHON* environment variables, e.g.PYTHONPATH andPYTHONHOME, that might be set.

-i

When a script is passed as first argument or the-c option is used,enter interactive mode after executing the script or the command, even whensys.stdin does not appear to be a terminal. ThePYTHONSTARTUP file is not read.

This can be useful to inspect global variables or a stack trace when a scriptraises an exception. See alsoPYTHONINSPECT.

-I

Run Python in isolated mode. This also implies -E and -s.In isolated modesys.path contains neither the script’s directory northe user’s site-packages directory. AllPYTHON* environmentvariables are ignored, too. Further restrictions may be imposed to preventthe user from injecting malicious code.

New in version 3.4.

-O

Remove assert statements and any code conditional on the value of__debug__. Augment the filename for compiled(bytecode) files by adding.opt-1 before the.pycextension (seePEP 488). See alsoPYTHONOPTIMIZE.

Changed in version 3.5:Modify.pyc filenames according toPEP 488.

-OO

Do-O and also discard docstrings. Augment the filenamefor compiled (bytecode) files by adding.opt-2 before the.pyc extension (seePEP 488).

Changed in version 3.5:Modify.pyc filenames according toPEP 488.

-q

Don’t display the copyright and version messages even in interactive mode.

New in version 3.2.

-R

Turn on hash randomization. This option only has an effect if thePYTHONHASHSEED environment variable is set to0, since hashrandomization is enabled by default.

On previous versions of Python, this option turns on hash randomization,so that the__hash__() values of str and bytes objectsare “salted” with an unpredictable random value. Although they remainconstant within an individual Python process, they are not predictablebetween repeated invocations of Python.

Hash randomization is intended to provide protection against adenial-of-service caused by carefully-chosen inputs that exploit the worstcase performance of a dict construction, O(n2) complexity. Seehttp://www.ocert.org/advisories/ocert-2011-003.html for details.

PYTHONHASHSEED allows you to set a fixed value for the hashseed secret.

Changed in version 3.7:The option is no longer ignored.

New in version 3.2.3.

-s

Don’t add theusersite-packagesdirectory tosys.path.

See also

PEP 370 – Per user site-packages directory

-S

Disable the import of the modulesite and the site-dependentmanipulations ofsys.path that it entails. Also disable thesemanipulations ifsite is explicitly imported later (callsite.main() if you want them to be triggered).

-u

Force the stdout and stderr streams to be unbuffered. This option has noeffect on the stdin stream.

See alsoPYTHONUNBUFFERED.

Changed in version 3.7:The text layer of the stdout and stderr streams now is unbuffered.

-v

Print a message each time a module is initialized, showing the place(filename or built-in module) from which it is loaded. When given twice(-vv), print a message for each file that is checked for whensearching for a module. Also provides information on module cleanup at exit.See alsoPYTHONVERBOSE.

-W arg

Warning control. Python’s warning machinery by default prints warningmessages tosys.stderr. A typical warning message has the followingform:

file:line: category: message

By default, each warning is printed once for each source line where itoccurs. This option controls how often warnings are printed.

Multiple-W options may be given; when a warning matches more thanone option, the action for the last matching option is performed. Invalid-W options are ignored (though, a warning message is printed aboutinvalid options when the first warning is issued).

Warnings can also be controlled using thePYTHONWARNINGSenvironment variable and from within a Python program using thewarnings module.

The simplest settings apply a particular action unconditionally to allwarnings emitted by a process (even those that are otherwise ignored bydefault):

-Wdefault# Warn once per call location-Werror# Convert to exceptions-Walways# Warn every time-Wmodule# Warn once per calling module-Wonce# Warn once per Python process-Wignore# Never warn

The action names can be abbreviated as desired (e.g.-Wi,-Wd,-Wa,-We) and the interpreter will resolve them to the appropriateaction name.

SeeThe Warnings Filter andDescribing Warning Filters for moredetails.

-x

Skip the first line of the source, allowing use of non-Unix forms of#!cmd. This is intended for a DOS specific hack only.

-X

Reserved for various implementation-specific options. CPython currentlydefines the following possible values:

  • -Xfaulthandler to enablefaulthandler;

  • -Xoldparser: enable the traditional LL(1) parser. See alsoPYTHONOLDPARSER andPEP 617.

  • -Xshowrefcount to output the total reference count and number of usedmemory blocks when the program finishes or after each statement in theinteractive interpreter. This only works on debug builds.

  • -Xtracemalloc to start tracing Python memory allocations using thetracemalloc module. By default, only the most recent frame isstored in a traceback of a trace. Use-Xtracemalloc=NFRAME to starttracing with a traceback limit ofNFRAME frames. See thetracemalloc.start() for more information.

  • -Xint_max_str_digits configures theinteger string conversionlength limitation. See alsoPYTHONINTMAXSTRDIGITS.

  • -Ximporttime to show how long each import takes. It shows modulename, cumulative time (including nested imports) and self time (excludingnested imports). Note that its output may be broken in multi-threadedapplication. Typical usage ispython3-Ximporttime-c'importasyncio'. See alsoPYTHONPROFILEIMPORTTIME.

  • -Xdev: enablePython Development Mode, introducingadditional runtime checks that are too expensive to be enabled bydefault.

  • -Xutf8 enables UTF-8 mode for operating system interfaces, overridingthe default locale-aware mode.-Xutf8=0 explicitly disables UTF-8mode (even when it would otherwise activate automatically).SeePYTHONUTF8 for more details.

  • -Xpycache_prefix=PATH enables writing.pyc files to a paralleltree rooted at the given directory instead of to the code tree. See alsoPYTHONPYCACHEPREFIX.

It also allows passing arbitrary values and retrieving them through thesys._xoptions dictionary.

Changed in version 3.2:The-X option was added.

New in version 3.3:The-Xfaulthandler option.

New in version 3.4:The-Xshowrefcount and-Xtracemalloc options.

New in version 3.6:The-Xshowalloccount option.

New in version 3.7:The-Ximporttime,-Xdev and-Xutf8 options.

New in version 3.8:The-Xpycache_prefix option. The-Xdev option now logsclose() exceptions inio.IOBase destructor.

Changed in version 3.9:Using-Xdev option, checkencoding anderrors arguments onstring encoding and decoding operations.

The-Xshowalloccount option has been removed.

New in version 3.9.14:The-Xint_max_str_digits option.

Deprecated since version 3.9, will be removed in version 3.10:The-Xoldparser option.

1.1.4.Options you shouldn’t use

-J

Reserved for use byJython.

1.2.Environment variables

These environment variables influence Python’s behavior, they are processedbefore the command-line switches other than -E or -I. It is customary thatcommand-line switches override environmental variables where there is aconflict.

PYTHONHOME

Change the location of the standard Python libraries. By default, thelibraries are searched inprefix/lib/pythonversion andexec_prefix/lib/pythonversion, whereprefix andexec_prefix are installation-dependent directories, both defaultingto/usr/local.

WhenPYTHONHOME is set to a single directory, its value replacesbothprefix andexec_prefix. To specify different valuesfor these, setPYTHONHOME toprefix:exec_prefix.

PYTHONPATH

Augment the default search path for module files. The format is the same asthe shell’sPATH: one or more directory pathnames separated byos.pathsep (e.g. colons on Unix or semicolons on Windows).Non-existent directories are silently ignored.

In addition to normal directories, individualPYTHONPATH entriesmay refer to zipfiles containing pure Python modules (in either source orcompiled form). Extension modules cannot be imported from zipfiles.

The default search path is installation dependent, but generally begins withprefix/lib/pythonversion (seePYTHONHOME above). Itisalways appended toPYTHONPATH.

An additional directory will be inserted in the search path in front ofPYTHONPATH as described above underInterface options. The search path can be manipulated fromwithin a Python program as the variablesys.path.

PYTHONPLATLIBDIR

If this is set to a non-empty string, it overrides thesys.platlibdirvalue.

New in version 3.9.

PYTHONSTARTUP

If this is the name of a readable file, the Python commands in that file areexecuted before the first prompt is displayed in interactive mode. The fileis executed in the same namespace where interactive commands are executed sothat objects defined or imported in it can be used without qualification inthe interactive session. You can also change the promptssys.ps1 andsys.ps2 and the hooksys.__interactivehook__ in this file.

Raises anauditing eventcpython.run_startup withthe filename as the argument when called on startup.

PYTHONOPTIMIZE

If this is set to a non-empty string it is equivalent to specifying the-O option. If set to an integer, it is equivalent to specifying-O multiple times.

PYTHONBREAKPOINT

If this is set, it names a callable using dotted-path notation. The modulecontaining the callable will be imported and then the callable will be runby the default implementation ofsys.breakpointhook() which itself iscalled by built-inbreakpoint(). If not set, or set to the emptystring, it is equivalent to the value “pdb.set_trace”. Setting this to thestring “0” causes the default implementation ofsys.breakpointhook()to do nothing but return immediately.

New in version 3.7.

PYTHONDEBUG

If this is set to a non-empty string it is equivalent to specifying the-d option. If set to an integer, it is equivalent to specifying-d multiple times.

PYTHONOLDPARSER

If this is set to a non-empty string, enable the traditional LL(1) parser.

See also the-Xoldparser option andPEP 617.

Deprecated since version 3.9, will be removed in version 3.10.

PYTHONINSPECT

If this is set to a non-empty string it is equivalent to specifying the-i option.

This variable can also be modified by Python code usingos.environto force inspect mode on program termination.

Raises anauditing eventcpython.run_stdin with no arguments.

Changed in version 3.9.20:(also 3.8.20)Emits audit events.

PYTHONUNBUFFERED

If this is set to a non-empty string it is equivalent to specifying the-u option.

PYTHONVERBOSE

If this is set to a non-empty string it is equivalent to specifying the-v option. If set to an integer, it is equivalent to specifying-v multiple times.

PYTHONCASEOK

If this is set, Python ignores case inimport statements. Thisonly works on Windows and macOS.

PYTHONDONTWRITEBYTECODE

If this is set to a non-empty string, Python won’t try to write.pycfiles on the import of source modules. This is equivalent tospecifying the-B option.

PYTHONPYCACHEPREFIX

If this is set, Python will write.pyc files in a mirror directory treeat this path, instead of in__pycache__ directories within the sourcetree. This is equivalent to specifying the-Xpycache_prefix=PATH option.

New in version 3.8.

PYTHONHASHSEED

If this variable is not set or set torandom, a random value is usedto seed the hashes of str and bytes objects.

IfPYTHONHASHSEED is set to an integer value, it is used as a fixedseed for generating the hash() of the types covered by the hashrandomization.

Its purpose is to allow repeatable hashing, such as for selftests for theinterpreter itself, or to allow a cluster of python processes to share hashvalues.

The integer must be a decimal number in the range [0,4294967295]. Specifyingthe value 0 will disable hash randomization.

New in version 3.2.3.

PYTHONINTMAXSTRDIGITS

If this variable is set to an integer, it is used to configure theinterpreter’s globalinteger string conversion length limitation.

New in version 3.9.14.

PYTHONIOENCODING

If this is set before running the interpreter, it overrides the encoding usedfor stdin/stdout/stderr, in the syntaxencodingname:errorhandler. Boththeencodingname and the:errorhandler parts are optional and havethe same meaning as instr.encode().

For stderr, the:errorhandler part is ignored; the handler will always be'backslashreplace'.

Changed in version 3.4:Theencodingname part is now optional.

Changed in version 3.6:On Windows, the encoding specified by this variable is ignored for interactiveconsole buffers unlessPYTHONLEGACYWINDOWSSTDIO is also specified.Files and pipes redirected through the standard streams are not affected.

PYTHONNOUSERSITE

If this is set, Python won’t add theusersite-packagesdirectory tosys.path.

See also

PEP 370 – Per user site-packages directory

PYTHONUSERBASE

Defines theuserbasedirectory, which is used tocompute the path of theusersite-packagesdirectoryandDistutils installation paths forpythonsetup.pyinstall--user.

See also

PEP 370 – Per user site-packages directory

PYTHONEXECUTABLE

If this environment variable is set,sys.argv[0] will be set to itsvalue instead of the value got through the C runtime. Only works onmacOS.

PYTHONWARNINGS

This is equivalent to the-W option. If set to a commaseparated string, it is equivalent to specifying-W multipletimes, with filters later in the list taking precedence over those earlierin the list.

The simplest settings apply a particular action unconditionally to allwarnings emitted by a process (even those that are otherwise ignored bydefault):

PYTHONWARNINGS=default# Warn once per call locationPYTHONWARNINGS=error# Convert to exceptionsPYTHONWARNINGS=always# Warn every timePYTHONWARNINGS=module# Warn once per calling modulePYTHONWARNINGS=once# Warn once per Python processPYTHONWARNINGS=ignore# Never warn

SeeThe Warnings Filter andDescribing Warning Filters for moredetails.

PYTHONFAULTHANDLER

If this environment variable is set to a non-empty string,faulthandler.enable() is called at startup: install a handler forSIGSEGV,SIGFPE,SIGABRT,SIGBUS andSIGILL signals to dump the Python traceback. This is equivalent to-Xfaulthandler option.

New in version 3.3.

PYTHONTRACEMALLOC

If this environment variable is set to a non-empty string, start tracingPython memory allocations using thetracemalloc module. The value ofthe variable is the maximum number of frames stored in a traceback of atrace. For example,PYTHONTRACEMALLOC=1 stores only the most recentframe. See thetracemalloc.start() for more information.

New in version 3.4.

PYTHONPROFILEIMPORTTIME

If this environment variable is set to a non-empty string, Python willshow how long each import takes. This is exactly equivalent to setting-Ximporttime on the command line.

New in version 3.7.

PYTHONASYNCIODEBUG

If this environment variable is set to a non-empty string, enable thedebug mode of theasyncio module.

New in version 3.4.

PYTHONMALLOC

Set the Python memory allocators and/or install debug hooks.

Set the family of memory allocators used by Python:

Install debug hooks:

  • debug: install debug hooks on top of thedefault memoryallocators.

  • malloc_debug: same asmalloc but also install debug hooks.

  • pymalloc_debug: same aspymalloc but also install debug hooks.

See thedefault memory allocators and thePyMem_SetupDebugHooks() function (install debug hooks on Pythonmemory allocators).

Changed in version 3.7:Added the"default" allocator.

New in version 3.6.

PYTHONMALLOCSTATS

If set to a non-empty string, Python will print statistics of thepymalloc memory allocator every time a new pymalloc objectarena is created, and on shutdown.

This variable is ignored if thePYTHONMALLOC environment variableis used to force themalloc() allocator of the C library, or ifPython is configured withoutpymalloc support.

Changed in version 3.6:This variable can now also be used on Python compiled in release mode.It now has no effect if set to an empty string.

PYTHONLEGACYWINDOWSFSENCODING

If set to a non-empty string, the default filesystem encoding and errors modewill revert to their pre-3.6 values of ‘mbcs’ and ‘replace’, respectively.Otherwise, the new defaults ‘utf-8’ and ‘surrogatepass’ are used.

This may also be enabled at runtime withsys._enablelegacywindowsfsencoding().

Availability: Windows.

New in version 3.6:SeePEP 529 for more details.

PYTHONLEGACYWINDOWSSTDIO

If set to a non-empty string, does not use the new console reader andwriter. This means that Unicode characters will be encoded according tothe active console code page, rather than using utf-8.

This variable is ignored if the standard streams are redirected (to filesor pipes) rather than referring to console buffers.

Availability: Windows.

New in version 3.6.

PYTHONCOERCECLOCALE

If set to the value0, causes the main Python command line applicationto skip coercing the legacy ASCII-based C and POSIX locales to a morecapable UTF-8 based alternative.

If this variable isnot set (or is set to a value other than0), theLC_ALL locale override environment variable is also not set, and thecurrent locale reported for theLC_CTYPE category is either the defaultC locale, or else the explicitly ASCII-basedPOSIX locale, then thePython CLI will attempt to configure the following locales for theLC_CTYPE category in the order listed before loading the interpreterruntime:

  • C.UTF-8

  • C.utf8

  • UTF-8

If setting one of these locale categories succeeds, then theLC_CTYPEenvironment variable will also be set accordingly in the current processenvironment before the Python runtime is initialized. This ensures that inaddition to being seen by both the interpreter itself and other locale-awarecomponents running in the same process (such as the GNUreadlinelibrary), the updated setting is also seen in subprocesses (regardless ofwhether or not those processes are running a Python interpreter), as well asin operations that query the environment rather than the current C locale(such as Python’s ownlocale.getdefaultlocale()).

Configuring one of these locales (either explicitly or via the aboveimplicit locale coercion) automatically enables thesurrogateescapeerror handler forsys.stdin andsys.stdout (sys.stderr continues to usebackslashreplaceas it does in any other locale). This stream handling behavior can beoverridden usingPYTHONIOENCODING as usual.

For debugging purposes, settingPYTHONCOERCECLOCALE=warn will causePython to emit warning messages onstderr if either the locale coercionactivates, or else if a locale thatwould have triggered coercion isstill active when the Python runtime is initialized.

Also note that even when locale coercion is disabled, or when it fails tofind a suitable target locale,PYTHONUTF8 will still activate bydefault in legacy ASCII-based locales. Both features must be disabled inorder to force the interpreter to useASCII instead ofUTF-8 forsystem interfaces.

Availability: *nix.

New in version 3.7:SeePEP 538 for more details.

PYTHONDEVMODE

If this environment variable is set to a non-empty string, enablePython Development Mode, introducing additional runtimechecks that are too expensive to be enabled by default.

New in version 3.7.

PYTHONUTF8

If set to1, enables the interpreter’s UTF-8 mode, whereUTF-8 isused as the text encoding for system interfaces, regardless of thecurrent locale setting.

This means that:

As a consequence of the changes in those lower level APIs, other higherlevel APIs also exhibit different default behaviours:

  • Command line arguments, environment variables and filenames are decodedto text using the UTF-8 encoding.

  • os.fsdecode() andos.fsencode() use the UTF-8 encoding.

  • open(),io.open(), andcodecs.open() use the UTF-8encoding by default. However, they still use the strict error handler bydefault so that attempting to open a binary file in text mode is likelyto raise an exception rather than producing nonsense data.

Note that the standard stream settings in UTF-8 mode can be overridden byPYTHONIOENCODING (just as they can be in the default locale-awaremode).

If set to0, the interpreter runs in its default locale-aware mode.

Setting any other non-empty string causes an error during interpreterinitialisation.

If this environment variable is not set at all, then the interpreter defaultsto using the current locale settings,unless the current locale isidentified as a legacy ASCII-based locale(as described forPYTHONCOERCECLOCALE), and locale coercion iseither disabled or fails. In such legacy locales, the interpreter willdefault to enabling UTF-8 mode unless explicitly instructed not to do so.

Also available as the-Xutf8 option.

New in version 3.7:SeePEP 540 for more details.

1.2.1.Debug-mode variables

Setting these variables only has an effect in a debug build of Python.

PYTHONTHREADDEBUG

If set, Python will print threading debug info.

Need Python configured with the--with-pydebug build option.

PYTHONDUMPREFS

If set, Python will dump objects and reference counts still alive aftershutting down the interpreter.

Need Python configured with the--with-trace-refs build option.