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).

1.1.2.Generic options¶

-?¶

-h¶

--help¶

Print a short description of all command line options and correspondingenvironment variables and exit.

--help-env¶

Print a short description of Python-specific environment variablesand exit.

New in version 3.11.

--help-xoptions¶

Print a description of implementation-specific-X optionsand exit.

New in version 3.11.

--help-all¶

Print complete usage information and exit.

New in version 3.11.

-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 convertingbytes orbytearray tostr without specifying encoding or comparingbytes orbytearray withstr orbytes withint.Issue an error when the option is given twice (-bb).

Changed in version 3.5:Affects also comparisons ofbytes withint.

-B¶

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

--check-hash-based-pycsdefault|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.

See also the-P and-I (isolated) options.

-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,-Pand-s options.

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.

-P¶

Don’t prepend a potentially unsafe path tosys.path:

• python-mmodule command line: Don’t prepend the current workingdirectory.

• pythonscript.py command line: Don’t prepend the script’s directory.If it’s a symbolic link, resolve symbolic links.

• python-ccode andpython (REPL) command lines: Don’t prepend anempty string, which means the current working directory.

See also thePYTHONSAFEPATH environment variable, and-Eand-I (isolated) options.

New in version 3.11.

-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://ocert.org/advisories/ocert-2011-003.html for details.

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

New in version 3.2.3.

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

-s¶

Don’t add theusersite-packagesdirectory tosys.path.

See alsoPYTHONNOUSERSITE.

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.

Changed in version 3.10:Thesite module reports the site-specific pathsand.pth files being processed.

See alsoPYTHONVERBOSE.

-Warg¶

Warning control. Python’s warning machinery by default prints warningmessages tosys.stderr.

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 and the interpreter willresolve them to the appropriate action name. For example,-Wi is thesame as-Wignore.

The full form of argument is:

action:message:category:module:lineno

Empty fields match all values; trailing empty fields may be omitted. Forexample-Wignore::DeprecationWarning ignores all DeprecationWarningwarnings.

Theaction field is as explained above but only applies to warnings thatmatch the remaining fields.

Themessage field must match the whole warning message; this match iscase-insensitive.

Thecategory field matches the warning category(ex:DeprecationWarning). This must be a class name; the match testwhether the actual warning category of the message is a subclass of thespecified warning category.

Themodule field matches the (fully qualified) module name; this match iscase-sensitive.

Thelineno field matches the line number, where zero matches all linenumbers and is thus equivalent to an omitted line number.

Multiple-W options can 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. For example, thewarnings.filterwarnings()function can be used to use a regular expression on the warning message.

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.See alsoPYTHONFAULTHANDLER.

• -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 ondebug 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.Seetracemalloc.start() andPYTHONTRACEMALLOCfor 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. See alsoPYTHONDEVMODE.

• -Xutf8 enables thePython UTF-8 Mode.-Xutf8=0 explicitly disablesPython UTF-8 Mode(even when it would otherwise activate automatically).See alsoPYTHONUTF8.

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

• -Xwarn_default_encoding issues aEncodingWarning when thelocale-specific default encoding is used for opening files.See alsoPYTHONWARNDEFAULTENCODING.

• -Xno_debug_ranges disables the inclusion of the tables mapping extralocation information (end line, start column offset and end column offset)to every instruction in code objects. This is useful when smaller codeobjects and pyc files are desired as well as suppressing the extra visuallocation indicators when the interpreter displays tracebacks. See alsoPYTHONNODEBUGRANGES.

• -Xfrozen_modules determines whether or not frozen modules areignored by the import machinery. A value of “on” means they getimported and “off” means they are ignored. The default is “on”if this is an installed Python (the normal case). If it’s underdevelopment (running from the source tree) then the default is “off”.Note that the “importlib_bootstrap” and “importlib_bootstrap_external”frozen modules are always used, even if this flag is set to “off”.

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

New in version 3.2.

Changed in version 3.3:Added the-Xfaulthandler option.

Changed in version 3.4:Added the-Xshowrefcount and-Xtracemalloc options.

Changed in version 3.6:Added the-Xshowalloccount option.

Changed in version 3.7:Added the-Ximporttime,-Xdev and-Xutf8 options.

Changed in version 3.8:Added 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.

Changed in version 3.10:Added the-Xwarn_default_encoding option.Removed the-Xoldparser option.

Changed in version 3.11:Added the-Xno_debug_ranges,-Xfrozen_modules and-Xint_max_str_digits options.

1.1.4.Options you shouldn’t use¶

-J¶

Reserved for use byJython.

1.2.1.Debug-mode variables¶

PYTHONTHREADDEBUG¶

If set, Python will print threading debug info into stdout.

Need adebug build of Python.

Deprecated since version 3.10, will be removed in version 3.12.

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.

PYTHONDUMPREFSFILE=FILENAME¶

If set, Python will dump objects and reference counts still aliveafter shutting down the interpreter into a file calledFILENAME.

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

New in version 3.11.