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.

-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(n^2) 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;

• -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.

• -Xshowalloccount to output the total count of allocated objects foreach type when the program finishes. This only works when Python was built withCOUNT_ALLOCS defined.

• -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: enable CPython’s “development mode”, introducing additionalruntime checks which are too expensive to be enabled by default. It shouldnot be more verbose than the default if the code is correct: new warningsare only emitted when an issue is detected. Effect of the developer mode:

  • Adddefault warning filter, as-Wdefault.

  • Install debug hooks on memory allocators: see thePyMem_SetupDebugHooks() C function.

  • Enable thefaulthandler module to dump the Python tracebackon a crash.

  • Enableasyncio debug mode.

  • Set thedev_mode attribute ofsys.flags toTrue.

  • io.IOBase destructor logsclose() exceptions.

• -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.

New in version 3.8.14:The-Xint_max_str_digits option.

1.1.4.Options you shouldn’t use¶

-J¶

Reserved for use byJython.

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.