The CPython interpreter scans the command line and the environment for various settings.
CPython implementation detail: Other implementations’ command line schemes may differ. See
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 some additional methods of invocation:
When called with standard input connected to a tty device, it prompts for commands and executes them until an EOF (an end-of-file character, you can produce that with Ctrl-D on UNIX or Ctrl-Z,Enter on Windows) is read.
When called with a file name argument or with a file as standard input, it reads and executes a script from that file.
When called with a directory name argument, it reads and executes an appropriately named script from that directory.
When called with -ccommand, it executes the Python statement(s) given as command. Here command may contain multiple statements separated by newlines. Leading whitespace is significant in Python statements!
When called with -mmodule-name, the given module is located on the Python 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 in
– note that the first element, subscript zero (sys.argv[0]), is a string reflecting the program’s source.
-c<command>
Execute the Python code in command. command can be one or more statements separated by newlines, with significant leading whitespace as in normal module code.
If this option is given, the first element of
will be "-c" and the current directory will be added to the start of
(allowing modules in that directory to be imported as top level modules).
Raises an
cpython.run_command with argument command.
-m<module-name>
Search
for the named module and execute its contents as the
module.
Since the argument is a module name, you must not give a file extension (.py). The module name should be a valid absolute Python module name, but the implementation may not always enforce this (e.g. it may allow you to use a name that includes a hyphen).
Package names (including namespace packages) are also permitted. When a package name is supplied instead of a normal module, the interpreter will execute <pkg>.__main__ as the main module. This behaviour is deliberately similar to the handling of directories and zipfiles that are passed to the interpreter as the script argument.
Note
This option cannot be used with built-in modules and extension modules written in C, since they do not have Python module files. However, it can still be used for precompiled modules, even if the original source file is not available.
If this option is given, the first element of
will be the full path to the module file (while the module file is being located, the first element will be set to "-m"). As with the
option, the current directory will be added to the start of
.
option can be used to run the script in isolated mode where
contains neither the current directory nor the user’s site-packages directory. All PYTHON* environment variables are ignored, too.
Many standard library modules contain code that is invoked on their execution as a script. An example is the
module:
python-mtimeit-s'setup here''benchmarked code here' python-mtimeit-h# for detailsRaises an
cpython.run_module with argument module-name.
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 (
). If standard input is a terminal,
is implied.
If this option is given, the first element of
will be "-" and the current directory will be added to the start of
.
Raises an
cpython.run_stdin with no arguments.
<script>Execute the Python code contained in script, which must be a filesystem path (absolute or relative) referring to either a Python file, a directory containing a __main__.py file, or a zipfile containing a __main__.py file.
If this option is given, the first element of
will be the script name as given on the command line.
If the script name refers directly to a Python file, the directory containing that file is added to the start of
, and the file is executed as the
module.
If the script name refers to a directory or zipfile, the script name is added to the start of
and the __main__.py file in that location is executed as the
module.
option can be used to run the script in isolated mode where
contains neither the script’s directory nor the user’s site-packages directory. All PYTHON* environment variables are ignored, too.
Raises an
cpython.run_file with argument filename.
See also
Equivalent functionality directly available to Python code
If no interface option is given,
is implied, sys.argv[0] is an empty string ("") and the current directory will be added to the start of
. Also, tab-completion and history editing is automatically enabled, if available on your platform (see
).
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 and corresponding environment variables and exit.
--help-env
Print a short description of Python-specific environment variables and exit.
New in version 3.11.
--help-xoptions
Print a description of implementation-specific
options and 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 converting
or
to
without specifying encoding or comparing bytes or bytearray with str or bytes with
. Issue an error when the option is given twice (-bb).
Changed in version 3.5: Affects also comparisons of
with
.
-B
If given, Python won’t try to write .pyc files on the import of source modules. See also
.
--check-hash-based-pycsdefault|always|never
Control the validation behavior of hash-based .pyc files. See
. When set to default, checked and unchecked hash-based bytecode cache files are validated according to their default semantics. When set to always, all hash-based .pyc files, whether checked or unchecked, are validated against their corresponding source file. When set to never, hash-based .pyc files are not validated against their corresponding source files.
The semantics of timestamp-based .pyc files are unaffected by this option.
-d
Turn on parser debugging output (for expert only, depending on compilation options). See also
.
-E
Ignore all PYTHON* environment variables, e.g.
and
, that might be set.
See also the
and
(isolated) options.
-i
When a script is passed as first argument or the
option is used, enter interactive mode after executing the script or the command, even when
does not appear to be a terminal. The
file is not read.
This can be useful to inspect global variables or a stack trace when a script raises an exception. See also
.
-I
Run Python in isolated mode. This also implies
,
and
options.
In isolated mode
contains neither the script’s directory nor the user’s site-packages directory. All PYTHON* environment variables are ignored, too. Further restrictions may be imposed to prevent the user from injecting malicious code.
New in version 3.4.
-O
Remove assert statements and any code conditional on the value of
. Augment the filename for compiled (
) files by adding .opt-1 before the .pyc extension (see
). See also
.
Changed in version 3.5: Modify .pyc filenames according to
.
-OO
Do
and also discard docstrings. Augment the filename for compiled (
) files by adding .opt-2 before the .pyc extension (see
).
Changed in version 3.5: Modify .pyc filenames according to
.
-P
Don’t prepend a potentially unsafe path to
:
python-mmodule command line: Don’t prepend the current working directory.
pythonscript.py command line: Don’t prepend the script’s directory. If it’s a symbolic link, resolve symbolic links.
python-ccode and python (REPL) command lines: Don’t prepend an empty string, which means the current working directory.
See also the
environment variable, and
and
(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 the
environment variable is set to 0, since hash randomization is enabled by default.
On previous versions of Python, this option turns on hash randomization, so that the
values of str and bytes objects are “salted” with an unpredictable random value. Although they remain constant within an individual Python process, they are not predictable between repeated invocations of Python.
Hash randomization is intended to provide protection against a denial-of-service caused by carefully chosen inputs that exploit the worst case performance of a dict construction, O(n2) complexity. See
http://ocert.org/advisories/ocert-2011-003.html
for details.
allows you to set a fixed value for the hash seed secret.
New in version 3.2.3.
Changed in version 3.7: The option is no longer ignored.
-s
Don’t add the
to
.
See also
.
See also
– Per user site-packages directory
-S
Disable the import of the module
and the site-dependent manipulations of
that it entails. Also disable these manipulations if
is explicitly imported later (call
if you want them to be triggered).
-u
Force the stdout and stderr streams to be unbuffered. This option has no effect on the stdin stream.
See also
.
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 when searching for a module. Also provides information on module cleanup at exit.
Changed in version 3.10: The
module reports the site-specific paths and .pth files being processed.
See also
.
-Warg
Warning control. Python’s warning machinery by default prints warning messages to
.
The simplest settings apply a particular action unconditionally to all warnings emitted by a process (even those that are otherwise ignored by default):
-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 warnThe action names can be abbreviated as desired and the interpreter will resolve them to the appropriate action name. For example, -Wi is the same as -Wignore.
The full form of argument is:
action:message:category:module:lineno Empty fields match all values; trailing empty fields may be omitted. For example -Wignore::DeprecationWarning ignores all DeprecationWarning warnings.
The action field is as explained above but only applies to warnings that match the remaining fields.
The message field must match the whole warning message; this match is case-insensitive.
The category field matches the warning category (ex: DeprecationWarning). This must be a class name; the match test whether the actual warning category of the message is a subclass of the specified warning category.
The module field matches the (fully qualified) module name; this match is case-sensitive.
The lineno field matches the line number, where zero matches all line numbers and is thus equivalent to an omitted line number.
Multiple
options can be given; when a warning matches more than one option, the action for the last matching option is performed. Invalid
options are ignored (though, a warning message is printed about invalid options when the first warning is issued).
Warnings can also be controlled using the
environment variable and from within a Python program using the
module. For example, the
function can be used to use a regular expression on the warning message.
See
and
for more details.
-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 currently defines the following possible values:
-Xfaulthandler to enable
. See also
.
-Xshowrefcount to output the total reference count and number of used memory blocks when the program finishes or after each statement in the interactive interpreter. This only works on
.
-Xtracemalloc to start tracing Python memory allocations using the
module. By default, only the most recent frame is stored in a traceback of a trace. Use -Xtracemalloc=NFRAME to start tracing with a traceback limit of NFRAME frames. See
and
for more information.
-Xint_max_str_digits configures the
integer string conversion length limitation
. See also
.
-Ximporttime to show how long each import takes. It shows module name, cumulative time (including nested imports) and self time (excluding nested imports). Note that its output may be broken in multi-threaded application. Typical usage is python3-Ximporttime-c'importasyncio'. See also
.
-Xdev: enable
, introducing additional runtime checks that are too expensive to be enabled by default. See also
.
-Xutf8 enables the
. -Xutf8=0 explicitly disables
(even when it would otherwise activate automatically). See also
.
-Xpycache_prefix=PATH enables writing .pyc files to a parallel tree rooted at the given directory instead of to the code tree. See also
.
-Xwarn_default_encoding issues a
when the locale-specific default encoding is used for opening files. See also
.
-Xno_debug_ranges disables the inclusion of the tables mapping extra location information (end line, start column offset and end column offset) to every instruction in code objects. This is useful when smaller code objects and pyc files are desired as well as suppressing the extra visual location indicators when the interpreter displays tracebacks. See also
.
-Xfrozen_modules determines whether or not frozen modules are ignored by the import machinery. A value of “on” means they get imported and “off” means they are ignored. The default is “on” if this is an installed Python (the normal case). If it’s under development (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 the
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 logs close() exceptions in
destructor.
Changed in version 3.9: Using -Xdev option, check encoding and errors arguments on string 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 by
.
1.2. Environment variables
These environment variables influence Python’s behavior, they are processed before the command-line switches other than -E or -I. It is customary that command-line switches override environmental variables where there is a conflict.
PYTHONHOME
Change the location of the standard Python libraries. By default, the libraries are searched in prefix/lib/pythonversion and exec_prefix/lib/pythonversion, where prefix and exec_prefix are installation-dependent directories, both defaulting to /usr/local.
When
is set to a single directory, its value replaces both prefix and exec_prefix. To specify different values for these, set
to prefix:exec_prefix.
PYTHONPATH
Augment the default search path for module files. The format is the same as the shell’s PATH: one or more directory pathnames separated by
(e.g. colons on Unix or semicolons on Windows). Non-existent directories are silently ignored.
In addition to normal directories, individual
entries may refer to zipfiles containing pure Python modules (in either source or compiled form). Extension modules cannot be imported from zipfiles.
The default search path is installation dependent, but generally begins with prefix/lib/pythonversion (see
above). It is always appended to
.
An additional directory will be inserted in the search path in front of
as described above under
. The search path can be manipulated from within a Python program as the variable
.
PYTHONSAFEPATH
If this is set to a non-empty string, don’t prepend a potentially unsafe path to
: see the
option for details.
New in version 3.11.
PYTHONPLATLIBDIR
If this is set to a non-empty string, it overrides the
value.
New in version 3.9.
PYTHONSTARTUP
If this is the name of a readable file, the Python commands in that file are executed before the first prompt is displayed in interactive mode. The file is executed in the same namespace where interactive commands are executed so that objects defined or imported in it can be used without qualification in the interactive session. You can also change the prompts
and
and the hook
in this file.
Raises an
cpython.run_startup with the filename as the argument when called on startup.
PYTHONOPTIMIZE
If this is set to a non-empty string it is equivalent to specifying the
option. If set to an integer, it is equivalent to specifying
multiple times.
PYTHONBREAKPOINT
If this is set, it names a callable using dotted-path notation. The module containing the callable will be imported and then the callable will be run by the default implementation of
which itself is called by built-in
. If not set, or set to the empty string, it is equivalent to the value “pdb.set_trace”. Setting this to the string “0” causes the default implementation of
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
option. If set to an integer, it is equivalent to specifying
multiple times.
PYTHONINSPECT
If this is set to a non-empty string it is equivalent to specifying the
option.
This variable can also be modified by Python code using
to force inspect mode on program termination.
Raises an
cpython.run_stdin with no arguments.
Changed in version 3.11.10: (also 3.10.15, 3.9.20, and 3.8.20) Emits audit events.
PYTHONUNBUFFERED
If this is set to a non-empty string it is equivalent to specifying the
option.
PYTHONVERBOSE
If this is set to a non-empty string it is equivalent to specifying the
option. If set to an integer, it is equivalent to specifying
multiple times.
PYTHONCASEOK
If this is set, Python ignores case in
statements. This only works on Windows and macOS.
PYTHONDONTWRITEBYTECODE
If this is set to a non-empty string, Python won’t try to write .pyc files on the import of source modules. This is equivalent to specifying the
option.
PYTHONPYCACHEPREFIX
If this is set, Python will write .pyc files in a mirror directory tree at this path, instead of in __pycache__ directories within the source tree. This is equivalent to specifying the
pycache_prefix=PATH option.
New in version 3.8.
PYTHONHASHSEED
If this variable is not set or set to random, a random value is used to seed the hashes of str and bytes objects.
If
is set to an integer value, it is used as a fixed seed for generating the hash() of the types covered by the hash randomization.
Its purpose is to allow repeatable hashing, such as for selftests for the interpreter itself, or to allow a cluster of python processes to share hash values.
The integer must be a decimal number in the range [0,4294967295]. Specifying the 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 the interpreter’s global
integer string conversion length limitation
.
New in version 3.11.
PYTHONIOENCODING
If this is set before running the interpreter, it overrides the encoding used for stdin/stdout/stderr, in the syntax encodingname:errorhandler. Both the encodingname and the :errorhandler parts are optional and have the same meaning as in
.
For stderr, the :errorhandler part is ignored; the handler will always be 'backslashreplace'.
Changed in version 3.4: The encodingname part is now optional.
Changed in version 3.6: On Windows, the encoding specified by this variable is ignored for interactive console buffers unless
is also specified. Files and pipes redirected through the standard streams are not affected.
PYTHONNOUSERSITE
If this is set, Python won’t add the
to
.
See also
– Per user site-packages directory
PYTHONUSERBASE
Defines the
, which is used to compute the path of the
and
for python-mpipinstall--user.
See also
– Per user site-packages directory
PYTHONEXECUTABLE
If this environment variable is set, sys.argv[0] will be set to its value instead of the value got through the C runtime. Only works on macOS.
PYTHONWARNINGS
This is equivalent to the
option. If set to a comma separated string, it is equivalent to specifying
multiple times, with filters later in the list taking precedence over those earlier in the list.
The simplest settings apply a particular action unconditionally to all warnings emitted by a process (even those that are otherwise ignored by default):
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 warnSee
and
for more details.
PYTHONFAULTHANDLER
If this environment variable is set to a non-empty string,
is called at startup: install a handler for
,
,
,
and
signals to dump the Python traceback. This is equivalent to
faulthandler option.
New in version 3.3.
PYTHONTRACEMALLOC
If this environment variable is set to a non-empty string, start tracing Python memory allocations using the
module. The value of the variable is the maximum number of frames stored in a traceback of a trace. For example, PYTHONTRACEMALLOC=1 stores only the most recent frame. See the
function for more information. This is equivalent to setting the
tracemalloc option.
New in version 3.4.
PYTHONPROFILEIMPORTTIME
If this environment variable is set to a non-empty string, Python will show how long each import takes. This is equivalent to setting the
importtime option.
New in version 3.7.
PYTHONASYNCIODEBUG
If this environment variable is set to a non-empty string, enable the
of the
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:
default: use the
.
malloc: use the malloc() function of the C library for all domains (
,
,
).
pymalloc: use the
for
and
domains and use the malloc() function for the
domain.
Install
:
debug: install debug hooks on top of the
.
malloc_debug: same as malloc but also install debug hooks.
pymalloc_debug: same as pymalloc but also install debug hooks.
New in version 3.6.
Changed in version 3.7: Added the "default" allocator.
PYTHONMALLOCSTATS
If set to a non-empty string, Python will print statistics of the
every time a new pymalloc object arena is created, and on shutdown.
This variable is ignored if the
environment variable is used to force the malloc() allocator of the C library, or if Python is configured without pymalloc 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 error handler
mode will 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 with
sys._enablelegacywindowsfsencoding()
.
New in version 3.6: See
for more details.
PYTHONLEGACYWINDOWSSTDIO
If set to a non-empty string, does not use the new console reader and writer. This means that Unicode characters will be encoded according to the active console code page, rather than using utf-8.
This variable is ignored if the standard streams are redirected (to files or pipes) rather than referring to console buffers.
New in version 3.6.
PYTHONCOERCECLOCALE
If set to the value 0, causes the main Python command line application to skip coercing the legacy ASCII-based C and POSIX locales to a more capable UTF-8 based alternative.
If this variable is not set (or is set to a value other than 0), the LC_ALL locale override environment variable is also not set, and the current locale reported for the LC_CTYPE category is either the default C locale, or else the explicitly ASCII-based POSIX locale, then the Python CLI will attempt to configure the following locales for the LC_CTYPE category in the order listed before loading the interpreter runtime:
C.UTF-8
C.utf8
UTF-8
If setting one of these locale categories succeeds, then the LC_CTYPE environment variable will also be set accordingly in the current process environment before the Python runtime is initialized. This ensures that in addition to being seen by both the interpreter itself and other locale-aware components running in the same process (such as the GNU readline library), the updated setting is also seen in subprocesses (regardless of whether or not those processes are running a Python interpreter), as well as in operations that query the environment rather than the current C locale (such as Python’s own
).
Configuring one of these locales (either explicitly or via the above implicit locale coercion) automatically enables the surrogateescape
for
and
(
continues to use backslashreplace as it does in any other locale). This stream handling behavior can be overridden using
as usual.
For debugging purposes, setting PYTHONCOERCECLOCALE=warn will cause Python to emit warning messages on stderr if either the locale coercion activates, or else if a locale that would have triggered coercion is still active when the Python runtime is initialized.
Also note that even when locale coercion is disabled, or when it fails to find a suitable target locale,
will still activate by default in legacy ASCII-based locales. Both features must be disabled in order to force the interpreter to use ASCII instead of UTF-8 for system interfaces.
New in version 3.7: See
for more details.
PYTHONDEVMODE
If this environment variable is set to a non-empty string, enable
, introducing additional runtime checks that are too expensive to be enabled by default. This is equivalent to setting the
dev option.
New in version 3.7.
PYTHONUTF8
If set to 1, enable the
.
If set to 0, disable the
.
Setting any other non-empty string causes an error during interpreter initialisation.
New in version 3.7.
PYTHONWARNDEFAULTENCODING
If this environment variable is set to a non-empty string, issue a
when the locale-specific default encoding is used.
See
for details.
New in version 3.10.
PYTHONNODEBUGRANGES
If this variable is set, it disables the inclusion of the tables mapping extra location information (end line, start column offset and end column offset) to every instruction in code objects. This is useful when smaller code objects and pyc files are desired as well as suppressing the extra visual location indicators when the interpreter displays tracebacks.
New in version 3.11.
1.2.1. Debug-mode variables
PYTHONTHREADDEBUG
If set, Python will print threading debug info into stdout.
Need a
.
Deprecated since version 3.10, will be removed in version 3.12.
PYTHONDUMPREFS
If set, Python will dump objects and reference counts still alive after shutting down the interpreter.
Need Python configured with the
build option.
PYTHONDUMPREFSFILE=FILENAME
If set, Python will dump objects and reference counts still alive after shutting down the interpreter into a file called FILENAME.
Need Python configured with the
build option.
New in version 3.11.