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PYTHON(1) General Commands Manual PYTHON(1)
NAME
python - an interpreted, interactive, object-oriented programming language
SYNOPSIS
python [ -B ] [ -b ] [ -d ] [ -E ] [ -h ] [ -i ] [ -I ]
[ -m module-name ] [ -q ] [ -O ] [ -OO ] [ -s ] [ -S ] [ -u ]
[ -v ] [ -V ] [ -W argument ] [ -x ] [ -X option ] [ -? ]
[ --check-hash-based-pycs default | always | never ]
[ -c command | script | - ] [ arguments ]
DESCRIPTION
Python is an interpreted, interactive, object-oriented programming language that combines
remarkable power with very clear syntax. For an introduction to programming in Python,
see the Python Tutorial. The Python Library Reference documents built-in and standard
types, constants, functions and modules. Finally, the Python Reference Manual describes
the syntax and semantics of the core language in (perhaps too) much detail. (These docu-
ments may be located via the INTERNET RESOURCES below; they may be installed on your sys-
tem as well.)
Python's basic power can be extended with your own modules written in C or C++. On most
systems such modules may be dynamically loaded. Python is also adaptable as an extension
language for existing applications. See the internal documentation for hints.
Documentation for installed Python modules and packages can be viewed by running the pydoc
program.
COMMAND LINE OPTIONS
-B Don't write .pyc files on import. See also PYTHONDONTWRITEBYTECODE.
-b Issue warnings about str(bytes_instance), str(bytearray_instance) and comparing
bytes/bytearray with str. (-bb: issue errors)
-c command
Specify the command to execute (see next section). This terminates the option list
(following options are passed as arguments to the command).
--check-hash-based-pycs mode
Configure how Python evaluates the up-to-dateness of hash-based .pyc files.
-d Turn on parser debugging output (for expert only, depending on compilation op-
tions).
-E Ignore environment variables like PYTHONPATH and PYTHONHOME that modify the behav-
ior of the interpreter.
-h , -? , --help
Prints the usage for the interpreter executable and exits.
-i When a script is passed as first argument or the -c option is used, enter interac-
tive mode after executing the script or the command. It does not read the $PYTHON-
STARTUP file. This can be useful to inspect global variables or a stack trace when
a script raises an exception.
-I Run Python in isolated mode. This also implies -E and -s. In isolated mode sys.path
contains neither the script's directory nor the user's site-packages directory. All
PYTHON* environment variables are ignored, too. Further restrictions may be im-
posed to prevent the user from injecting malicious code.
-m module-name
Searches sys.path for the named module and runs the corresponding .py file as a
script. This terminates the option list (following options are passed as arguments
to the module).
-O Remove assert statements and any code conditional on the value of __debug__; aug-
ment the filename for compiled (bytecode) files by adding .opt-1 before the .pyc
extension.
-OO Do -O and also discard docstrings; change the filename for compiled (bytecode)
files by adding .opt-2 before the .pyc extension.
-q Do not print the version and copyright messages. These messages are also suppressed
in non-interactive mode.
-s Don't add user site directory to sys.path.
-S Disable the import of the module site and the site-dependent manipulations of
sys.path that it entails. Also disable these manipulations if site is explicitly
imported later.
-u Force the stdout and stderr streams to be unbuffered. This option has no effect on
the stdin stream.
-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, print a message for
each file that is checked for when searching for a module. Also provides informa-
tion on module cleanup at exit.
-V , --version
Prints the Python version number of the executable and exits. When given twice,
print more information about the build.
-W argument
Warning control. Python's warning machinery by default prints warning messages to
sys.stderr.
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 warn
The 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 -W
ignore::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 printed 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-sen-
sitive.
The lineno field matches the line number, where zero matches all line numbers and
is thus equivalent to an omitted line number.
Multiple -W options can be given; when a warning matches more than one option, the
action for the last matching option is performed. Invalid -W 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 PYTHONWARNINGS environment variable and
from within a Python program using the warnings module. For example, the warn-
ings.filterwarnings() function can be used to use a regular expression on the warn-
ing message.
-X option
Set implementation specific option. The following options are available:
-X faulthandler: enable faulthandler
-X showrefcount: 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 debug builds
-X tracemalloc: start tracing Python memory allocations using the
tracemalloc module. By default, only the most recent frame is stored in a
traceback of a trace. Use -X tracemalloc=NFRAME to start tracing with a
traceback limit of NFRAME frames
-X importtime: 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 -X importtime -c 'import asyncio'
-X dev: enable CPython's "development mode", introducing additional runtime
checks which are too expensive to be enabled by default. It will not be
more verbose than the default if the code is correct: new warnings are
only emitted when an issue is detected. Effect of the developer mode:
* Add default warning filter, as -W default
* Install debug hooks on memory allocators: see the PyMem_SetupDe-
bugHooks()
C function
* Enable the faulthandler module to dump the Python traceback on a crash
* Enable asyncio debug mode
* Set the dev_mode attribute of sys.flags to True
* io.IOBase destructor logs close() exceptions
-X utf8: enable UTF-8 mode for operating system interfaces, overriding the de-
fault
locale-aware mode. -X utf8=0 explicitly disables UTF-8 mode (even when it
would
otherwise activate automatically). See PYTHONUTF8 for more details
-X pycache_prefix=PATH: enable writing .pyc files to a parallel tree rooted at
the
given directory instead of to the code tree.
-X warn_default_encoding: enable opt-in EncodingWarning for 'encoding=None'
-X int_max_str_digits=number: limit the size of int<->str conversions.
This helps avoid denial of service attacks when parsing untrusted data.
The default is sys.int_info.default_max_str_digits. 0 disables.
-x Skip the first line of the source. This is intended for a DOS specific hack only.
Warning: the line numbers in error messages will be off by one!
INTERPRETER INTERFACE
The interpreter interface resembles that of the UNIX shell: when called with standard in-
put connected to a tty device, it prompts for commands and executes them until an EOF 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 -c command, 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! In non-interactive
mode, the entire input is parsed before it is executed.
If available, the script name and additional arguments thereafter are passed to the script
in the Python variable sys.argv, which is a list of strings (you must first import sys to
be able to access it). If no script name is given, sys.argv[0] is an empty string; if -c
is used, sys.argv[0] contains the string '-c'. Note that options interpreted by the
Python interpreter itself are not placed in sys.argv.
In interactive mode, the primary prompt is `>>>'; the second prompt (which appears when a
command is not complete) is `...'. The prompts can be changed by assignment to sys.ps1 or
sys.ps2. The interpreter quits when it reads an EOF at a prompt. When an unhandled ex-
ception occurs, a stack trace is printed and control returns to the primary prompt; in
non-interactive mode, the interpreter exits after printing the stack trace. The interrupt
signal raises the KeyboardInterrupt exception; other UNIX signals are not caught (except
that SIGPIPE is sometimes ignored, in favor of the IOError exception). Error messages are
written to stderr.
FILES AND DIRECTORIES
These are subject to difference depending on local installation conventions; ${prefix} and
${exec_prefix} are installation-dependent and should be interpreted as for GNU software;
they may be the same. On Debian GNU/{Hurd,Linux} the default for both is /usr.
${exec_prefix}/bin/python
Recommended location of the interpreter.
${prefix}/lib/python<version>
${exec_prefix}/lib/python<version>
Recommended locations of the directories containing the standard modules.
${prefix}/include/python<version>
${exec_prefix}/include/python<version>
Recommended locations of the directories containing the include files needed for
developing Python extensions and embedding the interpreter.
ENVIRONMENT VARIABLES
PYTHONHOME
Change the location of the standard Python libraries. By default, the libraries
are searched in ${prefix}/lib/python<version> and ${exec_prefix}/lib/python<ver-
sion>, where ${prefix} and ${exec_prefix} are installation-dependent directories,
both defaulting to /usr/local. When $PYTHONHOME is set to a single directory, its
value replaces both ${prefix} and ${exec_prefix}. To specify different values for
these, set $PYTHONHOME to ${prefix}:${exec_prefix}.
PYTHONPATH
Augments the default search path for module files. The format is the same as the
shell's $PATH: one or more directory pathnames separated by colons. Non-existent
directories are silently ignored. The default search path is installation depen-
dent, but generally begins with ${prefix}/lib/python<version> (see PYTHONHOME
above). The default search path is always appended to $PYTHONPATH. If a script
argument is given, the directory containing the script is inserted in the path in
front of $PYTHONPATH. The search path can be manipulated from within a Python pro-
gram as the variable sys.path.
PYTHONPLATLIBDIR
Override sys.platlibdir.
PYTHONSTARTUP
If this is the name of a readable file, the Python commands in that file are exe-
cuted before the first prompt is displayed in interactive mode. The file is exe-
cuted in the same name space where interactive commands are executed so that ob-
jects defined or imported in it can be used without qualification in the interac-
tive session. You can also change the prompts sys.ps1 and sys.ps2 in this file.
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.
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.
PYTHONDONTWRITEBYTECODE
If this is set to a non-empty string it is equivalent to specifying the -B option
(don't try to write .pyc files).
PYTHONINSPECT
If this is set to a non-empty string it is equivalent to specifying the -i option.
PYTHONIOENCODING
If this is set before running the interpreter, it overrides the encoding used for
stdin/stdout/stderr, in the syntax encodingname:errorhandler The errorhandler part
is optional and has the same meaning as in str.encode. For stderr, the errorhandler
part is ignored; the handler will always be 'backslashreplace'.
PYTHONNOUSERSITE
If this is set to a non-empty string it is equivalent to specifying the -s option
(Don't add the user site directory to sys.path).
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.
PYTHONWARNINGS
If this is set to a comma-separated string it is equivalent to specifying the -W
option for each separate value.
PYTHONHASHSEED
If this variable is set to "random", a random value is used to seed the hashes of
str and bytes objects.
If PYTHONHASHSEED is set to an integer value, it is used as a fixed seed for gener-
ating 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.
PYTHONINTMAXSTRDIGITS
Limit the maximum digit characters in an int value when converting from a string
and when converting an int back to a str. A value of 0 disables the limit. Con-
versions to or from bases 2, 4, 8, 16, and 32 are never limited.
PYTHONMALLOC
Set the Python memory allocators and/or install debug hooks. The available memory
allocators are malloc and pymalloc. The available debug hooks are debug, mal-
loc_debug, and pymalloc_debug.
When Python is compiled in debug mode, the default is pymalloc_debug and the debug
hooks are automatically used. Otherwise, the default is pymalloc.
PYTHONMALLOCSTATS
If set to a non-empty string, Python will print statistics of the pymalloc memory
allocator every time a new pymalloc object arena is created, and on shutdown.
This variable is ignored if the $PYTHONMALLOC environment variable is used to force
the malloc(3) allocator of the C library, or if Python is configured without pymal-
loc support.
PYTHONASYNCIODEBUG
If this environment variable is set to a non-empty string, enable the debug mode of
the asyncio module.
PYTHONTRACEMALLOC
If this environment variable is set to a non-empty string, start tracing Python
memory allocations using the tracemalloc 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.
PYTHONFAULTHANDLER
If this environment variable is set to a non-empty string, faulthandler.enable() is
called at startup: install a handler for SIGSEGV, SIGFPE, SIGABRT, SIGBUS and SIG-
ILL signals to dump the Python traceback.
This is equivalent to the -X faulthandler option.
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 Mac OS X.
PYTHONUSERBASE
Defines the user base directory, which is used to compute the path of the user
site-packages directory and Distutils installation paths for python setup.py in-
stall --user.
PYTHONPROFILEIMPORTTIME
If this environment variable is set to a non-empty string, Python will show how
long each import takes. This is exactly equivalent to setting -X importtime on the
command line.
PYTHONBREAKPOINT
If this environment variable is set to 0, it disables the default debugger. It can
be set to the callable of your debugger of choice.
Debug-mode variables
Setting these variables only has an effect in a debug build of Python, that is, if Python
was configured with the --with-pydebug build option.
PYTHONTHREADDEBUG
If this environment variable is set, Python will print threading debug info. The
feature is deprecated in Python 3.10 and will be removed in Python 3.12.
PYTHONDUMPREFS
If this environment variable is set, Python will dump objects and reference counts
still alive after shutting down the interpreter.
AUTHOR
The Python Software Foundation: https://www.python.org/psf/
INTERNET RESOURCES
Main website: https://www.python.org/
Documentation: https://docs.python.org/
Developer resources: https://devguide.python.org/
Downloads: https://www.python.org/downloads/
Module repository: https://pypi.org/
Newsgroups: comp.lang.python, comp.lang.python.announce
LICENSING
Python is distributed under an Open Source license. See the file "LICENSE" in the Python
source distribution for information on terms & conditions for accessing and otherwise us-
ing Python and for a DISCLAIMER OF ALL WARRANTIES.
PYTHON(1)
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