Manual: python3
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 ] [ -R ] [ -O ] [ -OO ] [ -P ] [ -s ] [ -S ] [ -u ]
[ -v ] [ -V ] [ -W argument ] [ -x ] [ -X option ] [ -? ]
[ --check-hash-based-pycs default | always | never ]
[ --help ] [ --help-env ] [ --help-xoptions ] [ --help-all ]
[ -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 deā
scribes the syntax and semantics of the core language in (perhaps too) much detail. (These documents may be located
via the INTERNET RESOURCES below; they may be installed on your system 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 options).
-E Ignore environment variables like PYTHONPATH and PYTHONHOME that modify the behavior of the interpreter.
-h, -?, --help
Prints the usage for the interpreter executable and exits.
--help-env
Prints help about Python-specific environment variables and exits.
--help-xoptions
Prints help about implementation-specific -X options and exits.
--help-all
Prints complete usage information and exits.
-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. It does not read the $PYTHONSTARTUP file. This can be useful to inspect global variā
ables or a stack trace when a script raises an exception.
-I Run Python in isolated mode. This also implies -E, -P 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 imposed 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__; augment 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.
-P Don't automatically prepend a potentially unsafe path to sys.path such as the current directory, the script's
directory or an empty string. See also the PYTHONSAFEPATH environment variable.
-q Do not print the version and copyright messages. These messages are also suppressed in non-interactive mode.
-R Turn on hash randomization. This option only has an effect if the PYTHONHASHSEED environment variable is set
to 0, since hash randomization is enabled by default.
-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 modā
ule. Also provides information 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
-Wall # Same as -Walways
-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-sensitive.
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 matchā
ing 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 warnings.filterwarnings() function can be used to use a regular
expression on the warning message.
-X option
Set implementation-specific option. The following options are available:
-X cpu_count=N
Override the return value of os.cpu_count(). -X cpu_count=default cancels overriding. See also
PYTHON_CPU_COUNT.
-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 warnā
ings are only emitted when an issue is detected. Effect of the developer mode:
ā¢ Add default warning filter, as -W default.
ā¢ Install debug hooks on memory allocators: see the PyMem_SetupDebugHooks() C function.
ā¢ Enable the faulthandler module to dump the Python traceback on a crash.
ā¢ Enable asyncio debug mode.
ā¢ Set the dev_mode attribute of sys.flags to True.
ā¢ io.IOBase destructor logs close() exceptions.
-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 importtime=2 enables additional output that indicates when an imported module has already been
loaded. In such cases, the string cached will be printed in both time columns.
-X faulthandler
Enable faulthandler.
-X frozen_modules=[on|off]
Whether or not frozen modules should be used. The default is "on" (or "off" if you are running a local
build).
-X gil=[0|1]
Enable (1) or disable (0) the GIL. See also PYTHON_GIL. Only available in builds configured with
--disable-gil.
-X int_max_str_digits=number
Limit the size of int<->str conversions. This helps avoid denial of service attacks when parsing unā
trusted data. The default is sys.int_info.default_max_str_digits. 0 disables.
-X no_debug_ranges
Disable 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 interā
preter displays tracebacks.
-X perf
Support the Linux "perf" profiler. See also PYTHONPERFSUPPORT=1.
-X perf_jit
Support the Linux "perf" profiler with DWARF support. See also PYTHON_PERF_JIT_SUPPORT=1.
-X presite=MOD
Import this module before site. See also PYTHON_PRESITE. This only works on debug builds.
-X pycache_prefix=PATH
Enable writing .pyc files to a parallel tree rooted at the given directory instead of to the code tree.
-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 utf8
Enable UTF-8 mode for operating system interfaces, overriding the default locale-aware mode. -X utf8=0
explicitly disables UTF-8 mode (even when it would otherwise activate automatically). See PYTHONUTF8
for more details.
-X warn_default_encoding
Enable opt-in EncodingWarning for 'encoding=None'.
-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 input connected to a tty deā
vice, 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 exā
ecuted.
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 inā
terpreted 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 exception 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 instalā
lation-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
${exec_prefix}/lib/python
Recommended locations of the directories containing the standard modules.
${prefix}/include/python
${exec_prefix}/include/python
Recommended locations of the directories containing the include files needed for developing Python extensions
and embedding the interpreter.
ENVIRONMENT VARIABLES
PYTHONASYNCIODEBUG
If this environment variable is set to a non-empty string, enable the debug mode of the asyncio module.
PYTHON_BASIC_REPL
If this variable is set to any value, the interpreter will not attempt to load the Python-based REPL that reā
quires curses and readline, and will instead use the traditional parser-based REPL.
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.
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.
PYTHON_COLORS
If this variable is set to 1, the interpreter will colorize various kinds of output. Setting it to 0 deactiā
vates this behavior.
PYTHON_CPU_COUNT
If this variable is set to a positive integer, it overrides the return values of os.cpu_count and
os.process_cpu_count.
See also the -X cpu_count option.
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.
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.
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 SIGILL signals to dump the Python traceback.
This is equivalent to the -X faulthandler option.
PYTHON_FROZEN_MODULES
If this variable is set to on or off, it 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 for non-
debug builds (the normal case) and off for debug builds.
See also the -X frozen_modules option.
PYTHON_GIL
If this variable is set to 1, the global interpreter lock (GIL) will be forced on. Setting it to 0 forces the
GIL off. Only available in builds configured with --disable-gil.
This is equivalent to the -X gil option.
PYTHON_HISTORY
This environment variable can be used to set the location of a history file (on Unix, it is ~/.python_history
by default).
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.
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).
PYTHONDEVMODE
If this environment variable is set to a non-empty string, enable Python's "development mode", introducing adā
ditional runtime checks that are too expensive to be enabled by default.
This is equivalent to the -X dev option.
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 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.
PYTHONHOME
Change the location of the standard Python libraries. By default, the libraries are searched in ${preā
fix}/lib/python and ${exec_prefix}/lib/python, where ${prefix} and ${exec_prefix} are inā
stallation-dependent directories, both defaulting to /usr/local. When $PYTHONHOME is set to a single direcā
tory, its value replaces both ${prefix} and ${exec_prefix}. To specify different values for these, set
$PYTHONHOME to ${prefix}:${exec_prefix}.
PYTHONINSPECT
If this is set to a non-empty string it is equivalent to specifying the -i option.
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. Conversions to or from bases 2, 4, 8, 16, and 32 are never
limited.
This is equivalent to the -X int_max_str_digits=NUMBER 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 Ā“backslashreplaceĀ“.
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, malloc_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 pymalloc support.
PYTHONNOUSERSITE
If this is set to a non-empty string it is equivalent to specifying the -s option (Don't add the user site diā
rectory to sys.path).
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.
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 dependent, but generally begins with ${prefix}/lib/python (see PYTHONHOME
above). The default search path is always appended to $PYTHONPATH. If a script argument is given, the direcā
tory containing the script is inserted in the path in front of $PYTHONPATH. The search path can be manipuā
lated from within a Python program as the variable sys.path.
PYTHON_PERF_JIT_SUPPORT
If this variable is set to a nonzero value, it enables support for the Linux perf profiler so Python calls can
be detected by it using DWARF information. Setting to 0 disables.
See also the -X perf_jit option.
PYTHONPERFSUPPORT
If this variable is set to a nonzero value, it enables support for the Linux perf profiler so Python calls can
be detected by it. Setting to 0 disables.
See also the -X perf option.
PYTHONPLATLIBDIR
Override sys.platlibdir.
PYTHONPROFILEIMPORTTIME
If this environment variable is set to 1, Python will show how long each import takes. If set to 2, Python
will include output for imported modules that have already been loaded. This is exactly equivalent to setting
-X importtime on the command line.
PYTHONPYCACHEPREFIX
If this is set, Python will write .pyc files in a mirror directory tree at this path, instead of in __pyā
cache__ directories within the source tree.
This is equivalent to specifying the -X pycache_prefix=PATH option.
PYTHONSAFEPATH
If this is set to a non-empty string, don't automatically prepend a potentially unsafe path to sys.path such
as the current directory, the script's directory or an empty string. See also the -P option.
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 name space where interactive commands are
executed so that objects defined or imported in it can be used without qualification in the interactive sesā
sion. You can also change the prompts sys.ps1 and sys.ps2 in this file.
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.
PYTHONUNBUFFERED
If this is set to a non-empty string it is equivalent to specifying the -u option.
PYTHONUSERBASE
Defines the user base directory, which is used to compute the path of the user site-packages directory and inā
stallation paths for python -m pip install --user.
PYTHONUTF8
If set to 1, enable the Python "UTF-8 Mode". Setting to 0 disables.
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.
PYTHONWARNDEFAULTENCODING
If this environment variable is set to a non-empty string, issue a EncodingWarning when the locale-specific
default encoding is used.
PYTHONWARNINGS
If this is set to a comma-separated string it is equivalent to specifying the -W option for each separate
value.
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.
PYTHONDUMPREFS
If this environment variable is set, Python will dump objects and reference counts still alive after shutting
down the interpreter.
PYTHONDUMPREFSFILE
If set, Python will dump objects and reference counts still alive after shutting down the interpreter into a
file under the path given as the value to this environment variable.
PYTHON_PRESITE
If this variable is set to a module, that module will be imported early in the interpreter lifecycle, before
the site module is executed, and before the __main__ module is created. This only works on debug builds.
This is equivalent to the -X presite=module option.
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 inā
formation on terms & conditions for accessing and otherwise using Python and for a DISCLAIMER OF ALL WARRANTIES.
PYTHON(1)