man
1 PYTHON
PYTHON(1) General Commands Manual PYTHON(1)
NAME
python - an interpreted, interactive, object-oriented programming lan-
guage
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 lan-
guage 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, con-
stants, functions and modules. Finally, the Python Reference Manual
describes the syntax and semantics of the core language in (perhaps
too) much detail. (These documents may be located via the INTERNET RE-
SOURCES 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 applica-
tions. 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 PYTHONDONTWRITEBYTE-
CODE.
-b Issue warnings about str(bytes_instance), str(bytearray_in-
stance) and comparing bytes/bytearray with str. (-bb: issue er-
rors)
-c command
Specify the command to execute (see next section). This termi-
nates 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.
-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 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 imposed
to prevent the user from injecting malicious code.
-m module-name
Searches sys.path for the named module and runs the correspond-
ing .py file as a script.
-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 com-
piled (bytecode) files by adding .opt-2 before the .pyc exten-
sion.
-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 ma-
nipulations of sys.path that it entails. Also disable these ma-
nipulations if site is explicitly imported later.
-u Force the stdout and stderr streams to be unbuffered. This op-
tion 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 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 sometimes prints warning message to
sys.stderr. A typical warning message has the following form:
file:line: category: message. By default, each warning is
printed once for each source line where it occurs. This option
controls how often warnings are printed. Multiple -W options
may be given; when a warning matches more than one option, the
action for the last matching option is performed. Invalid -W
options are ignored (a warning message is printed about invalid
options when the first warning is issued). Warnings can also be
controlled from within a Python program using the warnings mod-
ule.
The simplest form of argument is one of the following action
strings (or a unique abbreviation): ignore to ignore all warn-
ings; default to explicitly request the default behavior (print-
ing each warning once per source line); all to print a warning
each time it occurs (this may generate many messages if a warn-
ing is triggered repeatedly for the same source line, such as
inside a loop); module to print each warning only the first time
it occurs in each module; once to print each warning only the
first time it occurs in the program; or error to raise an excep-
tion instead of printing a warning message.
The full form of argument is action:message:category:mod-
ule:line. Here, action is as explained above but only applies
to messages that match the remaining fields. Empty fields match
all values; trailing empty fields may be omitted. The message
field matches the start of the warning message printed; this
match is case-insensitive. The category field matches the warn-
ing category. 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 full class name must be given.
The module field matches the (fully-qualified) module name; this
match is case-sensitive. The line field matches the line num-
ber, where zero matches all line numbers and is thus equivalent
to an omitted line number.
-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 us-
ing 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 ad-
ditional 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_SetupDebugHooks() 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 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 pycache_prefix=PATH: enable writing .pyc files to a par-
allel tree rooted at the
given directory instead of to the code tree.
-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 input 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 multi-
ple statements separated by newlines. Leading whitespace is signifi-
cant 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 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 ex-
ception; 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 conven-
tions; ${prefix} and ${exec_prefix} are installation-dependent and
should be interpreted as for GNU software; they may be the same. The
default for both is /usr/local.
${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 de-
fault, the libraries are searched in ${prefix}/lib/python<ver-
sion> and ${exec_prefix}/lib/python<version>, 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 ${pre-
fix}:${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 path-
names separated by colons. Non-existent directories are
silently ignored. The default search path is installation de-
pendent, but generally begins with ${prefix}/lib/python<version>
(see PYTHONHOME above). The default search path is always ap-
pended to $PYTHONPATH. If a script argument is given, the di-
rectory containing the script is inserted in the path in front
of $PYTHONPATH. The search path can be manipulated from within
a Python program 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 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 in-
teractive 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 speci-
fying 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 speci-
fying 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 speci-
fying the -B option (don't try to write .pyc files).
PYTHONINSPECT
If this is set to a non-empty string it is equivalent to speci-
fying the -i option.
PYTHONIOENCODING
If this is set before running the interpreter, it overrides the
encoding used for stdin/stdout/stderr, in the syntax encoding-
name: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 speci-
fying 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 speci-
fying the -u option.
PYTHONVERBOSE
If this is set to a non-empty string it is equivalent to speci-
fying 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 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 ran-
domization.
PYTHONINTMAXSTRDIGITS
Limit the maximum digit characters in an int value when convert-
ing 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.
PYTHONMALLOC
Set the Python memory allocators and/or install debug hooks. The
available memory allocators are malloc and pymalloc. The avail-
able debug hooks are debug, malloc_debug, and pymalloc_debug.
When Python is compiled in debug mode, the default is pymal-
loc_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 vari-
able is used to force the malloc(3) allocator of the C library,
or if Python is configured without pymalloc support.
PYTHONASYNCIODEBUG
If this environment variable is set to a non-empty string, en-
able 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 SIGILL 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 installa-
tion paths for python setup.py install --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 de-
fault 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.
PYTHONDUMPREFS
If this environment variable is set, Python will dump objects
and reference counts still alive after shutting down the inter-
preter.
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 "LI-
CENSE" in the Python source distribution for information on terms &
conditions for accessing and otherwise using Python and for a DIS-
CLAIMER OF ALL WARRANTIES.
PYTHON(1)