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python \- an interpreted, interactive, object-oriented programming language
.B python
.B \-B
.B \-b
.B \-d
.B \-E
.B \-h
.B \-i
.B \-I
.B \-m
.I module-name
.B \-q
.B \-O
.B \-OO
.B \-P
.B \-s
.B \-S
.B \-u
.B \-v
.B \-V
.B \-W
.I argument
.B \-x
.B \-X
.I option
.B \-?
.B \--check-hash-based-pycs
.I default
.I always
.I never
.B \--help
.B \--help-env
.B \--help-xoptions
.B \--help-all
.B \-c
.I command
.I script
.I arguments
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 documents may be located via the
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
See the internal documentation for hints.
Documentation for installed Python modules and packages can be
viewed by running the
.B pydoc
.B \-B
Don't write
.I .pyc
files on import. See also PYTHONDONTWRITEBYTECODE.
.B \-b
Issue warnings about str(bytes_instance), str(bytearray_instance)
and comparing bytes/bytearray with str. (-bb: issue errors)
.BI "\-c " command
Specify the command to execute (see next section).
This terminates the option list (following options are passed as
arguments to the command).
.BI "\-\-check-hash-based-pycs " mode
Configure how Python evaluates the up-to-dateness of hash-based .pyc files.
.B \-d
Turn on parser debugging output (for expert only, depending on
compilation options).
.B \-E
Ignore environment variables like PYTHONPATH and PYTHONHOME that modify
the behavior of the interpreter.
.B \-h ", " \-? ", "\-\-help
Prints the usage for the interpreter executable and exits.
.B "\-\-help\-env"
Prints help about Python-specific environment variables and exits.
.B "\-\-help\-xoptions"
Prints help about implementation-specific \fB\-X\fP options and exits.
.B "\-\-help\-all"
Prints complete usage information and exits.
.B \-i
When a script is passed as first argument or the \fB\-c\fP 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.
.B \-I
Run Python in isolated mode. This also implies \fB\-E\fP, \fB\-P\fP and \fB\-s\fP. 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.
.BI "\-m " module-name
.I sys.path
for the named module and runs the corresponding
.I .py
file as a script. This terminates the option list (following options
are passed as arguments to the module).
.B \-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.
.B \-OO
Do \fB-O\fP and also discard docstrings; change the filename for
compiled (bytecode) files by adding .opt-2 before the .pyc extension.
.B \-P
Don't automatically prepend a potentially unsafe path to \fBsys.path\fP such
as the current directory, the script's directory or an empty string. See also the
\fBPYTHONSAFEPATH\fP environment variable.
.B \-q
Do not print the version and copyright messages. These messages are
also suppressed in non-interactive mode.
.B \-s
Don't add user site directory to sys.path.
.B \-S
Disable the import of the module
.I site
and the site-dependent manipulations of
.I sys.path
that it entails. Also disable these manipulations if
.I site
is explicitly imported later.
.B \-u
Force the stdout and stderr streams to be unbuffered.
This option has no effect on the stdin stream.
.B \-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.
.B \-V ", " \-\-version
Prints the Python version number of the executable and exits. When given
twice, print more information about the build.
.BI "\-W " argument
Warning control. Python's warning machinery by default prints warning messages
.IR 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,
.B -Wi
is the same as
.B -Wignore .
The full form of argument is:
.IB action:message:category:module:lineno
Empty fields match all values; trailing empty fields may be omitted. For
.B -W ignore::DeprecationWarning
ignores all DeprecationWarning warnings.
.I action
field is as explained above but only applies to warnings that match
the remaining fields.
.I message
field must match the whole printed warning message; this match is
.I 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.
.I module
field matches the (fully-qualified) module name; this match is case-sensitive.
.I lineno
field matches the line number, where zero matches all line numbers and is thus
equivalent to an omitted line number.
.B -W
options can be given; when a warning matches more than one option, the action
for the last matching option is performed. Invalid
.B -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
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.
.BI "\-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_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 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 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 interpreter displays tracebacks.
-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 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.
.B \-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!
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
.I script
from that file;
when called with
.B \-c
.IR command ,
it executes the Python statement(s) given as
.IR command .
.I 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
If available, the script name and additional arguments thereafter are
passed to the script in the Python variable
.IR sys.argv ,
which is a list of strings (you must first
.I import sys
to be able to access it).
If no script name is given,
.I sys.argv[0]
is an empty string; if
.B \-c
is used,
.I sys.argv[0]
contains the string
.I '-c'.
Note that options interpreted by the Python interpreter itself
are not placed in
.IR 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
.I sys.ps1
.IR 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
.I Keyboard\%Interrupt
exception; other UNIX signals are not caught (except that SIGPIPE is
sometimes ignored, in favor of the
.I IOError
exception). Error messages are written to stderr.
These are subject to difference depending on local installation
conventions; ${prefix} and ${exec_prefix} are installation-dependent
and should be interpreted as for GNU software; they may be the same.
The default for both is \fI/usr/local\fP.
.IP \fI${exec_prefix}/bin/python\fP
Recommended location of the interpreter.
.I ${prefix}/lib/python<version>
.I ${exec_prefix}/lib/python<version>
Recommended locations of the directories containing the standard
.I ${prefix}/include/python<version>
.I ${exec_prefix}/include/python<version>
Recommended locations of the directories containing the include files
needed for developing Python extensions and embedding the
If this is set to a non-empty string, don't automatically prepend a potentially
unsafe path to \fBsys.path\fP such as the current directory, the script's
directory or an empty string. See also the \fB\-P\fP option.
Change the location of the standard Python libraries. By default, the
libraries are searched in ${prefix}/lib/python<version> and
${exec_prefix}/lib/python<version>, where ${prefix} and ${exec_prefix}
are installation-dependent directories, both defaulting to
\fI/usr/local\fP. 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}.
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<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 program as the
.IR sys.path .
Override sys.platlibdir.
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
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 session.
You can also change the prompts
.I sys.ps1
.I sys.ps2
in this file.
If this is set to a non-empty string it is equivalent to specifying
the \fB\-O\fP option. If set to an integer, it is equivalent to
specifying \fB\-O\fP multiple times.
If this is set to a non-empty string it is equivalent to specifying
the \fB\-d\fP option. If set to an integer, it is equivalent to
specifying \fB\-d\fP multiple times.
If this is set to a non-empty string it is equivalent to specifying
the \fB\-B\fP option (don't try to write
.I .pyc
If this is set to a non-empty string it is equivalent to specifying
the \fB\-i\fP option.
If this is set before running the interpreter, it overrides the encoding used
for stdin/stdout/stderr, in the syntax
.IB encodingname ":" errorhandler
.IB errorhandler
part is optional and has the same meaning as in str.encode. For stderr, the
.IB errorhandler
part is ignored; the handler will always be \'backslashreplace\'.
If this is set to a non-empty string it is equivalent to specifying the
\fB\-s\fP option (Don't add the user site directory to sys.path).
If this is set to a non-empty string it is equivalent to specifying
the \fB\-u\fP option.
If this is set to a non-empty string it is equivalent to specifying
the \fB\-v\fP option. If set to an integer, it is equivalent to
specifying \fB\-v\fP multiple times.
If this is set to a comma-separated string it is equivalent to
specifying the \fB\-W\fP option for each separate value.
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
The integer must be a decimal number in the range [0,4294967295]. Specifying
the value 0 will disable hash randomization.
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.
Set the Python memory allocators and/or install debug hooks. The available
memory allocators are
.IR malloc
.IR pymalloc .
The available debug hooks are
.IR debug ,
.IR malloc_debug ,
.IR pymalloc_debug .
When Python is compiled in debug mode, the default is
.IR pymalloc_debug
and the debug hooks are automatically used. Otherwise, the default is
.IR pymalloc .
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
This variable is ignored if the
environment variable is used to force the
.BR malloc (3)
allocator of the C library, or if Python is configured without pymalloc support.
If this environment variable is set to a non-empty string, enable the debug
mode of the asyncio module.
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,
stores only the most recent frame.
If this environment variable is set to a non-empty string,
.IR 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 \fB-X faulthandler\fP option.
If this environment variable is set,
.IB sys.argv[0]
will be set to its value instead of the value got through the C runtime. Only
works on Mac OS X.
Defines the user base directory, which is used to compute the path of the user
.IR site-packages
directory and installation paths for
.IR "python \-m pip install \-\-user" .
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
\fB\-X importtime\fP on the command line.
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.
If this variable is set to 1, the interpreter will colorize various kinds of
output. Setting it to 0 deactivates this behavior.
This environment variable can be used to set the location of a history file
(on Unix, it is \fI~/.python_history\fP by default).
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 \fB--disable-gil\fP.
.SS Debug-mode variables
Setting these variables only has an effect in a debug build of Python, that is,
if Python was configured with the
\fB\--with-pydebug\fP build option.
If this environment variable is set, Python will dump objects and reference
counts still alive after shutting down the interpreter.
The Python Software Foundation:
Main website:
Developer resources:
Module repository:
Newsgroups: comp.lang.python, comp.lang.python.announce
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 using Python and for a