cpython/Misc/python.man at 3.9 · python-emscripten/cpython · GitHub

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.TH PYTHON "1"

.\" To view this file while editing, run it through groff:

.\" groff -Tascii -man python.man | less

.SH NAME

python \- an interpreted, interactive, object-oriented programming language

.SH SYNOPSIS

.B python

[

.B \-B

]

[

.B \-b

]

[

.B \-d

]

[

.B \-E

]

[

.B \-h

]

[

.B \-i

]

[

.B \-I

]

.br

[

.B \-m

.I module-name

]

[

.B \-q

]

[

.B \-O

]

[

.B \-OO

]

[

.B \-s

]

[

.B \-S

]

[

.B \-u

]

.br

[

.B \-v

]

[

.B \-V

]

[

.B \-W

.I argument

]

[

.B \-x

]

[

.B \-X

.I option

]

.B \-?

]

.br

[

.B \--check-hash-based-pycs

.I default

.I always

.I never

]

.br

[

.B \-c

.I command

.I script

]

[

.I arguments

]

.SH 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 documents may be located via the

.B "INTERNET RESOURCES"

below; they may be installed on your system as well.)

.PP

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.

.PP

Documentation for installed Python modules and packages can be

viewed by running the

.B pydoc

program.

.SH COMMAND LINE OPTIONS

.TP

.B \-B

Don't write

.I .pyc

files on import. See also PYTHONDONTWRITEBYTECODE.

.TP

.B \-b

Issue warnings about str(bytes_instance), str(bytearray_instance)

and comparing bytes/bytearray with str. (-bb: issue errors)

.TP

.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).

.TP

.BI "\-\-check-hash-based-pycs " mode

Configure how Python evaluates the up-to-dateness of hash-based .pyc files.

.TP

.B \-d

Turn on parser debugging output (for expert only, depending on

compilation options).

.TP

.B \-E

Ignore environment variables like PYTHONPATH and PYTHONHOME that modify

the behavior of the interpreter.

.TP

.B \-h ", " \-? ", "\-\-help

Prints the usage for the interpreter executable and exits.

.TP

.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.

.TP

.B \-I

Run Python in isolated mode. This also implies \fB\-E\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.

.TP

.BI "\-m " module-name

Searches

.I sys.path

for the named module and runs the corresponding

.I .py

file as a script.

.TP

.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.

.TP

.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.

.TP

.B \-q

Do not print the version and copyright messages. These messages are

also suppressed in non-interactive mode.

.TP

.B \-s

Don't add user site directory to sys.path.

.TP

.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.

.TP

.B \-u

Force the stdout and stderr streams to be unbuffered.

This option has no effect on the stdin stream.

.TP

.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.

.TP

.B \-V ", " \-\-version

Prints the Python version number of the executable and exits. When given

twice, print more information about the build.

.TP

.BI "\-W " argument

Warning control. Python sometimes prints warning message to

.IR sys.stderr .

A typical warning message has the following form:

.IB 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

.B \-W

options may 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 (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

.I warnings

module.

The simplest form of

.I argument

is one of the following

.I action

strings (or a unique abbreviation):

.B ignore

to ignore all warnings;

.B default

to explicitly request the default behavior (printing each warning once

per source line);

.B all

to print a warning each time it occurs (this may generate many

messages if a warning is triggered repeatedly for the same source

line, such as inside a loop);

.B module

to print each warning only the first time it occurs in each

module;

.B once

to print each warning only the first time it occurs in the program; or

.B error

to raise an exception instead of printing a warning message.

The full form of

.I argument

.IB action : message : category : module : line.

Here,

.I 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

.I message

field matches the start of the warning message printed; this match is

case-insensitive. The

.I category

field matches the warning 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

.I module

field matches the (fully-qualified) module name; this match is

case-sensitive. The

.I line

field matches the line number, where zero matches all line numbers and

is thus equivalent to an omitted line number.

.TP

.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.

.TP

.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!

.SH 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

.I script

from that file;

when called with

.B \-c

.IR command ,

it executes the Python statement(s) given as

.IR command .

Here

.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

executed.

.PP

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 .

.PP

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.

.SH 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.

The default for both is \fI/usr/local\fP.

.IP \fI${exec_prefix}/bin/python\fP

Recommended location of the interpreter.

.PP

.I ${prefix}/lib/python<version>

.br

.I ${exec_prefix}/lib/python<version>

.RS

Recommended locations of the directories containing the standard

modules.

.RE

.PP

.I ${prefix}/include/python<version>

.br

.I ${exec_prefix}/include/python<version>

.RS

Recommended locations of the directories containing the include files

needed for developing Python extensions and embedding the

interpreter.

.RE

.SH ENVIRONMENT VARIABLES

.IP 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<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}.

.IP 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<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

variable

.IR sys.path .

.IP PYTHONPLATLIBDIR

Override sys.platlibdir.

.IP 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 session.

You can also change the prompts

.I sys.ps1

and

.I sys.ps2

in this file.

.IP PYTHONOPTIMIZE

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.

.IP PYTHONDEBUG

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.

.IP PYTHONDONTWRITEBYTECODE

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

files).

.IP PYTHONINSPECT

If this is set to a non-empty string it is equivalent to specifying

the \fB\-i\fP option.

.IP PYTHONIOENCODING

If this is set before running the interpreter, it overrides the encoding used

for stdin/stdout/stderr, in the syntax

.IB encodingname ":" errorhandler

The

.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\'.

.IP PYTHONNOUSERSITE

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).

.IP PYTHONUNBUFFERED

If this is set to a non-empty string it is equivalent to specifying

the \fB\-u\fP option.

.IP PYTHONVERBOSE

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.

.IP PYTHONWARNINGS

If this is set to a comma-separated string it is equivalent to

specifying the \fB\-W\fP option for each separate value.

.IP 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.

.IP PYTHONMALLOC

Set the Python memory allocators and/or install debug hooks. The available

memory allocators are

.IR malloc

and

.IR pymalloc .

The available debug hooks are

.IR debug ,

.IR malloc_debug ,

and

.IR pymalloc_debug .

.IP

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 .

.IP 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.

.IP

This variable is ignored if the

.RB $ PYTHONMALLOC

environment variable is used to force the

.BR malloc (3)

allocator of the C library, or if Python is configured without pymalloc support.

.IP PYTHONASYNCIODEBUG

If this environment variable is set to a non-empty string, enable the debug

mode of the asyncio module.

.IP PYTHONTRACEMALLOC

If this environment variable is set to a non-empty string, start tracing

Python memory allocations using the tracemalloc module.

.IP

The value of the variable is the maximum number of frames stored in a

traceback of a trace. For example,

.IB PYTHONTRACEMALLOC=1

stores only the most recent frame.

.IP PYTHONFAULTHANDLER

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.

.IP

This is equivalent to the \fB-X faulthandler\fP option.

.IP PYTHONEXECUTABLE

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.

.IP PYTHONUSERBASE

Defines the user base directory, which is used to compute the path of the user

.IR site-packages

directory and Distutils installation paths for

.IR "python setup\.py install \-\-user" .

.IP 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

\fB\-X importtime\fP on the command line.

.IP 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.

.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.

.IP PYTHONTHREADDEBUG

If this environment variable is set, Python will print threading debug info.

.IP PYTHONDUMPREFS

If this environment variable is set, Python will dump objects and reference

counts still alive after shutting down the interpreter.

.SH AUTHOR

The Python Software Foundation: https://www.python.org/psf/

.SH INTERNET RESOURCES

Main website: https://www.python.org/

.br

Documentation: https://docs.python.org/

.br

Developer resources: https://devguide.python.org/

.br

Downloads: https://www.python.org/downloads/

.br

Module repository: https://pypi.org/

.br

Newsgroups: comp.lang.python, comp.lang.python.announce

.SH 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 using Python and for a

DISCLAIMER OF ALL WARRANTIES.

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