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.. highlightlang:: sh
.. ATTENTION: You probably should update Misc/python.man, too, if you modify
this file.
.. _using-on-general:
Command line and environment
============================
The CPython interpreter scans the command line and the environment for various
settings.
.. impl-detail::
Other implementations' command line schemes may differ. See
:ref:`implementations` for further resources.
.. _using-on-cmdline:
Command line
------------
When invoking Python, you may specify any of these options::
python [-bBdEhiIOqsSuvVWx?] [-c command | -m module-name | script | - ] [args]
The most common use case is, of course, a simple invocation of a script::
python myscript.py
.. _using-on-interface-options:
Interface options
~~~~~~~~~~~~~~~~~
The interpreter interface resembles that of the UNIX shell, but provides some
additional methods of invocation:
* When called with standard input connected to a tty device, it prompts for
commands and executes them until an EOF (an end-of-file character, you can
produce that with :kbd:`Ctrl-D` on UNIX or :kbd:`Ctrl-Z, Enter` on Windows) 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 a directory name argument, it reads and executes an
appropriately named script from that directory.
* 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!
* When called with ``-m module-name``, the given module is located on the
Python module path and executed as a script.
In non-interactive mode, the entire input is parsed before it is executed.
An interface option terminates the list of options consumed by the interpreter,
all consecutive arguments will end up in :data:`sys.argv` -- note that the first
element, subscript zero (``sys.argv[0]``), is a string reflecting the program's
source.
.. cmdoption:: -c <command>
Execute the Python code in *command*. *command* can be one or more
statements separated by newlines, with significant leading whitespace as in
normal module code.
If this option is given, the first element of :data:`sys.argv` will be
``"-c"`` and the current directory will be added to the start of
:data:`sys.path` (allowing modules in that directory to be imported as top
level modules).
.. cmdoption:: -m <module-name>
Search :data:`sys.path` for the named module and execute its contents as
the :mod:`__main__` module.
Since the argument is a *module* name, you must not give a file extension
(``.py``). The module name should be a valid absolute Python module name, but
the implementation may not always enforce this (e.g. it may allow you to
use a name that includes a hyphen).
Package names (including namespace packages) are also permitted. When a
package name is supplied instead
of a normal module, the interpreter will execute ``<pkg>.__main__`` as
the main module. This behaviour is deliberately similar to the handling
of directories and zipfiles that are passed to the interpreter as the
script argument.
.. note::
This option cannot be used with built-in modules and extension modules
written in C, since they do not have Python module files. However, it
can still be used for precompiled modules, even if the original source
file is not available.
If this option is given, the first element of :data:`sys.argv` will be the
full path to the module file (while the module file is being located, the
first element will be set to ``"-m"``). As with the :option:`-c` option,
the current directory will be added to the start of :data:`sys.path`.
Many standard library modules contain code that is invoked on their execution
as a script. An example is the :mod:`timeit` module::
python -mtimeit -s 'setup here' 'benchmarked code here'
python -mtimeit -h # for details
.. seealso::
:func:`runpy.run_module`
Equivalent functionality directly available to Python code
:pep:`338` -- Executing modules as scripts
.. versionchanged:: 3.1
Supply the package name to run a ``__main__`` submodule.
.. versionchanged:: 3.4
namespace packages are also supported
.. describe:: -
Read commands from standard input (:data:`sys.stdin`). If standard input is
a terminal, :option:`-i` is implied.
If this option is given, the first element of :data:`sys.argv` will be
``"-"`` and the current directory will be added to the start of
:data:`sys.path`.
.. describe:: <script>
Execute the Python code contained in *script*, which must be a filesystem
path (absolute or relative) referring to either a Python file, a directory
containing a ``__main__.py`` file, or a zipfile containing a
``__main__.py`` file.
If this option is given, the first element of :data:`sys.argv` will be the
script name as given on the command line.
If the script name refers directly to a Python file, the directory
containing that file is added to the start of :data:`sys.path`, and the
file is executed as the :mod:`__main__` module.
If the script name refers to a directory or zipfile, the script name is
added to the start of :data:`sys.path` and the ``__main__.py`` file in
that location is executed as the :mod:`__main__` module.
.. seealso::
:func:`runpy.run_path`
Equivalent functionality directly available to Python code
If no interface option is given, :option:`-i` is implied, ``sys.argv[0]`` is
an empty string (``""``) and the current directory will be added to the
start of :data:`sys.path`. Also, tab-completion and history editing is
automatically enabled, if available on your platform (see
:ref:`rlcompleter-config`).
.. seealso:: :ref:`tut-invoking`
.. versionchanged:: 3.4
Automatic enabling of tab-completion and history editing.
Generic options
~~~~~~~~~~~~~~~
.. cmdoption:: -?
-h
--help
Print a short description of all command line options.
.. cmdoption:: -V
--version
Print the Python version number and exit. Example output could be:
.. code-block:: none
Python 3.7.0b2+
When given twice, print more information about the build, like:
.. code-block:: none
Python 3.7.0b2+ (3.7:0c076caaa8, Sep 22 2018, 12:04:24)
[GCC 6.2.0 20161005]
.. versionadded:: 3.6
The ``-VV`` option.
.. _using-on-misc-options:
Miscellaneous options
~~~~~~~~~~~~~~~~~~~~~
.. cmdoption:: -b
Issue a warning when comparing :class:`bytes` or :class:`bytearray` with
:class:`str` or :class:`bytes` with :class:`int`. Issue an error when the
option is given twice (:option:`!-bb`).
.. versionchanged:: 3.5
Affects comparisons of :class:`bytes` with :class:`int`.
.. cmdoption:: -B
If given, Python won't try to write ``.pyc`` files on the
import of source modules. See also :envvar:`PYTHONDONTWRITEBYTECODE`.
.. cmdoption:: --check-hash-based-pycs default|always|never
Control the validation behavior of hash-based ``.pyc`` files. See
:ref:`pyc-invalidation`. When set to ``default``, checked and unchecked
hash-based bytecode cache files are validated according to their default
semantics. When set to ``always``, all hash-based ``.pyc`` files, whether
checked or unchecked, are validated against their corresponding source
file. When set to ``never``, hash-based ``.pyc`` files are not validated
against their corresponding source files.
The semantics of timestamp-based ``.pyc`` files are unaffected by this
option.
.. cmdoption:: -d
Turn on parser debugging output (for expert only, depending on compilation
options). See also :envvar:`PYTHONDEBUG`.
.. cmdoption:: -E
Ignore all :envvar:`PYTHON*` environment variables, e.g.
:envvar:`PYTHONPATH` and :envvar:`PYTHONHOME`, that might be set.
.. cmdoption:: -i
When a script is passed as first argument or the :option:`-c` option is used,
enter interactive mode after executing the script or the command, even when
:data:`sys.stdin` does not appear to be a terminal. The
:envvar:`PYTHONSTARTUP` file is not read.
This can be useful to inspect global variables or a stack trace when a script
raises an exception. See also :envvar:`PYTHONINSPECT`.
.. cmdoption:: -I
Run Python in isolated mode. This also implies -E and -s.
In isolated mode :data:`sys.path` contains neither the script's directory nor
the user's site-packages directory. All :envvar:`PYTHON*` environment
variables are ignored, too. Further restrictions may be imposed to prevent
the user from injecting malicious code.
.. versionadded:: 3.4
.. cmdoption:: -O
Remove assert statements and any code conditional on the value of
:const:`__debug__`. Augment the filename for compiled
(:term:`bytecode`) files by adding ``.opt-1`` before the ``.pyc``
extension (see :pep:`488`). See also :envvar:`PYTHONOPTIMIZE`.
.. versionchanged:: 3.5
Modify ``.pyc`` filenames according to :pep:`488`.
.. cmdoption:: -OO
Do :option:`-O` and also discard docstrings. Augment the filename
for compiled (:term:`bytecode`) files by adding ``.opt-2`` before the
``.pyc`` extension (see :pep:`488`).
.. versionchanged:: 3.5
Modify ``.pyc`` filenames according to :pep:`488`.
.. cmdoption:: -q
Don't display the copyright and version messages even in interactive mode.
.. versionadded:: 3.2
.. cmdoption:: -R
Turn on hash randomization. This option only has an effect if the
:envvar:`PYTHONHASHSEED` environment variable is set to ``0``, since hash
randomization is enabled by default.
On previous versions of Python, this option turns on hash randomization,
so that the :meth:`__hash__` values of str, bytes and datetime
are "salted" with an unpredictable random value. Although they remain
constant within an individual Python process, they are not predictable
between repeated invocations of Python.
Hash randomization is intended to provide protection against a
denial-of-service caused by carefully-chosen inputs that exploit the worst
case performance of a dict construction, O(n^2) complexity. See
http://www.ocert.org/advisories/ocert-2011-003.html for details.
:envvar:`PYTHONHASHSEED` allows you to set a fixed value for the hash
seed secret.
.. versionchanged:: 3.7
The option is no longer ignored.
.. versionadded:: 3.2.3
.. cmdoption:: -s
Don't add the :data:`user site-packages directory <site.USER_SITE>` to
:data:`sys.path`.
.. seealso::
:pep:`370` -- Per user site-packages directory
.. cmdoption:: -S
Disable the import of the module :mod:`site` and the site-dependent
manipulations of :data:`sys.path` that it entails. Also disable these
manipulations if :mod:`site` is explicitly imported later (call
:func:`site.main` if you want them to be triggered).
.. cmdoption:: -u
Force the stdout and stderr streams to be unbuffered. This option has no
effect on the stdin stream.
See also :envvar:`PYTHONUNBUFFERED`.
.. versionchanged:: 3.7
The text layer of the stdout and stderr streams now is unbuffered.
.. cmdoption:: -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
(:option:`!-vv`), print a message for each file that is checked for when
searching for a module. Also provides information on module cleanup at exit.
See also :envvar:`PYTHONVERBOSE`.
.. _using-on-warnings:
.. cmdoption:: -W arg
Warning control. Python's warning machinery by default prints warning
messages to :data:`sys.stderr`. A typical warning message has the following
form:
.. code-block:: none
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 :option:`-W` options may be given; when a warning matches more than
one option, the action for the last matching option is performed. Invalid
:option:`-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 :envvar:`PYTHONWARNINGS`
environment variable and from within a Python program using the
:mod:`warnings` module.
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 (e.g. ``-Wi``, ``-Wd``,
``-Wa``, ``-We``) and the interpreter will resolve them to the appropriate
action name.
See :ref:`warning-filter` and :ref:`describing-warning-filters` for more
details.
.. cmdoption:: -x
Skip the first line of the source, allowing use of non-Unix forms of
``#!cmd``. This is intended for a DOS specific hack only.
.. cmdoption:: -X
Reserved for various implementation-specific options. CPython currently
defines the following possible values:
* ``-X faulthandler`` to enable :mod:`faulthandler`;
* ``-X showrefcount`` to 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`` to start tracing Python memory allocations using the
:mod:`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. See the
:func:`tracemalloc.start` for more information.
* ``-X showalloccount`` to output the total count of allocated objects for
each type when the program finishes. This only works when Python was built with
``COUNT_ALLOCS`` defined.
* ``-X importtime`` to 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'``. See also :envvar:`PYTHONPROFILEIMPORTTIME`.
* ``-X dev``: enable CPython's "development mode", introducing additional
runtime checks which are too expensive to be enabled by default. It should
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 :option:`-W` ``default``.
* Install debug hooks on memory allocators: see the
:c:func:`PyMem_SetupDebugHooks` C function.
* Enable the :mod:`faulthandler` module to dump the Python traceback
on a crash.
* Enable :ref:`asyncio debug mode <asyncio-debug-mode>`.
* Set the :attr:`~sys.flags.dev_mode` attribute of :attr:`sys.flags` to
``True``
* ``-X utf8`` enables 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 :envvar:`PYTHONUTF8` for more details.
It also allows passing arbitrary values and retrieving them through the
:data:`sys._xoptions` dictionary.
.. versionchanged:: 3.2
The :option:`-X` option was added.
.. versionadded:: 3.3
The ``-X faulthandler`` option.
.. versionadded:: 3.4
The ``-X showrefcount`` and ``-X tracemalloc`` options.
.. versionadded:: 3.6
The ``-X showalloccount`` option.
.. versionadded:: 3.7
The ``-X importtime``, ``-X dev`` and ``-X utf8`` options.
Options you shouldn't use
~~~~~~~~~~~~~~~~~~~~~~~~~
.. cmdoption:: -J
Reserved for use by Jython_.
.. _Jython: http://www.jython.org/
.. _using-on-envvars:
Environment variables
---------------------
These environment variables influence Python's behavior, they are processed
before the command-line switches other than -E or -I. It is customary that
command-line switches override environmental variables where there is a
conflict.
.. envvar:: PYTHONHOME
Change the location of the standard Python libraries. By default, the
libraries are searched in :file:`{prefix}/lib/python{version}` and
:file:`{exec_prefix}/lib/python{version}`, where :file:`{prefix}` and
:file:`{exec_prefix}` are installation-dependent directories, both defaulting
to :file:`/usr/local`.
When :envvar:`PYTHONHOME` is set to a single directory, its value replaces
both :file:`{prefix}` and :file:`{exec_prefix}`. To specify different values
for these, set :envvar:`PYTHONHOME` to :file:`{prefix}:{exec_prefix}`.
.. envvar:: PYTHONPATH
Augment the default search path for module files. The format is the same as
the shell's :envvar:`PATH`: one or more directory pathnames separated by
:data:`os.pathsep` (e.g. colons on Unix or semicolons on Windows).
Non-existent directories are silently ignored.
In addition to normal directories, individual :envvar:`PYTHONPATH` entries
may refer to zipfiles containing pure Python modules (in either source or
compiled form). Extension modules cannot be imported from zipfiles.
The default search path is installation dependent, but generally begins with
:file:`{prefix}/lib/python{version}` (see :envvar:`PYTHONHOME` above). It
is *always* appended to :envvar:`PYTHONPATH`.
An additional directory will be inserted in the search path in front of
:envvar:`PYTHONPATH` as described above under
:ref:`using-on-interface-options`. The search path can be manipulated from
within a Python program as the variable :data:`sys.path`.
.. envvar:: 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 namespace 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 :data:`sys.ps1` and
:data:`sys.ps2` and the hook :data:`sys.__interactivehook__` in this file.
.. envvar:: PYTHONOPTIMIZE
If this is set to a non-empty string it is equivalent to specifying the
:option:`-O` option. If set to an integer, it is equivalent to specifying
:option:`-O` multiple times.
.. envvar:: PYTHONBREAKPOINT
If this is set, it names a callable using dotted-path notation. The module
containing the callable will be imported and then the callable will be run
by the default implementation of :func:`sys.breakpointhook` which itself is
called by built-in :func:`breakpoint`. If not set, or set to the empty
string, it is equivalent to the value "pdb.set_trace". Setting this to the
string "0" causes the default implementation of :func:`sys.breakpointhook`
to do nothing but return immediately.
.. versionadded:: 3.7
.. envvar:: PYTHONDEBUG
If this is set to a non-empty string it is equivalent to specifying the
:option:`-d` option. If set to an integer, it is equivalent to specifying
:option:`-d` multiple times.
.. envvar:: PYTHONINSPECT
If this is set to a non-empty string it is equivalent to specifying the
:option:`-i` option.
This variable can also be modified by Python code using :data:`os.environ`
to force inspect mode on program termination.
.. envvar:: PYTHONUNBUFFERED
If this is set to a non-empty string it is equivalent to specifying the
:option:`-u` option.
.. envvar:: PYTHONVERBOSE
If this is set to a non-empty string it is equivalent to specifying the
:option:`-v` option. If set to an integer, it is equivalent to specifying
:option:`-v` multiple times.
.. envvar:: PYTHONCASEOK
If this is set, Python ignores case in :keyword:`import` statements. This
only works on Windows and OS X.
.. envvar:: PYTHONDONTWRITEBYTECODE
If this is set to a non-empty string, Python won't try to write ``.pyc``
files on the import of source modules. This is equivalent to
specifying the :option:`-B` option.
.. envvar:: PYTHONHASHSEED
If this variable is not set or set to ``random``, a random value is used
to seed the hashes of str, bytes and datetime objects.
If :envvar:`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.
.. versionadded:: 3.2.3
.. envvar:: PYTHONIOENCODING
If this is set before running the interpreter, it overrides the encoding used
for stdin/stdout/stderr, in the syntax ``encodingname:errorhandler``. Both
the ``encodingname`` and the ``:errorhandler`` parts are optional and have
the same meaning as in :func:`str.encode`.
For stderr, the ``:errorhandler`` part is ignored; the handler will always be
``'backslashreplace'``.
.. versionchanged:: 3.4
The ``encodingname`` part is now optional.
.. versionchanged:: 3.6
On Windows, the encoding specified by this variable is ignored for interactive
console buffers unless :envvar:`PYTHONLEGACYWINDOWSSTDIO` is also specified.
Files and pipes redirected through the standard streams are not affected.
.. envvar:: PYTHONNOUSERSITE
If this is set, Python won't add the :data:`user site-packages directory
<site.USER_SITE>` to :data:`sys.path`.
.. seealso::
:pep:`370` -- Per user site-packages directory
.. envvar:: PYTHONUSERBASE
Defines the :data:`user base directory <site.USER_BASE>`, which is used to
compute the path of the :data:`user site-packages directory <site.USER_SITE>`
and :ref:`Distutils installation paths <inst-alt-install-user>` for
``python setup.py install --user``.
.. seealso::
:pep:`370` -- Per user site-packages directory
.. envvar:: 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.
.. envvar:: PYTHONWARNINGS
This is equivalent to the :option:`-W` option. If set to a comma
separated string, it is equivalent to specifying :option:`-W` multiple
times, with filters later in the list taking precedence over those earlier
in the list.
The simplest settings apply a particular action unconditionally to all
warnings emitted by a process (even those that are otherwise ignored by
default)::
PYTHONWARNINGS=default # Warn once per call location
PYTHONWARNINGS=error # Convert to exceptions
PYTHONWARNINGS=always # Warn every time
PYTHONWARNINGS=module # Warn once per calling module
PYTHONWARNINGS=once # Warn once per Python process
PYTHONWARNINGS=ignore # Never warn
See :ref:`warning-filter` and :ref:`describing-warning-filters` for more
details.
.. envvar:: PYTHONFAULTHANDLER
If this environment variable is set to a non-empty string,
:func:`faulthandler.enable` is called at startup: install a handler for
:const:`SIGSEGV`, :const:`SIGFPE`, :const:`SIGABRT`, :const:`SIGBUS` and
:const:`SIGILL` signals to dump the Python traceback. This is equivalent to
:option:`-X` ``faulthandler`` option.
.. versionadded:: 3.3
.. envvar:: PYTHONTRACEMALLOC
If this environment variable is set to a non-empty string, start tracing
Python memory allocations using the :mod:`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. See the :func:`tracemalloc.start` for more information.
.. versionadded:: 3.4
.. envvar:: 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.
.. versionadded:: 3.7
.. envvar:: PYTHONASYNCIODEBUG
If this environment variable is set to a non-empty string, enable the
:ref:`debug mode <asyncio-debug-mode>` of the :mod:`asyncio` module.
.. versionadded:: 3.4
.. envvar:: PYTHONMALLOC
Set the Python memory allocators and/or install debug hooks.
Set the family of memory allocators used by Python:
* ``default``: use the :ref:`default memory allocators
<default-memory-allocators>`.
* ``malloc``: use the :c:func:`malloc` function of the C library
for all domains (:c:data:`PYMEM_DOMAIN_RAW`, :c:data:`PYMEM_DOMAIN_MEM`,
:c:data:`PYMEM_DOMAIN_OBJ`).
* ``pymalloc``: use the :ref:`pymalloc allocator <pymalloc>` for
:c:data:`PYMEM_DOMAIN_MEM` and :c:data:`PYMEM_DOMAIN_OBJ` domains and use
the :c:func:`malloc` function for the :c:data:`PYMEM_DOMAIN_RAW` domain.
Install debug hooks:
* ``debug``: install debug hooks on top of the :ref:`default memory
allocators <default-memory-allocators>`.
* ``malloc_debug``: same as ``malloc`` but also install debug hooks
* ``pymalloc_debug``: same as ``pymalloc`` but also install debug hooks
See the :ref:`default memory allocators <default-memory-allocators>` and the
:c:func:`PyMem_SetupDebugHooks` function (install debug hooks on Python
memory allocators).
.. versionchanged:: 3.7
Added the ``"default"`` allocator.
.. versionadded:: 3.6
.. envvar:: PYTHONMALLOCSTATS
If set to a non-empty string, Python will print statistics of the
:ref:`pymalloc memory allocator <pymalloc>` every time a new pymalloc object
arena is created, and on shutdown.
This variable is ignored if the :envvar:`PYTHONMALLOC` environment variable
is used to force the :c:func:`malloc` allocator of the C library, or if
Python is configured without ``pymalloc`` support.
.. versionchanged:: 3.6
This variable can now also be used on Python compiled in release mode.
It now has no effect if set to an empty string.
.. envvar:: PYTHONLEGACYWINDOWSFSENCODING
If set to a non-empty string, the default filesystem encoding and errors mode
will revert to their pre-3.6 values of 'mbcs' and 'replace', respectively.
Otherwise, the new defaults 'utf-8' and 'surrogatepass' are used.
This may also be enabled at runtime with
:func:`sys._enablelegacywindowsfsencoding()`.
.. availability:: Windows.
.. versionadded:: 3.6
See :pep:`529` for more details.
.. envvar:: PYTHONLEGACYWINDOWSSTDIO
If set to a non-empty string, does not use the new console reader and
writer. This means that Unicode characters will be encoded according to
the active console code page, rather than using utf-8.
This variable is ignored if the standard streams are redirected (to files
or pipes) rather than referring to console buffers.
.. availability:: Windows.
.. versionadded:: 3.6
.. envvar:: 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.
If this variable is *not* set (or is set to a value other than ``0``), the
``LC_ALL`` locale override environment variable is also not set, and the
current locale reported for the ``LC_CTYPE`` category is either the default
``C`` locale, or else the explicitly ASCII-based ``POSIX`` locale, then the
Python CLI will attempt to configure the following locales for the
``LC_CTYPE`` category in the order listed before loading the interpreter
runtime:
* ``C.UTF-8``
* ``C.utf8``
* ``UTF-8``
If setting one of these locale categories succeeds, then the ``LC_CTYPE``
environment variable will also be set accordingly in the current process
environment before the Python runtime is initialized. This ensures that in
addition to being seen by both the interpreter itself and other locale-aware
components running in the same process (such as the GNU ``readline``
library), the updated setting is also seen in subprocesses (regardless of
whether or not those processes are running a Python interpreter), as well as
in operations that query the environment rather than the current C locale
(such as Python's own :func:`locale.getdefaultlocale`).
Configuring one of these locales (either explicitly or via the above
implicit locale coercion) automatically enables the ``surrogateescape``
:ref:`error handler <error-handlers>` for :data:`sys.stdin` and
:data:`sys.stdout` (:data:`sys.stderr` continues to use ``backslashreplace``
as it does in any other locale). This stream handling behavior can be
overridden using :envvar:`PYTHONIOENCODING` as usual.
For debugging purposes, setting ``PYTHONCOERCECLOCALE=warn`` will cause
Python to emit warning messages on ``stderr`` if either the locale coercion
activates, or else if a locale that *would* have triggered coercion is
still active when the Python runtime is initialized.
Also note that even when locale coercion is disabled, or when it fails to
find a suitable target locale, :envvar:`PYTHONUTF8` will still activate by
default in legacy ASCII-based locales. Both features must be disabled in
order to force the interpreter to use ``ASCII`` instead of ``UTF-8`` for
system interfaces.
.. availability:: \*nix.
.. versionadded:: 3.7
See :pep:`538` for more details.
.. envvar:: PYTHONDEVMODE
If this environment variable is set to a non-empty string, enable the
CPython "development mode". See the :option:`-X` ``dev`` option.
.. versionadded:: 3.7
.. envvar:: PYTHONUTF8
If set to ``1``, enables the interpreter's UTF-8 mode, where ``UTF-8`` is
used as the text encoding for system interfaces, regardless of the
current locale setting.
This means that:
* :func:`sys.getfilesystemencoding()` returns ``'UTF-8'`` (the locale
encoding is ignored).
* :func:`locale.getpreferredencoding()` returns ``'UTF-8'`` (the locale
encoding is ignored, and the function's ``do_setlocale`` parameter has no
effect).
* :data:`sys.stdin`, :data:`sys.stdout`, and :data:`sys.stderr` all use
UTF-8 as their text encoding, with the ``surrogateescape``
:ref:`error handler <error-handlers>` being enabled for :data:`sys.stdin`
and :data:`sys.stdout` (:data:`sys.stderr` continues to use
``backslashreplace`` as it does in the default locale-aware mode)
As a consequence of the changes in those lower level APIs, other higher
level APIs also exhibit different default behaviours:
* Command line arguments, environment variables and filenames are decoded
to text using the UTF-8 encoding.
* :func:`os.fsdecode()` and :func:`os.fsencode()` use the UTF-8 encoding.
* :func:`open()`, :func:`io.open()`, and :func:`codecs.open()` use the UTF-8
encoding by default. However, they still use the strict error handler by
default so that attempting to open a binary file in text mode is likely
to raise an exception rather than producing nonsense data.
Note that the standard stream settings in UTF-8 mode can be overridden by
:envvar:`PYTHONIOENCODING` (just as they can be in the default locale-aware
mode).
If set to ``0``, the interpreter runs in its default locale-aware mode.
Setting any other non-empty string causes an error during interpreter
initialisation.
If this environment variable is not set at all, then the interpreter defaults
to using the current locale settings, *unless* the current locale is
identified as a legacy ASCII-based locale
(as described for :envvar:`PYTHONCOERCECLOCALE`), and locale coercion is
either disabled or fails. In such legacy locales, the interpreter will
default to enabling UTF-8 mode unless explicitly instructed not to do so.
Also available as the :option:`-X` ``utf8`` option.
.. availability:: \*nix.
.. versionadded:: 3.7
See :pep:`540` for more details.
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.
.. envvar:: PYTHONTHREADDEBUG
If set, Python will print threading debug info.
.. envvar:: PYTHONDUMPREFS
If set, Python will dump objects and reference counts still alive after
shutting down the interpreter.

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.. _using-index:
##########################
Python Setup and Usage
##########################
This part of the documentation is devoted to general information on the setup
of the Python environment on different platforms, the invocation of the
interpreter and things that make working with Python easier.
.. toctree::
:numbered:
cmdline.rst
unix.rst
windows.rst
mac.rst

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.. _using-on-mac:
***************************
Using Python on a Macintosh
***************************
:Author: Bob Savage <bobsavage@mac.com>
Python on a Macintosh running Mac OS X is in principle very similar to Python on
any other Unix platform, but there are a number of additional features such as
the IDE and the Package Manager that are worth pointing out.
.. _getting-osx:
Getting and Installing MacPython
================================
Mac OS X 10.8 comes with Python 2.7 pre-installed by Apple. If you wish, you
are invited to install the most recent version of Python 3 from the Python
website (https://www.python.org). A current "universal binary" build of Python,
which runs natively on the Mac's new Intel and legacy PPC CPU's, is available
there.
What you get after installing is a number of things:
* A :file:`Python 3.7` folder in your :file:`Applications` folder. In here
you find IDLE, the development environment that is a standard part of official
Python distributions; PythonLauncher, which handles double-clicking Python
scripts from the Finder; and the "Build Applet" tool, which allows you to
package Python scripts as standalone applications on your system.
* A framework :file:`/Library/Frameworks/Python.framework`, which includes the
Python executable and libraries. The installer adds this location to your shell
path. To uninstall MacPython, you can simply remove these three things. A
symlink to the Python executable is placed in /usr/local/bin/.
The Apple-provided build of Python is installed in
:file:`/System/Library/Frameworks/Python.framework` and :file:`/usr/bin/python`,
respectively. You should never modify or delete these, as they are
Apple-controlled and are used by Apple- or third-party software. Remember that
if you choose to install a newer Python version from python.org, you will have
two different but functional Python installations on your computer, so it will
be important that your paths and usages are consistent with what you want to do.
IDLE includes a help menu that allows you to access Python documentation. If you
are completely new to Python you should start reading the tutorial introduction
in that document.
If you are familiar with Python on other Unix platforms you should read the
section on running Python scripts from the Unix shell.
How to run a Python script
--------------------------
Your best way to get started with Python on Mac OS X is through the IDLE
integrated development environment, see section :ref:`ide` and use the Help menu
when the IDE is running.
If you want to run Python scripts from the Terminal window command line or from
the Finder you first need an editor to create your script. Mac OS X comes with a
number of standard Unix command line editors, :program:`vim` and
:program:`emacs` among them. If you want a more Mac-like editor,
:program:`BBEdit` or :program:`TextWrangler` from Bare Bones Software (see
http://www.barebones.com/products/bbedit/index.html) are good choices, as is
:program:`TextMate` (see https://macromates.com/). Other editors include
:program:`Gvim` (http://macvim-dev.github.io/macvim/) and :program:`Aquamacs`
(http://aquamacs.org/).
To run your script from the Terminal window you must make sure that
:file:`/usr/local/bin` is in your shell search path.
To run your script from the Finder you have two options:
* Drag it to :program:`PythonLauncher`
* Select :program:`PythonLauncher` as the default application to open your
script (or any .py script) through the finder Info window and double-click it.
:program:`PythonLauncher` has various preferences to control how your script is
launched. Option-dragging allows you to change these for one invocation, or use
its Preferences menu to change things globally.
.. _osx-gui-scripts:
Running scripts with a GUI
--------------------------
With older versions of Python, there is one Mac OS X quirk that you need to be
aware of: programs that talk to the Aqua window manager (in other words,
anything that has a GUI) need to be run in a special way. Use :program:`pythonw`
instead of :program:`python` to start such scripts.
With Python 3.7, you can use either :program:`python` or :program:`pythonw`.
Configuration
-------------
Python on OS X honors all standard Unix environment variables such as
:envvar:`PYTHONPATH`, but setting these variables for programs started from the
Finder is non-standard as the Finder does not read your :file:`.profile` or
:file:`.cshrc` at startup. You need to create a file
:file:`~/.MacOSX/environment.plist`. See Apple's Technical Document QA1067 for
details.
For more information on installation Python packages in MacPython, see section
:ref:`mac-package-manager`.
.. _ide:
The IDE
=======
MacPython ships with the standard IDLE development environment. A good
introduction to using IDLE can be found at
http://www.hashcollision.org/hkn/python/idle_intro/index.html.
.. _mac-package-manager:
Installing Additional Python Packages
=====================================
There are several methods to install additional Python packages:
* Packages can be installed via the standard Python distutils mode (``python
setup.py install``).
* Many packages can also be installed via the :program:`setuptools` extension
or :program:`pip` wrapper, see https://pip.pypa.io/.
GUI Programming on the Mac
==========================
There are several options for building GUI applications on the Mac with Python.
*PyObjC* is a Python binding to Apple's Objective-C/Cocoa framework, which is
the foundation of most modern Mac development. Information on PyObjC is
available from https://pypi.org/project/pyobjc/.
The standard Python GUI toolkit is :mod:`tkinter`, based on the cross-platform
Tk toolkit (https://www.tcl.tk). An Aqua-native version of Tk is bundled with OS
X by Apple, and the latest version can be downloaded and installed from
https://www.activestate.com; it can also be built from source.
*wxPython* is another popular cross-platform GUI toolkit that runs natively on
Mac OS X. Packages and documentation are available from https://www.wxpython.org.
*PyQt* is another popular cross-platform GUI toolkit that runs natively on Mac
OS X. More information can be found at
https://riverbankcomputing.com/software/pyqt/intro.
Distributing Python Applications on the Mac
===========================================
The "Build Applet" tool that is placed in the MacPython 3.6 folder is fine for
packaging small Python scripts on your own machine to run as a standard Mac
application. This tool, however, is not robust enough to distribute Python
applications to other users.
The standard tool for deploying standalone Python applications on the Mac is
:program:`py2app`. More information on installing and using py2app can be found
at http://undefined.org/python/#py2app.
Other Resources
===============
The MacPython mailing list is an excellent support resource for Python users and
developers on the Mac:
https://www.python.org/community/sigs/current/pythonmac-sig/
Another useful resource is the MacPython wiki:
https://wiki.python.org/moin/MacPython

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.. highlightlang:: sh
.. _using-on-unix:
********************************
Using Python on Unix platforms
********************************
.. sectionauthor:: Shriphani Palakodety
Getting and installing the latest version of Python
===================================================
On Linux
--------
Python comes preinstalled on most Linux distributions, and is available as a
package on all others. However there are certain features you might want to use
that are not available on your distro's package. You can easily compile the
latest version of Python from source.
In the event that Python doesn't come preinstalled and isn't in the repositories as
well, you can easily make packages for your own distro. Have a look at the
following links:
.. seealso::
https://www.debian.org/doc/manuals/maint-guide/first.en.html
for Debian users
https://en.opensuse.org/Portal:Packaging
for OpenSuse users
https://docs-old.fedoraproject.org/en-US/Fedora_Draft_Documentation/0.1/html/RPM_Guide/ch-creating-rpms.html
for Fedora users
http://www.slackbook.org/html/package-management-making-packages.html
for Slackware users
On FreeBSD and OpenBSD
----------------------
* FreeBSD users, to add the package use::
pkg install python3
* OpenBSD users, to add the package use::
pkg_add -r python
pkg_add ftp://ftp.openbsd.org/pub/OpenBSD/4.2/packages/<insert your architecture here>/python-<version>.tgz
For example i386 users get the 2.5.1 version of Python using::
pkg_add ftp://ftp.openbsd.org/pub/OpenBSD/4.2/packages/i386/python-2.5.1p2.tgz
On OpenSolaris
--------------
You can get Python from `OpenCSW <https://www.opencsw.org/>`_. Various versions
of Python are available and can be installed with e.g. ``pkgutil -i python27``.
.. _building-python-on-unix:
Building Python
===============
If you want to compile CPython yourself, first thing you should do is get the
`source <https://www.python.org/downloads/source/>`_. You can download either the
latest release's source or just grab a fresh `clone
<https://devguide.python.org/setup/#getting-the-source-code>`_. (If you want
to contribute patches, you will need a clone.)
The build process consists in the usual ::
./configure
make
make install
invocations. Configuration options and caveats for specific Unix platforms are
extensively documented in the :source:`README.rst` file in the root of the Python
source tree.
.. warning::
``make install`` can overwrite or masquerade the :file:`python3` binary.
``make altinstall`` is therefore recommended instead of ``make install``
since it only installs :file:`{exec_prefix}/bin/python{version}`.
Python-related paths and files
==============================
These are subject to difference depending on local installation conventions;
:envvar:`prefix` (``${prefix}``) and :envvar:`exec_prefix` (``${exec_prefix}``)
are installation-dependent and should be interpreted as for GNU software; they
may be the same.
For example, on most Linux systems, the default for both is :file:`/usr`.
+-----------------------------------------------+------------------------------------------+
| File/directory | Meaning |
+===============================================+==========================================+
| :file:`{exec_prefix}/bin/python3` | Recommended location of the interpreter. |
+-----------------------------------------------+------------------------------------------+
| :file:`{prefix}/lib/python{version}`, | Recommended locations of the directories |
| :file:`{exec_prefix}/lib/python{version}` | containing the standard modules. |
+-----------------------------------------------+------------------------------------------+
| :file:`{prefix}/include/python{version}`, | Recommended locations of the directories |
| :file:`{exec_prefix}/include/python{version}` | containing the include files needed for |
| | developing Python extensions and |
| | embedding the interpreter. |
+-----------------------------------------------+------------------------------------------+
Miscellaneous
=============
To easily use Python scripts on Unix, you need to make them executable,
e.g. with
.. code-block:: shell-session
$ chmod +x script
and put an appropriate Shebang line at the top of the script. A good choice is
usually ::
#!/usr/bin/env python3
which searches for the Python interpreter in the whole :envvar:`PATH`. However,
some Unices may not have the :program:`env` command, so you may need to hardcode
``/usr/bin/python3`` as the interpreter path.
To use shell commands in your Python scripts, look at the :mod:`subprocess` module.
Editors and IDEs
================
There are a number of IDEs that support Python programming language.
Many editors and IDEs provide syntax highlighting, debugging tools, and :pep:`8` checks.
Please go to `Python Editors <https://wiki.python.org/moin/PythonEditors>`_ and
`Integrated Development Environments <https://wiki.python.org/moin/IntegratedDevelopmentEnvironments>`_
for a comprehensive list.

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