cpython/Doc/tutorial/interpreter.rst

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.. _tut-using:
****************************
Using the Python Interpreter
****************************
.. _tut-invoking:
Invoking the Interpreter
========================
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The Python interpreter is usually installed as :file:`/usr/local/bin/python3.3`
on those machines where it is available; putting :file:`/usr/local/bin` in your
Unix shell's search path makes it possible to start it by typing the command:
.. code-block:: text
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python3.3
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to the shell. [#]_ Since the choice of the directory where the interpreter lives
is an installation option, other places are possible; check with your local
Python guru or system administrator. (E.g., :file:`/usr/local/python` is a
popular alternative location.)
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On Windows machines, the Python installation is usually placed in
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:file:`C:\\Python33`, though you can change this when you're running the
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installer. To add this directory to your path, you can type the following
command into the command prompt in a DOS box::
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set path=%path%;C:\python33
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Typing an end-of-file character (:kbd:`Control-D` on Unix, :kbd:`Control-Z` on
Windows) at the primary prompt causes the interpreter to exit with a zero exit
status. If that doesn't work, you can exit the interpreter by typing the
Merged revisions 74779-74786,74793,74795,74811,74860-74861,74863,74876,74886,74896,74901,74903,74908,74912,74930,74933,74943,74946,74952-74955,75015,75019,75032,75068,75076,75095,75098,75102,75129,75139,75230 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r74779 | michael.foord | 2009-09-13 11:13:36 -0500 (Sun, 13 Sep 2009) | 1 line Change to tutorial wording for reading text / binary files on Windows. Issue #6301. ........ r74780 | michael.foord | 2009-09-13 11:40:02 -0500 (Sun, 13 Sep 2009) | 1 line Objects that compare equal automatically pass or fail assertAlmostEqual and assertNotAlmostEqual tests on unittest.TestCase. Issue 6567. ........ r74781 | michael.foord | 2009-09-13 11:46:19 -0500 (Sun, 13 Sep 2009) | 1 line Note that sys._getframe is not guaranteed to exist in all implementations of Python, and a corresponding note in inspect.currentframe. Issue 6712. ........ r74782 | michael.foord | 2009-09-13 12:07:46 -0500 (Sun, 13 Sep 2009) | 1 line Tutorial tweaks. Issue 6849. ........ r74783 | michael.foord | 2009-09-13 12:28:35 -0500 (Sun, 13 Sep 2009) | 1 line unittest.TestLoader.loadTestsFromName honors the loader suiteClass attribute. Issue 6866. ........ r74784 | georg.brandl | 2009-09-13 13:15:07 -0500 (Sun, 13 Sep 2009) | 1 line Typo fix. ........ r74785 | michael.foord | 2009-09-13 14:07:03 -0500 (Sun, 13 Sep 2009) | 1 line Test discovery in unittest will only attempt to import modules that are importable; i.e. their names are valid Python identifiers. If an import fails during discovery this will be recorded as an error and test discovery will continue. Issue 6568. ........ r74786 | michael.foord | 2009-09-13 14:08:18 -0500 (Sun, 13 Sep 2009) | 1 line Remove an extraneous space in unittest documentation. ........ r74793 | georg.brandl | 2009-09-14 09:50:47 -0500 (Mon, 14 Sep 2009) | 1 line #6908: fix association of hashlib hash attributes. ........ r74795 | benjamin.peterson | 2009-09-14 22:36:26 -0500 (Mon, 14 Sep 2009) | 1 line Py_SetPythonHome uses static storage #6913 ........ r74811 | georg.brandl | 2009-09-15 15:26:59 -0500 (Tue, 15 Sep 2009) | 1 line Add Armin Ronacher. ........ r74860 | benjamin.peterson | 2009-09-16 21:46:54 -0500 (Wed, 16 Sep 2009) | 1 line kill bare except ........ r74861 | benjamin.peterson | 2009-09-16 22:18:28 -0500 (Wed, 16 Sep 2009) | 1 line pep 8 defaults ........ r74863 | benjamin.peterson | 2009-09-16 22:27:33 -0500 (Wed, 16 Sep 2009) | 1 line rationalize a bit ........ r74876 | georg.brandl | 2009-09-17 11:15:53 -0500 (Thu, 17 Sep 2009) | 1 line #6932: remove paragraph that advises relying on __del__ being called. ........ r74886 | benjamin.peterson | 2009-09-17 16:33:46 -0500 (Thu, 17 Sep 2009) | 1 line use macros ........ r74896 | georg.brandl | 2009-09-18 02:22:41 -0500 (Fri, 18 Sep 2009) | 1 line #6936: for interactive use, quit() is just fine. ........ r74901 | georg.brandl | 2009-09-18 04:14:52 -0500 (Fri, 18 Sep 2009) | 1 line #6905: use better exception messages in inspect when the argument is of the wrong type. ........ r74903 | georg.brandl | 2009-09-18 04:18:27 -0500 (Fri, 18 Sep 2009) | 1 line #6938: "ident" is always a string, so use a format code which works. ........ r74908 | georg.brandl | 2009-09-18 08:57:11 -0500 (Fri, 18 Sep 2009) | 1 line Use str.format() to fix beginner's mistake with %-style string formatting. ........ r74912 | georg.brandl | 2009-09-18 11:19:56 -0500 (Fri, 18 Sep 2009) | 1 line Optimize optimization and fix method name in docstring. ........ r74930 | georg.brandl | 2009-09-18 16:21:41 -0500 (Fri, 18 Sep 2009) | 1 line #6925: rewrite docs for locals() and vars() a bit. ........ r74933 | georg.brandl | 2009-09-18 16:35:59 -0500 (Fri, 18 Sep 2009) | 1 line #6930: clarify description about byteorder handling in UTF decoder routines. ........ r74943 | georg.brandl | 2009-09-19 02:35:07 -0500 (Sat, 19 Sep 2009) | 1 line #6944: the argument to PyArg_ParseTuple should be a tuple, otherwise a SystemError is set. Also clean up another usage of PyArg_ParseTuple. ........ r74946 | georg.brandl | 2009-09-19 03:43:16 -0500 (Sat, 19 Sep 2009) | 1 line Update bug tracker reference. ........ r74952 | georg.brandl | 2009-09-19 05:42:34 -0500 (Sat, 19 Sep 2009) | 1 line #6946: fix duplicate index entries for datetime classes. ........ r74953 | georg.brandl | 2009-09-19 07:04:16 -0500 (Sat, 19 Sep 2009) | 1 line Fix references to threading.enumerate(). ........ r74954 | georg.brandl | 2009-09-19 08:13:56 -0500 (Sat, 19 Sep 2009) | 1 line Add Doug. ........ r74955 | georg.brandl | 2009-09-19 08:20:49 -0500 (Sat, 19 Sep 2009) | 1 line Add Mark Summerfield. ........ r75015 | georg.brandl | 2009-09-22 05:55:08 -0500 (Tue, 22 Sep 2009) | 1 line Fix encoding name. ........ r75019 | vinay.sajip | 2009-09-22 12:23:41 -0500 (Tue, 22 Sep 2009) | 1 line Fixed a typo, and added sections on optimization and using arbitrary objects as messages. ........ r75032 | benjamin.peterson | 2009-09-22 17:15:28 -0500 (Tue, 22 Sep 2009) | 1 line fix typos/rephrase ........ r75068 | benjamin.peterson | 2009-09-25 21:57:59 -0500 (Fri, 25 Sep 2009) | 1 line comment out ugly xxx ........ r75076 | vinay.sajip | 2009-09-26 09:53:32 -0500 (Sat, 26 Sep 2009) | 1 line Tidied up name of parameter in StreamHandler ........ r75095 | michael.foord | 2009-09-27 14:15:41 -0500 (Sun, 27 Sep 2009) | 1 line Test creation moved from TestProgram.parseArgs to TestProgram.createTests exclusively. Issue 6956. ........ r75098 | michael.foord | 2009-09-27 15:08:23 -0500 (Sun, 27 Sep 2009) | 1 line Documentation improvement for load_tests protocol in unittest. Issue 6515. ........ r75102 | skip.montanaro | 2009-09-27 21:12:27 -0500 (Sun, 27 Sep 2009) | 3 lines Patch from Thomas Barr so that csv.Sniffer will set doublequote property. Closes issue 6606. ........ r75129 | vinay.sajip | 2009-09-29 02:08:54 -0500 (Tue, 29 Sep 2009) | 1 line Issue #7014: logging: Improved IronPython 2.6 compatibility. ........ r75139 | raymond.hettinger | 2009-09-29 13:53:24 -0500 (Tue, 29 Sep 2009) | 3 lines Issue 7008: Better document str.title and show how to work around the apostrophe problem. ........ r75230 | benjamin.peterson | 2009-10-04 08:38:38 -0500 (Sun, 04 Oct 2009) | 1 line test logging ........
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following command: ``quit()``.
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The interpreter's line-editing features usually aren't very sophisticated. On
Unix, whoever installed the interpreter may have enabled support for the GNU
readline library, which adds more elaborate interactive editing and history
features. Perhaps the quickest check to see whether command line editing is
supported is typing Control-P to the first Python prompt you get. If it beeps,
you have command line editing; see Appendix :ref:`tut-interacting` for an
introduction to the keys. If nothing appears to happen, or if ``^P`` is echoed,
command line editing isn't available; you'll only be able to use backspace to
remove characters from the current line.
The interpreter operates somewhat like the Unix shell: when called with standard
input connected to a tty device, it reads and executes commands interactively;
when called with a file name argument or with a file as standard input, it reads
and executes a *script* from that file.
A second way of starting the interpreter is ``python -c command [arg] ...``,
which executes the statement(s) in *command*, analogous to the shell's
:option:`-c` option. Since Python statements often contain spaces or other
Merged revisions 63724,63726,63732,63744,63754-63755,63757-63758,63760,63775,63781-63782,63787,63805-63808,63818-63819,63823-63824 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r63724 | gregory.p.smith | 2008-05-26 22:22:14 +0200 (Mon, 26 May 2008) | 6 lines Fixes issue2791: subprocess.Popen.communicate leaked a file descripton until the last reference to the Popen instance was dropped. Adding explicit close() calls fixes it. Candidate for backport to release25-maint. ........ r63726 | benjamin.peterson | 2008-05-26 22:43:24 +0200 (Mon, 26 May 2008) | 2 lines fix minor grammar typo ........ r63732 | benjamin.peterson | 2008-05-26 23:44:26 +0200 (Mon, 26 May 2008) | 2 lines remove duplication in test module ........ r63744 | lars.gustaebel | 2008-05-27 14:39:23 +0200 (Tue, 27 May 2008) | 3 lines Do not close external file objects passed to tarfile.open(mode='w:bz2') when the TarFile is closed. ........ r63754 | benjamin.peterson | 2008-05-28 03:12:35 +0200 (Wed, 28 May 2008) | 2 lines update tutorial function with more appropiate one from Eric Smith ........ r63755 | mark.hammond | 2008-05-28 03:54:55 +0200 (Wed, 28 May 2008) | 2 lines bdist_wininst now works correctly when both --skip-build and --plat-name are specified. ........ r63757 | georg.brandl | 2008-05-28 13:21:39 +0200 (Wed, 28 May 2008) | 2 lines #2989: add PyType_Modified(). ........ r63758 | benjamin.peterson | 2008-05-28 13:51:41 +0200 (Wed, 28 May 2008) | 2 lines fix spelling ........ r63760 | georg.brandl | 2008-05-28 17:41:36 +0200 (Wed, 28 May 2008) | 2 lines #2990: prevent inconsistent state while updating method cache. ........ r63775 | georg.brandl | 2008-05-29 09:18:17 +0200 (Thu, 29 May 2008) | 2 lines Two fixes in bytearray docs. ........ r63781 | georg.brandl | 2008-05-29 09:38:37 +0200 (Thu, 29 May 2008) | 2 lines #2988: add note about catching CookieError when parsing untrusted cookie data. ........ r63782 | georg.brandl | 2008-05-29 09:45:26 +0200 (Thu, 29 May 2008) | 2 lines #2985: allow i8 in XMLRPC responses. ........ r63787 | georg.brandl | 2008-05-29 16:35:39 +0200 (Thu, 29 May 2008) | 2 lines Revert #2990 patch; it's not necessary as Armin showed. ........ r63805 | raymond.hettinger | 2008-05-30 08:37:27 +0200 (Fri, 30 May 2008) | 1 line Issue 2784: fix leaks in exception exit. ........ r63806 | raymond.hettinger | 2008-05-30 08:49:47 +0200 (Fri, 30 May 2008) | 1 line Issue 2855: Fix obscure crasher by slowing down the entire module. Mimics what was done to dictionaries in r59223. ........ r63807 | raymond.hettinger | 2008-05-30 09:16:53 +0200 (Fri, 30 May 2008) | 1 line Issue 2903: Add __name__ in globals for namedtuple namespace. ........ r63808 | georg.brandl | 2008-05-30 09:54:16 +0200 (Fri, 30 May 2008) | 2 lines #2999: fix name of third parameter in unicode.replace()'s docstring. ........ r63818 | georg.brandl | 2008-05-30 21:12:13 +0200 (Fri, 30 May 2008) | 2 lines getloadavg() is not available on Windows. ........ r63819 | georg.brandl | 2008-05-30 21:17:29 +0200 (Fri, 30 May 2008) | 2 lines Better quote with single quotes. ........ r63823 | benjamin.peterson | 2008-05-30 22:44:39 +0200 (Fri, 30 May 2008) | 2 lines fix grammar ........ r63824 | marc-andre.lemburg | 2008-05-30 22:52:18 +0200 (Fri, 30 May 2008) | 5 lines Update the locale module alias table. Closes #3011. ........
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characters that are special to the shell, it is usually advised to quote
*command* in its entirety with single quotes.
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Some Python modules are also useful as scripts. These can be invoked using
``python -m module [arg] ...``, which executes the source file for *module* as
if you had spelled out its full name on the command line.
When a script file is used, it is sometimes useful to be able to run the script
and enter interactive mode afterwards. This can be done by passing :option:`-i`
before the script.
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.. _tut-argpassing:
Argument Passing
----------------
When known to the interpreter, the script name and additional arguments
thereafter are turned into a list of strings and assigned to the ``argv``
variable in the ``sys`` module. You can access this list by executing ``import
sys``. The length of the list is at least one; when no script and no arguments
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are given, ``sys.argv[0]`` is an empty string. When the script name is given as
``'-'`` (meaning standard input), ``sys.argv[0]`` is set to ``'-'``. When
:option:`-c` *command* is used, ``sys.argv[0]`` is set to ``'-c'``. When
:option:`-m` *module* is used, ``sys.argv[0]`` is set to the full name of the
located module. Options found after :option:`-c` *command* or :option:`-m`
*module* are not consumed by the Python interpreter's option processing but
left in ``sys.argv`` for the command or module to handle.
.. _tut-interactive:
Interactive Mode
----------------
When commands are read from a tty, the interpreter is said to be in *interactive
mode*. In this mode it prompts for the next command with the *primary prompt*,
usually three greater-than signs (``>>>``); for continuation lines it prompts
with the *secondary prompt*, by default three dots (``...``). The interpreter
prints a welcome message stating its version number and a copyright notice
before printing the first prompt::
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$ python3.3
Python 3.3 (default, Sep 24 2012, 09:25:04)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
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>>>
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.. XXX update for new releases
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Continuation lines are needed when entering a multi-line construct. As an
example, take a look at this :keyword:`if` statement::
>>> the_world_is_flat = 1
>>> if the_world_is_flat:
... print("Be careful not to fall off!")
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...
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Be careful not to fall off!
.. _tut-interp:
The Interpreter and Its Environment
===================================
.. _tut-error:
Error Handling
--------------
When an error occurs, the interpreter prints an error message and a stack trace.
In interactive mode, it then returns to the primary prompt; when input came from
a file, it exits with a nonzero exit status after printing the stack trace.
(Exceptions handled by an :keyword:`except` clause in a :keyword:`try` statement
are not errors in this context.) Some errors are unconditionally fatal and
cause an exit with a nonzero exit; this applies to internal inconsistencies and
some cases of running out of memory. All error messages are written to the
standard error stream; normal output from executed commands is written to
standard output.
Typing the interrupt character (usually Control-C or DEL) to the primary or
secondary prompt cancels the input and returns to the primary prompt. [#]_
Typing an interrupt while a command is executing raises the
:exc:`KeyboardInterrupt` exception, which may be handled by a :keyword:`try`
statement.
.. _tut-scripts:
Executable Python Scripts
-------------------------
On BSD'ish Unix systems, Python scripts can be made directly executable, like
shell scripts, by putting the line ::
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#! /usr/bin/env python3.3
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(assuming that the interpreter is on the user's :envvar:`PATH`) at the beginning
of the script and giving the file an executable mode. The ``#!`` must be the
first two characters of the file. On some platforms, this first line must end
with a Unix-style line ending (``'\n'``), not a Windows (``'\r\n'``) line
ending. Note that the hash, or pound, character, ``'#'``, is used to start a
comment in Python.
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The script can be given an executable mode, or permission, using the
:program:`chmod` command::
$ chmod +x myscript.py
Merged revisions 60143-60149 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r60143 | georg.brandl | 2008-01-20 15:50:05 +0100 (Sun, 20 Jan 2008) | 3 lines Switch mmap from old Py_FindMethod to new PyObject_GenericGetAttr attribute access. Fixes #1087735. ........ r60145 | georg.brandl | 2008-01-20 20:40:58 +0100 (Sun, 20 Jan 2008) | 2 lines Add blurb about executable scripts on Windows. #760657. ........ r60146 | georg.brandl | 2008-01-20 20:48:40 +0100 (Sun, 20 Jan 2008) | 2 lines #1219903: fix tp_richcompare docs. ........ r60147 | georg.brandl | 2008-01-20 22:10:08 +0100 (Sun, 20 Jan 2008) | 2 lines Fix markup. ........ r60148 | gregory.p.smith | 2008-01-21 08:11:11 +0100 (Mon, 21 Jan 2008) | 14 lines Provide a sanity check during PyThreadState_DeleteCurrent() and PyThreadState_Delete() to avoid an infinite loop when the tstate list is messed up and has somehow becomes circular and does not contain the current thread. I don't know how this happens but it does, *very* rarely. On more than one hardware platform. I have not been able to reproduce it manually. Attaching to a process where its happening: it has always been in an infinite loop over a single element tstate list that is not the tstate we're looking to delete. It has been in t_bootstrap()'s call to PyThreadState_DeleteCurrent() as a pthread is exiting. ........ r60149 | georg.brandl | 2008-01-21 11:24:59 +0100 (Mon, 21 Jan 2008) | 2 lines #1269: fix a bug in pstats.add_callers() and add a unit test file for pstats. ........
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On Windows systems, there is no notion of an "executable mode". The Python
installer automatically associates ``.py`` files with ``python.exe`` so that
a double-click on a Python file will run it as a script. The extension can
also be ``.pyw``, in that case, the console window that normally appears is
suppressed.
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.. _tut-source-encoding:
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Source Code Encoding
--------------------
By default, Python source files are treated as encoded in UTF-8. In that
encoding, characters of most languages in the world can be used simultaneously
in string literals, identifiers and comments --- although the standard library
only uses ASCII characters for identifiers, a convention that any portable code
should follow. To display all these characters properly, your editor must
recognize that the file is UTF-8, and it must use a font that supports all the
characters in the file.
It is also possible to specify a different encoding for source files. In order
to do this, put one more special comment line right after the ``#!`` line to
define the source file encoding::
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# -*- coding: encoding -*-
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With that declaration, everything in the source file will be treated as having
the encoding *encoding* instead of UTF-8. The list of possible encodings can be
found in the Python Library Reference, in the section on :mod:`codecs`.
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For example, if your editor of choice does not support UTF-8 encoded files and
insists on using some other encoding, say Windows-1252, you can write::
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# -*- coding: cp-1252 -*-
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and still use all characters in the Windows-1252 character set in the source
files. The special encoding comment must be in the *first or second* line
within the file.
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.. _tut-startup:
The Interactive Startup File
----------------------------
When you use Python interactively, it is frequently handy to have some standard
commands executed every time the interpreter is started. You can do this by
setting an environment variable named :envvar:`PYTHONSTARTUP` to the name of a
file containing your start-up commands. This is similar to the :file:`.profile`
feature of the Unix shells.
.. XXX This should probably be dumped in an appendix, since most people
don't use Python interactively in non-trivial ways.
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This file is only read in interactive sessions, not when Python reads commands
from a script, and not when :file:`/dev/tty` is given as the explicit source of
commands (which otherwise behaves like an interactive session). It is executed
in the same namespace where interactive commands are executed, so that objects
that it defines or imports can be used without qualification in the interactive
session. You can also change the prompts ``sys.ps1`` and ``sys.ps2`` in this
file.
If you want to read an additional start-up file from the current directory, you
can program this in the global start-up file using code like ``if
os.path.isfile('.pythonrc.py'): exec(open('.pythonrc.py').read())``.
If you want to use the startup file in a script, you must do this explicitly
in the script::
import os
filename = os.environ.get('PYTHONSTARTUP')
if filename and os.path.isfile(filename):
exec(open(filename).read())
.. _tut-customize:
The Customization Modules
-------------------------
Python provides two hooks to let you customize it: :mod:`sitecustomize` and
:mod:`usercustomize`. To see how it works, you need first to find the location
of your user site-packages directory. Start Python and run this code:
>>> import site
>>> site.getusersitepackages()
'/home/user/.local/lib/python3.2/site-packages'
Now you can create a file named :file:`usercustomize.py` in that directory and
put anything you want in it. It will affect every invocation of Python, unless
it is started with the :option:`-s` option to disable the automatic import.
:mod:`sitecustomize` works in the same way, but is typically created by an
administrator of the computer in the global site-packages directory, and is
imported before :mod:`usercustomize`. See the documentation of the :mod:`site`
module for more details.
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.. rubric:: Footnotes
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.. [#] On Unix, the Python 3.x interpreter is by default not installed with the
executable named ``python``, so that it does not conflict with a
simultaneously installed Python 2.x executable.
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.. [#] A problem with the GNU Readline package may prevent this.