cpython/Doc/tutorial/interpreter.rst

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.. _tut-using:
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****************************
Using the Python Interpreter
****************************
.. _tut-invoking:
Invoking the Interpreter
========================
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The Python interpreter is usually installed as :file:`/usr/local/bin/python3.7`
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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python3.7
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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
:file:`C:\\Python36`, 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::
set path=%path%;C:\python36
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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 include interactive editing, history
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substitution and code completion on systems that support readline. Perhaps the
quickest check to see whether command line editing is supported is typing
:kbd:`Control-P` to the first Python prompt you get. If it beeps, you have command
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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.
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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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All command line options are described in :ref:`using-on-general`.
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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:
.. code-block:: shell-session
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$ python3.7
Python 3.7 (default, Sep 16 2015, 09:25:04)
[GCC 4.8.2] on linux
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 = True
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>>> 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!
For more on interactive mode, see :ref:`tut-interac`.
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.. _tut-interp:
The Interpreter and Its Environment
===================================
.. _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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.. 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.