254 lines
10 KiB
ReStructuredText
254 lines
10 KiB
ReStructuredText
.. _tut-using:
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****************************
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Using the Python Interpreter
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****************************
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.. _tut-invoking:
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Invoking the Interpreter
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========================
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The Python interpreter is usually installed as :file:`/usr/local/bin/python` on
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those machines where it is available; putting :file:`/usr/local/bin` in your
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Unix shell's search path makes it possible to start it by typing the command ::
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python
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to the shell. Since the choice of the directory where the interpreter lives is
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an installation option, other places are possible; check with your local Python
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guru or system administrator. (E.g., :file:`/usr/local/python` is a popular
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alternative location.)
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On Windows machines, the Python installation is usually placed in
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:file:`C:\\Python26`, 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
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command into the command prompt in a DOS box::
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set path=%path%;C:\python26
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Typing an end-of-file character (:kbd:`Control-D` on Unix, :kbd:`Control-Z` on
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Windows) at the primary prompt causes the interpreter to exit with a zero exit
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status. If that doesn't work, you can exit the interpreter by typing the
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following commands: ``import sys; sys.exit()``.
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The interpreter's line-editing features usually aren't very sophisticated. On
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Unix, whoever installed the interpreter may have enabled support for the GNU
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readline library, which adds more elaborate interactive editing and history
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features. Perhaps the quickest check to see whether command line editing is
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supported is typing Control-P to the first Python prompt you get. If it beeps,
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you have command line editing; see Appendix :ref:`tut-interacting` for an
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introduction to the keys. If nothing appears to happen, or if ``^P`` is echoed,
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command line editing isn't available; you'll only be able to use backspace to
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remove characters from the current line.
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The interpreter operates somewhat like the Unix shell: when called with standard
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input connected to a tty device, it reads and executes commands interactively;
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when called with a file name argument or with a file as standard input, it reads
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and executes a *script* from that file.
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A second way of starting the interpreter is ``python -c command [arg] ...``,
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which executes the statement(s) in *command*, analogous to the shell's
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:option:`-c` option. Since Python statements often contain spaces or other
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characters that are special to the shell, it is usually advised to quote
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*command* in its entirety with single quotes.
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Some Python modules are also useful as scripts. These can be invoked using
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``python -m module [arg] ...``, which executes the source file for *module* as
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if you had spelled out its full name on the command line.
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Note that there is a difference between ``python file`` and ``python <file``.
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In the latter case, input requests from the program, such as calls to
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:func:`input` and :func:`raw_input`, are satisfied from *file*. Since this file
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has already been read until the end by the parser before the program starts
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executing, the program will encounter end-of-file immediately. In the former
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case (which is usually what you want) they are satisfied from whatever file or
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device is connected to standard input of the Python interpreter.
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When a script file is used, it is sometimes useful to be able to run the script
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and enter interactive mode afterwards. This can be done by passing :option:`-i`
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before the script. (This does not work if the script is read from standard
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input, for the same reason as explained in the previous paragraph.)
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.. _tut-argpassing:
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Argument Passing
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----------------
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When known to the interpreter, the script name and additional arguments
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thereafter are passed to the script in the variable ``sys.argv``, which is a
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list of strings. Its length 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
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``'-'`` (meaning standard input), ``sys.argv[0]`` is set to ``'-'``. When
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:option:`-c` *command* is used, ``sys.argv[0]`` is set to ``'-c'``. When
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:option:`-m` *module* is used, ``sys.argv[0]`` is set to the full name of the
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located module. Options found after :option:`-c` *command* or :option:`-m`
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*module* are not consumed by the Python interpreter's option processing but
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left in ``sys.argv`` for the command or module to handle.
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.. _tut-interactive:
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Interactive Mode
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----------------
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When commands are read from a tty, the interpreter is said to be in *interactive
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mode*. In this mode it prompts for the next command with the *primary prompt*,
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usually three greater-than signs (``>>>``); for continuation lines it prompts
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with the *secondary prompt*, by default three dots (``...``). The interpreter
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prints a welcome message stating its version number and a copyright notice
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before printing the first prompt::
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python
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Python 2.6 (#1, Feb 28 2007, 00:02:06)
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Type "help", "copyright", "credits" or "license" for more information.
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>>>
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Continuation lines are needed when entering a multi-line construct. As an
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example, take a look at this :keyword:`if` statement::
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>>> the_world_is_flat = 1
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>>> if the_world_is_flat:
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... print "Be careful not to fall off!"
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...
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Be careful not to fall off!
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.. _tut-interp:
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The Interpreter and Its Environment
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===================================
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.. _tut-error:
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Error Handling
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--------------
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When an error occurs, the interpreter prints an error message and a stack trace.
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In interactive mode, it then returns to the primary prompt; when input came from
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a file, it exits with a nonzero exit status after printing the stack trace.
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(Exceptions handled by an :keyword:`except` clause in a :keyword:`try` statement
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are not errors in this context.) Some errors are unconditionally fatal and
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cause an exit with a nonzero exit; this applies to internal inconsistencies and
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some cases of running out of memory. All error messages are written to the
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standard error stream; normal output from executed commands is written to
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standard output.
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Typing the interrupt character (usually Control-C or DEL) to the primary or
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secondary prompt cancels the input and returns to the primary prompt. [#]_
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Typing an interrupt while a command is executing raises the
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:exc:`KeyboardInterrupt` exception, which may be handled by a :keyword:`try`
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statement.
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.. _tut-scripts:
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Executable Python Scripts
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-------------------------
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On BSD'ish Unix systems, Python scripts can be made directly executable, like
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shell scripts, by putting the line ::
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#! /usr/bin/env python
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(assuming that the interpreter is on the user's :envvar:`PATH`) at the beginning
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of the script and giving the file an executable mode. The ``#!`` must be the
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first two characters of the file. On some platforms, this first line must end
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with a Unix-style line ending (``'\n'``), not a Windows (``'\r\n'``) line
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ending. Note that the hash, or pound, character, ``'#'``, is used to start a
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comment in Python.
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The script can be given an executable mode, or permission, using the
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:program:`chmod` command::
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$ chmod +x myscript.py
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On Windows systems, there is no notion of an "executable mode". The Python
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installer automatically associates ``.py`` files with ``python.exe`` so that
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a double-click on a Python file will run it as a script. The extension can
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also be ``.pyw``, in that case, the console window that normally appears is
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suppressed.
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Source Code Encoding
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--------------------
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It is possible to use encodings different than ASCII in Python source files. The
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best way to do it is to put one more special comment line right after the ``#!``
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line to define the source file encoding::
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# -*- coding: encoding -*-
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With that declaration, all characters in the source file will be treated as
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having the encoding *encoding*, and it will be possible to directly write
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Unicode string literals in the selected encoding. The list of possible
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encodings can be found in the Python Library Reference, in the section on
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:mod:`codecs`.
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For example, to write Unicode literals including the Euro currency symbol, the
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ISO-8859-15 encoding can be used, with the Euro symbol having the ordinal value
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164. This script will print the value 8364 (the Unicode codepoint corresponding
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to the Euro symbol) and then exit::
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# -*- coding: iso-8859-15 -*-
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currency = u"€"
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print ord(currency)
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If your editor supports saving files as ``UTF-8`` with a UTF-8 *byte order mark*
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(aka BOM), you can use that instead of an encoding declaration. IDLE supports
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this capability if ``Options/General/Default Source Encoding/UTF-8`` is set.
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Notice that this signature is not understood in older Python releases (2.2 and
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earlier), and also not understood by the operating system for script files with
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``#!`` lines (only used on Unix systems).
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By using UTF-8 (either through the signature or an encoding declaration),
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characters of most languages in the world can be used simultaneously in string
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literals and comments. Using non-ASCII characters in identifiers is not
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supported. To display all these characters properly, your editor must recognize
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that the file is UTF-8, and it must use a font that supports all the characters
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in the file.
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.. _tut-startup:
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The Interactive Startup File
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----------------------------
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When you use Python interactively, it is frequently handy to have some standard
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commands executed every time the interpreter is started. You can do this by
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setting an environment variable named :envvar:`PYTHONSTARTUP` to the name of a
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file containing your start-up commands. This is similar to the :file:`.profile`
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feature of the Unix shells.
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.. XXX This should probably be dumped in an appendix, since most people
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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
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from a script, and not when :file:`/dev/tty` is given as the explicit source of
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commands (which otherwise behaves like an interactive session). It is executed
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in the same namespace where interactive commands are executed, so that objects
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that it defines or imports can be used without qualification in the interactive
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session. You can also change the prompts ``sys.ps1`` and ``sys.ps2`` in this
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file.
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If you want to read an additional start-up file from the current directory, you
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can program this in the global start-up file using code like ``if
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os.path.isfile('.pythonrc.py'): execfile('.pythonrc.py')``. If you want to use
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the startup file in a script, you must do this explicitly in the script::
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import os
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filename = os.environ.get('PYTHONSTARTUP')
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if filename and os.path.isfile(filename):
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execfile(filename)
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.. rubric:: Footnotes
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.. [#] A problem with the GNU Readline package may prevent this.
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