
Using the Python Interpreter
****************************


Invoking the Interpreter
========================

The Python interpreter is usually installed as
``/usr/local/bin/python3.2`` on those machines where it is available;
putting ``/usr/local/bin`` in your Unix shell's search path makes it
possible to start it by typing the command:

   python3.2

to the shell. [1] 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., ``/usr/local/python`` is a popular alternative location.)

On Windows machines, the Python installation is usually placed in
``C:\Python32``, though you can change this when you're running the
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:\python32

Typing an end-of-file character (``Control-D`` on Unix, ``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 following command: ``quit()``.

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 *Interactive Input Editing and
History Substitution* 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 *-c* option.  Since Python statements often contain spaces or
other characters that are special to the shell, it is usually advised
to quote *command* in its entirety with single quotes.

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 *-i* before the script.


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 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 *-c* *command* is
used, ``sys.argv[0]`` is set to ``'-c'``.  When *-m* *module* is used,
``sys.argv[0]``  is set to the full name of the located module.
Options found after  *-c* *command* or *-m* *module* are not consumed
by the Python interpreter's option processing but left in ``sys.argv``
for  the command or module to handle.


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:

   $ python3.2
   Python 3.2.3 (default, May  3 2012, 15:54:42)
   [GCC 4.6.3] on linux2
   Type "help", "copyright", "credits" or "license" for more information.
   >>>

Continuation lines are needed when entering a multi-line construct. As
an example, take a look at this ``if`` statement:

   >>> the_world_is_flat = 1
   >>> if the_world_is_flat:
   ...     print("Be careful not to fall off!")
   ...
   Be careful not to fall off!


The Interpreter and Its Environment
===================================


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
``except`` clause in a ``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. [2] Typing an interrupt while a command is executing
raises the ``KeyboardInterrupt`` exception, which may be handled by a
``try`` statement.


Executable Python Scripts
-------------------------

On BSD'ish Unix systems, Python scripts can be made directly
executable, like shell scripts, by putting the line

   #! /usr/bin/env python3.2

(assuming that the interpreter is on the user's ``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.

The script can be given an executable mode, or permission, using the
**chmod** command:

   $ chmod +x myscript.py

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.


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:

   # -*- coding: encoding -*-

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 ``codecs``.

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:

   # -*- coding: cp-1252 -*-

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.


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 ``PYTHONSTARTUP``
to the name of a file containing your start-up commands.  This is
similar to the ``.profile`` feature of the Unix shells.

This file is only read in interactive sessions, not when Python reads
commands from a script, and not when ``/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())


The Customization Modules
-------------------------

Python provides two hooks to let you customize it: ``sitecustomize``
and ``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 ``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 *-s* option to disable the
automatic import.

``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 ``usercustomize``.  See the
documentation of the ``site`` module for more details.

-[ Footnotes ]-

[1] 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.

[2] A problem with the GNU Readline package may prevent this.
