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.. _tut-using:
****************************
Using the Python Interpreter
****************************
.. _tut-invoking:
Invoking the Interpreter
========================
The Python interpreter is usually installed as :file:`/usr/local/bin/python3.8`
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
python3.8
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.)
On Windows machines, the Python installation is usually placed in
:file:`C:\\Python36`, 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:\python36
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
following command: ``quit()``.
The interpreter's line-editing features include interactive editing, history
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
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
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 :option:`-i`
before the script.
All command line options are described in :ref:`using-on-general`.
.. _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
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
$ python3.8
Python 3.8 (default, Sep 16 2015, 09:25:04)
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
.. XXX update for new releases
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
>>> if the_world_is_flat:
... print("Be careful not to fall off!")
...
Be careful not to fall off!
For more on interactive mode, see :ref:`tut-interac`.
.. _tut-interp:
The Interpreter and Its Environment
===================================
.. _tut-source-encoding:
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.
To declare an encoding other than the default one, a special comment line
should be added as the *first* line of the file. The syntax is as follows::
# -*- coding: encoding -*-
where *encoding* is one of the valid :mod:`codecs` supported by Python.
For example, to declare that Windows-1252 encoding is to be used, the first
line of your source code file should be::
# -*- coding: cp-1252 -*-
One exception to the *first line* rule is when the source code starts with a
:ref:`UNIX "shebang" line <tut-scripts>`. In this case, the encoding
declaration should be added as the second line of the file. For example::
#!/usr/bin/env python3
# -*- coding: cp-1252 -*-
.. rubric:: Footnotes
.. [#] 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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