2. Using the Python Interpreter
*******************************


2.1. Invoking the Interpreter
=============================

The Python interpreter is usually installed as
"/usr/local/bin/python3.9" 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:

   python3.9

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 where you have installed Python from the Microsoft
Store, the "python3.9" command will be available. If you have the
py.exe launcher installed, you can use the "py" command. See Excursus:
Setting environment variables for other ways to launch Python.

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 include interactive editing,
history substitution and code completion on systems that support the
GNU Readline library. 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.

All command line options are described in Command line and
environment.


2.1.1. 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.


2.1.2. 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:

   $ python3.9
   Python 3.9 (default, June 4 2019, 09:25:04)
   [GCC 4.8.2] on linux
   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 = 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 Interactive Mode.


2.2. The Interpreter and Its Environment
========================================


2.2.1. 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 "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: cp1252 -*-

One exception to the *first line* rule is when the source code starts
with a UNIX “shebang” line.  In this case, the encoding declaration
should be added as the second line of the file.  For example:

   #!/usr/bin/env python3
   # -*- coding: cp1252 -*-

-[ 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.
