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


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

The Python interpreter is usually installed as
"/usr/local/bin/python3.5" 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:

   python3.5

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:\Python35", 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:\python35

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
readline.  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:

   $ python3.5
   Python 3.5 (default, Sep 16 2015, 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.

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.

-[ 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.
