2. Using the Python Interpreter¶
2.1. Invoking the Interpreter¶
The Python interpreter is usually installed as /usr/local/bin/python 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
python
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., /usr/local/python is a popular
alternative location.)
On Windows machines, the Python installation is usually placed in
C:\Python27, 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:\python27
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.
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:
python
Python 2.7 (#1, Feb 28 2010, 00:02:06)
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!
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 ASCII. 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 python
# -*- coding: cp1252 -*-