2.Using the Python Interpreter

2.1.Invoking the Interpreter

The Python interpreter is usually installed as/usr/local/bin/python onthose machines where it is available; putting/usr/local/bin in yourUnix 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 isan installation option, other places are possible; check with your local Pythonguru or system administrator. (E.g.,/usr/local/python is a popularalternative location.)

On Windows machines, the Python installation is usually placed inC:\Python27, though you can change this when you’re running theinstaller. To add this directory to your path, you can type the followingcommand into the command prompt in a DOS box:

setpath=%path%;C:\python27

Typing an end-of-file character (Control-D on Unix,Control-Z onWindows) at the primary prompt causes the interpreter to exit with a zero exitstatus. If that doesn’t work, you can exit the interpreter by typing thefollowing command:quit().

The interpreter’s line-editing features usually aren’t very sophisticated. OnUnix, whoever installed the interpreter may have enabled support for the GNUreadline library, which adds more elaborate interactive editing and historyfeatures. Perhaps the quickest check to see whether command line editing issupported is typingControl-P to the first Python prompt you get. If it beeps,you have command line editing; see AppendixInteractive Input Editing and History Substitution for anintroduction 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 toremove characters from the current line.

The interpreter operates somewhat like the Unix shell: when called with standardinput 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 readsand executes ascript from that file.

A second way of starting the interpreter ispython-ccommand[arg]...,which executes the statement(s) incommand, analogous to the shell’s-c option. Since Python statements often contain spaces or othercharacters that are special to the shell, it is usually advised to quotecommand in its entirety with single quotes.

Some Python modules are also useful as scripts. These can be invoked usingpython-mmodule[arg]..., which executes the source file formodule asif 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 scriptand enter interactive mode afterwards. This can be done by passing-ibefore the script.

All command-line options are described inCommand line and environment.

2.1.1.Argument Passing

When known to the interpreter, the script name and additional argumentsthereafter are turned into a list of strings and assigned to theargvvariable in thesys module. You can access this list by executingimportsys. The length of the list is at least one; when no script and no argumentsare 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-ccommand is used,sys.argv[0] is set to'-c'. When-mmodule is used,sys.argv[0] is set to the full name of thelocated module. Options found after-ccommand or-mmodule are not consumed by the Python interpreter’s option processing butleft insys.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 ininteractivemode. In this mode it prompts for the next command with theprimary prompt,usually three greater-than signs (>>>); for continuation lines it promptswith thesecondary prompt, by default three dots (...). The interpreterprints a welcome message stating its version number and a copyright noticebefore printing the first prompt:

pythonPython 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 anexample, take a look at thisif statement:

>>>the_world_is_flat=1>>>ifthe_world_is_flat:...print"Be careful not to fall off!"...Be careful not to fall off!

For more on interactive mode, seeInteractive 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 lineshould be added as thefirst line of the file. The syntax is as follows:

# -*- coding: encoding -*-

whereencoding is one of the validcodecs supported by Python.

For example, to declare that Windows-1252 encoding is to be used, the firstline of your source code file should be:

# -*- coding: cp1252 -*-

One exception to thefirst line rule is when the source code starts with aUNIX “shebang” line. In this case, the encodingdeclaration should be added as the second line of the file. For example:

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