2. Using the Python Interpreter¶
2.1. Invoking the Interpreter¶
The Python interpreter is usually installed as
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:
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:\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:
Typing an end-of-file character (
Control-D on Unix,
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
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
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
variable in the
sys module. You can access this list by executing
sys. The length of the list is at least one; when no script and no arguments
sys.argv is an empty string. When the script name is given as
'-' (meaning standard input),
sys.argv is set to
-c command is used,
sys.argv is set to
-m module is used,
sys.argv is set to the full name of the
located module. Options found after
-c command or
module are not consumed by the Python interpreter’s option processing but
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
>>> 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
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.
|||On Unix, the Python 3.x interpreter is by default not installed with the
executable named |