2. Using the Python Interpreter

2.1. Invoking the Interpreter

The Python interpreter is usually installed as /usr/local/bin/python3.14 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:

python3.14

to the shell. [1] Since the choice of the directory where the interpreter lives is an installation optionother 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 Storethe python3.14 command will be available. If you have the py.exe launcher installedyou can use the py command. See Python install manager for other ways to launch Python.

Typing an end-of-file character (Control-D on UnixControl-Z on Windows) at the primary prompt causes the interpreter to exit with a zero exit status. If that doesn’t workyou can exit the interpreter by typing the following command: quit().

The interpreter’s line-editing features include interactive editinghistory substitution and code completion on most systems. Perhaps the quickest check to see whether command line editing is supported is typing a word in on the Python promptthen pressing Left arrow (or Control-b). If the cursor movesyou have command line editing; see Appendix Interactive Input Editing and History Substitution for an introduction to the keys. If nothing appears to happenor if a sequence like ^[[D or ^B appears, 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 deviceit reads and executes commands interactively; when called with a file name argument or with a file as standard inputit 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 commandanalogous to the shell’s -c option. Since Python statements often contain spaces or other characters that are special to the shellit is usually advised to quote command in its entirety.

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 usedit 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 interpreterthe 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 givensys.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 usedsys.argv[0] is set to '-c'. When -m module is usedsys.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 ttythe 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 promptby default three dots (...). The interpreter prints a welcome message stating its version number and a copyright notice before printing the first prompt:

$ python3.14
Python 3.14 (defaultApril 4 202409:25:04)
[GCC 10.2.0] on linux
Type "help""copyright""credits" or "license" for more information.
>>>

Continuation lines are needed when entering a multi-line construct. As an exampletake 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 modesee Interactive Mode.

2.2. The Interpreter and Its Environment

2.2.1. Source Code Encoding

By defaultPython source files are treated as encoded in UTF-8. In that encodingcharacters of most languages in the world can be used simultaneously in string literalsidentifiers and comments — although the standard library only uses ASCII characters for identifiersa convention that any portable code should follow. To display all these characters properlyyour editor must recognize that the file is UTF-8and it must use a font that supports all the characters in the file.

To declare an encoding other than the default onea 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 exampleto declare that Windows-1252 encoding is to be usedthe 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 casethe encoding declaration should be added as the second line of the file. For example:

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

Footnotes

[1]

On Unixthe Python 3.x interpreter is by default not installed with the executable named pythonso that it does not conflict with a simultaneously installed Python 2.x executable.