python_doc_md/Beginner/tutorial/appendix.md

Appendix {#tut-appendix}

Interactive Mode {#tut-interac}

There are two variants of the interactive REPL{.interpreted-text role="term"}. The classic basic interpreter is supported on all platforms with minimal line control capabilities.

Since Python 3.13, a new interactive shell is used by default. This one supports color, multiline editing, history browsing, and paste mode. To disable color, see using-on-controlling-color{.interpreted-text role="ref"} for details. Function keys provide some additional functionality. F1{.interpreted-text role="kbd"} enters the interactive help browser pydoc{.interpreted-text role="mod"}. F2{.interpreted-text role="kbd"} allows for browsing command-line history with neither output nor the >>>{.interpreted-text role="term"} and ...{.interpreted-text role="term"} prompts. F3{.interpreted-text role="kbd"} enters "paste mode", which makes pasting larger blocks of code easier. Press F3{.interpreted-text role="kbd"} to return to the regular prompt.

When using the new interactive shell, exit the shell by typing exit{.interpreted-text role="kbd"} or quit{.interpreted-text role="kbd"}. Adding call parentheses after those commands is not required.

If the new interactive shell is not desired, it can be disabled via the PYTHON_BASIC_REPL{.interpreted-text role="envvar"} environment variable.

Error Handling {#tut-error}

When an error occurs, the interpreter prints an error message and a stack trace. In interactive mode, it then returns to the primary prompt; when input came from a file, it exits with a nonzero exit status after printing the stack trace. (Exceptions handled by an except{.interpreted-text role="keyword"} clause in a try{.interpreted-text role="keyword"} statement are not errors in this context.) Some errors are unconditionally fatal and cause an exit with a nonzero exit status; this applies to internal inconsistencies and some cases of running out of memory. All error messages are written to the standard error stream; normal output from executed commands is written to standard output.

Typing the interrupt character (usually Control-C{.interpreted-text role="kbd"} or Delete{.interpreted-text role="kbd"}) to the primary or secondary prompt cancels the input and returns to the primary prompt.[^1] Typing an interrupt while a command is executing raises the KeyboardInterrupt{.interpreted-text role="exc"} exception, which may be handled by a try{.interpreted-text role="keyword"} statement.

Executable Python Scripts {#tut-scripts}

On BSD'ish Unix systems, Python scripts can be made directly executable, like shell scripts, by putting the line :

#!/usr/bin/env python3

(assuming that the interpreter is on the user's PATH{.interpreted-text role="envvar"}) at the beginning of the script and giving the file an executable mode. The #! must be the first two characters of the file. On some platforms, this first line must end with a Unix-style line ending ('\n'), not a Windows ('\r\n') line ending. Note that the hash, or pound, character, '#', is used to start a comment in Python.

The script can be given an executable mode, or permission, using the chmod{.interpreted-text role="program"} command.

$ chmod +x myscript.py

On Windows systems, there is no notion of an "executable mode". The Python installer automatically associates .py files with python.exe so that a double-click on a Python file will run it as a script. The extension can also be .pyw, in that case, the console window that normally appears is suppressed.

The Interactive Startup File {#tut-startup}

When you use Python interactively, it is frequently handy to have some standard commands executed every time the interpreter is started. You can do this by setting an environment variable named PYTHONSTARTUP{.interpreted-text role="envvar"} to the name of a file containing your start-up commands. This is similar to the .profile{.interpreted-text role="file"} feature of the Unix shells.

This file is only read in interactive sessions, not when Python reads commands from a script, and not when /dev/tty{.interpreted-text role="file"} is given as the explicit source of commands (which otherwise behaves like an interactive session). It is executed in the same namespace where interactive commands are executed, so that objects that it defines or imports can be used without qualification in the interactive session. You can also change the prompts sys.ps1 and sys.ps2 in this file.

If you want to read an additional start-up file from the current directory, you can program this in the global start-up file using code like if os.path.isfile('.pythonrc.py'): exec(open('.pythonrc.py').read()). If you want to use the startup file in a script, you must do this explicitly in the script:

import os
filename = os.environ.get('PYTHONSTARTUP')
if filename and os.path.isfile(filename):
    with open(filename) as fobj:
        startup_file = fobj.read()
    exec(startup_file)

The Customization Modules {#tut-customize}

Python provides two hooks to let you customize it: sitecustomize{.interpreted-text role="index"} and usercustomize{.interpreted-text role="index"}. To see how it works, you need first to find the location of your user site-packages directory. Start Python and run this code:

>>> import site
>>> site.getusersitepackages()
'/home/user/.local/lib/python3.x/site-packages'

Now you can create a file named usercustomize.py{.interpreted-text role="file"} in that directory and put anything you want in it. It will affect every invocation of Python, unless it is started with the -s{.interpreted-text role="option"} option to disable the automatic import.

sitecustomize{.interpreted-text role="index"} works in the same way, but is typically created by an administrator of the computer in the global site-packages directory, and is imported before usercustomize{.interpreted-text role="index"}. See the documentation of the site{.interpreted-text role="mod"} module for more details.

Footnotes

[^1]: A problem with the GNU Readline package may prevent this.