16. Appendix
************


16.1. Interactive Mode
======================


16.1.1. Error Handling
----------------------

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" clause in a "try" 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" or "Delete") 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" exception, which may be handled by a
"try" statement.


16.1.2. Executable Python 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") 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** 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.


16.1.3. The Interactive Startup File
------------------------------------

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"
to the name of a file containing your start-up commands.  This is
similar to the ".profile" 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" 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)


16.1.4. The Customization Modules
---------------------------------

Python provides two hooks to let you customize it: sitecustomize and
usercustomize.  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" in that directory
and put anything you want in it.  It will affect every invocation of
Python, unless it is started with the "-s" option to disable the
automatic import.

sitecustomize 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.  See the documentation of the
"site" module for more details.

-[ Footnotes ]-

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