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<TITLE>Using Python on the Macintosh</TITLE>
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<H1>Using Python on the Macintosh</H1>
<EM>(preliminary)</EM>
<HR>
This document is an introduction to using Python on the Apple Macintosh.
It does not introduce the language itself, for this you should refer
to the <A HREF="http://www.python.org/doc/tut/tut.html">Python Tutorial</A>
by Guido van Rossum. This guide
more-or-less replaces chapter two of the tutorial, and provides some
additional material. <p>
The document refers to Python 1.3.3 or higher, some of the features (like
setting applet options) will not work in earlier versions of Python. <p>
<h2>Invoking the interpreter</h2>
The name of the interpreter may differ on different installations: it may
be called <CODE>Python</CODE>, <CODE>PythonPPC</CODE> (for powerpc macs) or
<CODE>Python68K</CODE> (indeed, for 68K macs). It will always be recognizable by
the "16 ton" icon, though. You start the interpreter in interactive mode by
double-clicking it. <p>
<img src="html.icons/python.gif"><p>
This should give you a text window with an informative version string and a prompt,
something like the following:
<PRE>
Python 1.3.3 (Apr 7 1996) [CW PPC w/GUSI]
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
&gt;&gt;&gt;
</PRE>
The version string tells you the version of Python, whether it was built for
PPC or 68K macs and possibly some options used to build the interpreter. If
you find a bug or have a question about how the interpreter works it is a good
idea to include the version information in your message. <p>
At the prompt you can type interactive python commands. See the tutorial for
more information. The interactive window works more-or-less like a Communication
Toolbox or Telnet window: you type commands at the bottom and terminate them with
the <EM>[return]</EM> or <EM>[enter]</EM> key. Interpreter feedback also appears
at the bottom of the window, and the contents scroll as output is added. You can
use copy and paste in the normal way, but be sure to paste only at the bottom
of the document.
<h2>Creating Python scripts</h2>
The Python interpreter works in a way that is different from what you would
expect of a macintosh program: the interpreter is just that: an interpreter.
There is no builtin editor or other development support. Hence, to create
a Python script you need an external text editor. For a first script you
can use any editor that can create plain, unstyled text files, such as
<CODE>SimpleText</CODE>. <p>
For more serious scripts, though, it is advisable to use a programmers editor,
such as <CODE>BBEdit</CODE> or <CODE>Alpha</CODE>. BBEdit is my favorite: it comes in a
commercial version but also in a fully-functional free version
<CODE>BBEdit Lite</CODE>. You can download it from the
<A HREF="http://www.barebones.com/">BareBones</A> site.
The free version will probably provide all the functionality you will ever need.
Besides the standard edit facilities it has multi-file searches and many other
goodies that can be very handy when editing programs. <p>
After you have created your script in the editor of your choice you drop it on
the interpreter. This will start the interpreter executing the script, again with
a console window in which the output appears and in which you can type input if
the script requires it. Normally the interpreter will close the window and quit
as soon as the script is done executing, see below under
<A HREF="#startup">startup options</A>
for a way to change this. <p>
It is a good idea to have the names of all your scripts end in <CODE>.py</CODE>. While
this is not necessary for standalone scripts it is needed for modules, and it is
probably a good idea to start the habit now. <p>
<h2>Clickable python scripts</h2>
If you create your script with the correct creator and type, creator <CODE>'Pyth'</CODE>
and type <CODE>'TEXT'</CODE>, you can double-click your script and it will automatically
invoke the interpreter. If you use BBEdit you can tell it about the Python file
type by adding it to the "file types" sections of the preferences. Then, if you save
a file for the first time you can tell BBEdit to save the file as a Python script
through the "options" choice of the save dialog. <p>
The <CODE>Scripts</CODE> folder contains a script <CODE>fixfiletypes</CODE> that will
recursively traverse a folder and set the correct creator and type for all files
ending in <CODE>.py</CODE>. <p>
<h2>Interaction with the user</h2>
Normally, the interpreter will check for user input (mouse clicks, keyboard
input) every once in a while, so it is possible to switch to other applications
while a script runs. It is also possible to interrupt the interpreter with
the standard command-period keypress, this will raise the <CODE>KeyboardInterrupt</CODE>
exception. Scripts may, however, turn off this behaviour to facilitate their
own event handling. Such scripts can only be killed with the command-option-escape
shortcut.
<h2><A NAME="startup">startup options</A></h2>
If the <EM>option</EM> key is depressed when Python starts executing the
interpreter will bring up an options dialog thru which you can influence the way
the interpreter behaves. Keep the option key depressed until the dialog comes up. <p>
<img src="html.icons/options.gif"><p>
The options modify the interpreters behaviour in the following way:
<ul>
<li> the interpreter goes to interactive mode (in stead of
exiting) after a script has terminated normally,
<li> for every module imported a line is printed telling you where the
module was loaded from,
<li> do not print the values of expressions executed as statements in an
interactive python,
<li> do not buffer stdout and stderr,
<li> print some debugging output during the parsing phase,
<li> keep the output window open when a script terminates.
</ul>
In addition, you can enter a unix-style command line which is passed to the script
in <CODE>sys.argv</CODE>. Sys.argv[0] is always the name of the script being executed,
additional values can be passed here. Quoting works as expected. <p>
The default options are also settable on a system-wide basis, see the section on
<A HREF="#preferences">editing preferences</A>. <p>
<h2>Module search path</h2>
The module search path, <CODE>sys.path</CODE>, contains the folders python will search
when you import a module. The path is settable on a system-wide basis (see the
preferences section), and normally comprises the current folder (where the script
lives), the <CODE>Lib</CODE> folder and some of its subfolders and possibly some more. <p>
<h2>Working folder</h2>
The unix concept of a <I>working directory</I> does not translate directly to
a similar concept on the Macintosh. To facilitate easy porting and the use of
relative pathnames in scripts the interpreter simulates a working directory. When
a script is started the initial working directory is the folder where the script
lives. In case of an interactive interpreter the working directory is the folder
where the interpreter lives. The "standard file" folder does <EM>not</EM> follow
the working directory, it follows the standard MacOS rules (which are settable
through a control panel since MacOS 7.5).
<h2>Interactive startup file</h2>
If the folder containing the interpreter contains a file named <CODE>PythonStartup</CODE>
this file is executed when you start an interactive interpreter. In this file you
could import modules you often use and other such things. <p>
<h2>Compiled python scripts</h2>
Once a python module has been imported the interpreter creates a compiled version
which is stored in a file with the ".py" extension replaced by ".pyc". These
compiled files, with creator <CODE>'Pyth'</CODE> and type <CODE>'PYC '</CODE> load faster
when imported (because they do not have to be parsed). The <CODE>Lib</CODE> folder
contains a script <CODE>compileall.py</CODE>, running this script will cause all modules
along the python search path to be precompiled, which will speed up your programs.
Compiled files are also double-clickable. <p>
<h2>Python resources</h2>
MacPython has the ability to collect a number of compiled modules together
in the resource fork of a single file. This feature is useful if you
distribute a python program and want to minimize clutter: you can put all the
needed modules in a single file (which could even be the interpreter itself). <p>
If the module search path contains a filename as one of its entries (as opposed to
a folder name, which is the normal case) this file will be searched for a resource
with type <CODE>'PYC '</CODE> and a name matching the module being imported. <p>
The <CODE>scripts</CODE> folder contains a script <CODE>PackLibDir</CODE> which will convert
a number of modules (or possibly a complete subtree full of modules) into such a
resource file.
<h2><A NAME="preferences">Setting interpreter preferences</A></h2>
The python interpreter keeps a preferences file in the standard location in the
system folder. In this preferences file it remembers the default module search
path and the default settings for the runtime options. The preferences are settable
via <CODE>EditPythonPrefs</CODE>. For PPC python this is a standalone program living
in the main Python folder, for 68K python it is a script in the <CODE>Scripts</CODE>
folder. <p>
The interface to edit the preferences is rather clunky for the current release. <p>
<img src="html.icons/preferences.gif"><p>
In the editable text field at the top you enter the initial module search path,
using newline as a separator. There are two special values you can use here:
an initial substring <CODE>$(PYTHON)</CODE> will expand to the Python home folder
and a value of <CODE>$(APPLICATION)</CODE> will expand to the the python application
itself. Note that the text field may extend "beyond the bottom" even though it
does not have a scroll bar. Using the arrow keys works, though.<p>
The Python home folder $(PYTHON) is initially, when you execute the interpreter
for the first time, set to the folder where the interpreter lives. You can change it
here. <p>
Finally, you can set the default startup options here, through a sub-dialog.
<h2>Applets</h2>
An applet is a fullblown application written in Python, similar to an AppleScript
applet (and completely different from a Java applet). Applets are currently only
supported on PowerPC macintoshes, and are created using the <CODE>mkapplet</CODE>
program. You create an applet by dropping the python source script onto mkapplet.
The <CODE>Demo</CODE> folder contains an example of a more involved applet with its
own resource file, etc. <p>
Note that while an applet behaves as a fullblown Macintosh application it is
not self-sufficient, so distributing it to a machine without an installed Python
interpreter will not work: it needs the shared python execution engine
<CODE>PythonCore</CODE>, and probably various modules from the Lib and PlugIns folders. <p>
<h2>Customizing applets</h2>
Applets can have their own settings for the startup options and module search
path. Dropping an applet on the <CODE>EditPythonPrefs</CODE>
application allows you to set
these, in the same way as double-clicking EditPythonPrefs allows you to set
the system-wide defaults. <p>
Actually, not only applets but also the interpreter itself can have non-default
settings for path and options. If you make a copy of the interpreter and drop
this copy onto EditPythonPrefs you will have an interpreter that has a different
set of default settings.
<h2>Where to go from here</h2>
The previously mentioned <A HREF="http://www.python.org/doc/tut/tut.html">Python
Tutorial</A> is an excellent place to start reading if you have never used
Python before. Other documentation such as the library reference manual is
indexed at the <A HREF="http://www.python.org/doc/">Python Documentation</A>
page. <p>
There are some <A HREF="index.html">annotated sample programs</A> available
that show some mac-specific issues, like use of various toolboxes and creation
of Python applets. <p>
Finally, the <CODE>Demo</CODE> folder in the Macintosh distribution contains
a number of other example programs. Most of these are only very lightly documented,
but they may help you to understand some aspects of using Python. <p>
The best way to contact fellow Macintosh Python programmers is to join
the MacPython Special Interest Group mailing list. Send a message with "info"
in the body to <A HREF="mailto:pythonmac-sig-request@python.org">pythonmac-sig-request@python.org</A>
or view the <A HREF="http://www.python.org/sigs/pythonmac-sig/">Pythonmac SIG page</A> on the
<A HREF="http://www.python.org">www.python.org</A> WWW server. <p>
<h2>Troubleshooting</h2>
Python is a rather safe language, and hence it should be difficult to crash the
interpreter of the system with a Python script. There is an exception to this rule,
though: the modules that interface to the system toolboxes (windowing, quickdraw,
etc) do very little error checking and therefore a misbehaving program using these
modules may indeed crash the system. Such programs are unfortunately rather
difficult to debug, since the crash does not generate the standard Python stack
trace, obviously, and since debugging print statements will often interfere with
the operation of the program. There is little to do about this currently. <p>
Probably the most common cause of problems with modules ported from other
systems is the Mac end-of-line convention. Where unix uses linefeed, 0x0d, to
separate lines the mac uses carriage return, 0x0a. To complicate matters more
a lot of mac programming editors like BBEdit and emacs will work happily with
both conventions, so the file will appear to be correct in the editor but cause
strange errors when imported. BBEdit has a popup menu which allows you to inspect
(and set) the end-of-line convention used in a file. <p>
<HR>
<A HREF="http://www.cwi.nl/~jack">Jack Jansen</A>,
<A HREF="mailto:jack@cwi.nl">jack@cwi.nl</A>, 7-Apr-1996.
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