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4.2 KiB
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<HTML><HEAD><TITLE>Using python to create Macintosh applications, part zero</TITLE></HEAD>
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<BODY>
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<H1>Using python to create Macintosh applications, part zero</H1>
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<HR>
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This document will show you how to create a simple mac-style
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application using Python. We will glance at how to use file dialogs and
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messages. <p>
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Our example program <a href="example0/checktext.py">checktext.py</a> asks
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the user for a text file and checks what style end-of-lines the file has.
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This may need a little explanation: ASCII text files are almost identical
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on different machines, with one exception:
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<ul>
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<li> Unix systems terminate lines with the "linefeed" character, <code>0x0a</code>,
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<li> Macintoshes terminate lines with the "carriage return" character,
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<code>0x0d</code> and
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<li> MSDOS systems terminate lines with first a carriage return and then a linefeed.
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</ul>
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Let us have a look at the program. The first interesting statement in the main
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program is the call to <code>macfs.PromptGetFile</code>. This is one of the routines
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that allow you to ask the user to specify a file. You pass it one required
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argument, the prompt string. There are up to four optional MacOS <em>file type</em> arguments
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you can pass, as 4-byte strings. Specifying no file
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type will allow the user to select any file, specifying one or more types restricts
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the user to files of this type. File types are explained in most books on the Mac. <p>
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<code>PromptGetFile</code> returns two values: an <em>FSSpec</em> object and a
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success indicator. The FSSpec object is the "official" MacOS way of specifying a
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file, more on it later. The success indicator tells you whether the user clicked OK
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or Cancel. In the event of Cancel we simply exit back to the finder. <p>
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<code>PromptGetFile</code> has a number of friends that do similar things:
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<ul>
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<li> <code>StandardGetFile</code> is identical to <code>PromptGetFile</code> but
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without the prompt. It has up to four optional filetype arguments.
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<li> <code>StandardPutFile</code> asks the user for an output file. It will
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warn the user when she tries to overwrite an existing file. The routine has one
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mandatory argument: a prompt string. Pass the empty string if you do not want a prompt.
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<li> <code>GetDirectory</code> asks the user for a folder (or directory, in unix terms).
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It has one optional argument: a prompt string.
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</ul>
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All routines return an FSSpec and a success indicator. <p>
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There are many things you can do with FSSpec objects (see the
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<a href="http://www.python.org/doc/lib/macfs.html">macfs</a> section in the
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<a href="http://www.python.org/doc/lib/Top.html">Python Library Reference</a>
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for details), but passing them to <code>open</code> is not
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one of them. For this, we first have to convert the FSSpec object to a pathname, with
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the <code>as_pathname</code> method. This returns a standard MacOS-style pathname with
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colon-separated components. This can then be passed to <code>open</code>. Note that
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we call open with mode parameter <code>'rb'</code>: we want to read the file in binary
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mode. Python, like C and C++, uses unix-style line endings internally and opening a
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file in text mode (<code>'r'</code>) would result in conversion of carriage-returns to
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linefeeds upon reading. This is something that Mac and DOS programmers are usually aware
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of but that never ceases to amaze unix buffs. <p>
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After we open the file we attempt to read all data into memory. If this fails we use
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<code>EasyDialogs.Message</code> to display a message in a standard dialog box and exit.
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The EasyDialogs module has a few more useful simple dialog routines, more on that in
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<a href="example1.html">example 1</a>. <p>
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The rest of the code is pretty straightforward: we check that the file actually contains
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data, count the number of linefeeds and returns and display a message with our guess of the
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end-of-line convention used in the file. <p>
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The <a href="example0">example0</a> folder has three text files in Mac, Unix and DOS style
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for you to try the program on. After that, you can continue with <a href="example1.html">example 1</a>
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or go back to the <a href="index.html">index</a> to find another interesting topic. <p>
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<HR>
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<A HREF="http://www.cwi.nl/~jack">Jack Jansen</A>,
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<A HREF="mailto:jack@cwi.nl">jack@cwi.nl</A>, 18-July-1996.
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</BODY></HTML>
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