Serge NOEL
111 lines
5.3 KiB
Plaintext
111 lines
5.3 KiB
Plaintext
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<title>JMRI: The Python/Jython language</title>
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<h1>JMRI: The Python/Jython language</h1>
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<a href="https://www.python.org/" target="_blank">Python</a> is a widely used scripting
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language that's available on many types of computers. A Java-based variant, called <a href=
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"https://www.jython.org/" target="_blank">Jython</a>, has been integrated with JMRI to make it
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easy to control a model railroad from the command line of a computer. For our purposes, the
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two languages are completely the same.
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<p>If like me you prefer to read off paper, there are lots of books available on Python.
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Perhaps one of the best for beginners is <a href="https://www.oreilly.com/library/view/learning-python-2nd/0596002815/"
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target="_blank">"Learning Python"</a> published by O'Reilly. It contains more than you really
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need to know, though.</p>
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<p>The <a href="https://www.jython.org" target="_blank">jython.org site</a> has some
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introductory information, also see the support on their Wiki, <a href="https://wiki.python.org/jython/"
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target="_blank">https://wiki.python.org/jython/</a>.</p>
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<p>Non-programmers might want to start with this <a href=
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"https://wiki.python.org/moin/BeginnersGuide" target="_blank">Beginners Guide</a> listed on
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the Jython wiki.</p>
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<p>Looking at the examples in the "jython" directory in the JMRI distribution will also be of
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value. See the <a href="Examples.shtml">examples page</a> for links to these sample
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scripts.</p>
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<h2>How do Jython and Python differ?</h2>
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<div class="para">
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<p>For the purposes of writing JMRI scripts, they don't differ very much. Most of the
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differences involve what happens in somewhat contrived error cases. There are also some
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restrictions on what you can do with the computer's configuration information, etc, in
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Jython, but these are not things a JMRI script is likely to need.</p>
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</div>
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<h2>IMPORTANT INFORMATION ABOUT PROGRAM FORMATTING: INDENTATION MATTERS</h2>
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<div class="para">
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The single oddest thing about Python is that the indentation matters. Instead of using {
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and } characters to indicate the beginning and end of a block or function, that's done with
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indentation in Python. Of course, in a C-like language people usually indent blocks anyway,
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but it takes a little getting used to that you <em>have</em> to do it in Python. You should
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indent with four spaces for each level in your code. (Four is a convention, but it really
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helps readability; using tabs can cause lots of confusion and is not recommended. Mixing
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spaces and tabs is likely to cause lots of confusion; please don't do that)
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<p>For example, this is a syntax error:</p>
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<pre>
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a = 15
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print a
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b = 21
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</pre>because those statements, though logically grouped at the same level in the program, aren't
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indented the same. This sounds like a pain at first, but you rapidly get used to it. Then it makes
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things like the following pretty easy to read, without having to worry about where the { and } go:
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<pre>
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if ( now == -1 ) :
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done = 1
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else :
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done = 0
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print done
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</pre>If you do get a message about "Syntax error", look at the indicated line number to see if
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your indentation isn't lined up.
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</div>
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<a id="newJython"></a>
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<h2>Is JMRI's support for Python complete?</h2>
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JMRI scripting uses <a href="https://www.jython.org" target="_blank">Jython</a>, a
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Java-implemented form of the Python language. The basic language is pretty complete, but not
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all of the Python libraries are available. Some "import" statements that you might read in a
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book might not work because of missing libraries.
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<p>Support is improving all the time, though, and you might want to try a more modern version
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of Jython than the one that JMRI distributes. To do that, install Jython on you computer,
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then add a python.properties file in your <a href="../../setup/Files.shtml">user files
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directory</a> or <a href="../../setup/Files.shtml">preferences directory</a> that sets the
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python configuration variables, e.g. on Windows:</p>
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<pre style="font-family: monospace;">
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python.path = C:\\jython2.7.0\\Lib\\site\-packages
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python.startup =
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</pre>
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<p>Note that the double-back-slashes are necessary. They'll appear as single-back-slashes in
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the final value, which you can check with the "Context" item from the main JMRI Help menu. On
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Mac use something like:</p>
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<pre style="font-family: monospace;">
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python.path = /Users/username/jython2.7.0/Lib/site-packages
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python.startup =
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</pre>or where-ever you've installed the new Jython. Linux is similar.
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