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<title>Chapter 5. Objects and Object-Orientation</title>
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<div class="chapter" lang="en">
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<h2 class="title"><a name="fileinfo"></a>Chapter 5. Objects and Object-Orientation
</h2>
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<div class="toc">
<ul>
<li><span class="section"><a href="index.html#fileinfo.divein">5.1. Diving In</a></span></li>
<li><span class="section"><a href="importing_modules.html">5.2. Importing Modules Using from module import</a></span></li>
<li><span class="section"><a href="defining_classes.html">5.3. Defining Classes</a></span><ul>
<li><span class="section"><a href="defining_classes.html#d0e11720">5.3.1. Initializing and Coding Classes</a></span></li>
<li><span class="section"><a href="defining_classes.html#d0e11896">5.3.2. Knowing When to Use self and __init__</a></span></li>
</ul>
</li>
<li><span class="section"><a href="instantiating_classes.html">5.4. Instantiating Classes</a></span><ul>
<li><span class="section"><a href="instantiating_classes.html#d0e12165">5.4.1. Garbage Collection</a></span></li>
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<li><span class="section"><a href="userdict.html">5.5. Exploring UserDict: A Wrapper Class</a></span></li>
<li><span class="section"><a href="special_class_methods.html">5.6. Special Class Methods</a></span><ul>
<li><span class="section"><a href="special_class_methods.html#d0e12822">5.6.1. Getting and Setting Items</a></span></li>
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<li><span class="section"><a href="special_class_methods2.html">5.7. Advanced Special Class Methods</a></span></li>
<li><span class="section"><a href="class_attributes.html">5.8. Introducing Class Attributes</a></span></li>
<li><span class="section"><a href="private_functions.html">5.9. Private Functions</a></span></li>
<li><span class="section"><a href="summary.html">5.10. Summary</a></span></li>
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<div class="abstract">
<p>This chapter, and pretty much every chapter after this, deals with object-oriented <span class="application">Python</span> programming.
</p>
</div>
<div class="section" lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a name="fileinfo.divein"></a>5.1. Diving In
</h2>
</div>
</div>
<div></div>
</div>
<div class="abstract">
<p>Here is a complete, working <span class="application">Python</span> program. Read the <a href="../getting_to_know_python/documenting_functions.html" title="2.3. Documenting Functions"><tt class="literal">doc string</tt>s</a> of the module, the classes, and the functions to get an overview of what this program does and how it works. As usual, don't
worry about the stuff you don't understand; that's what the rest of the chapter is for.
</p>
</div>
<div class="example"><a name="d0e11177"></a><h3 class="title">Example 5.1. <tt class="filename">fileinfo.py</tt></h3>
<p>If you have not already done so, you can <a href="http://diveintopython.org/download/diveintopython-examples-5.4.zip" title="Download example scripts">download this and other examples</a> used in this book.
</p><pre class="programlisting">
<span class='pystring'>"""Framework for getting filetype-specific metadata.
Instantiate appropriate class with filename. Returned object acts like a
dictionary, with key-value pairs for each piece of metadata.
import fileinfo
info = fileinfo.MP3FileInfo("/music/ap/mahadeva.mp3")
print "\\n".join(["%s=%s" % (k, v) for k, v in info.items()])
Or use listDirectory function to get info on all files in a directory.
for info in fileinfo.listDirectory("/music/ap/", [".mp3"]):
...
Framework can be extended by adding classes for particular file types, e.g.
HTMLFileInfo, MPGFileInfo, DOCFileInfo. Each class is completely responsible for
parsing its files appropriately; see MP3FileInfo for example.
"""</span>
<span class='pykeyword'>import</span> os
<span class='pykeyword'>import</span> sys
<span class='pykeyword'>from</span> UserDict <span class='pykeyword'>import</span> UserDict
<span class='pykeyword'>def</span><span class='pyclass'> stripnulls</span>(data):
<span class='pystring'>"strip whitespace and nulls"</span>
<span class='pykeyword'>return</span> data.replace(<span class='pystring'>"\00"</span>, <span class='pystring'>""</span>).strip()
<span class='pykeyword'>class</span><span class='pyclass'> FileInfo</span>(UserDict):
<span class='pystring'>"store file metadata"</span>
<span class='pykeyword'>def</span><span class='pyclass'> __init__</span>(self, filename=None):
UserDict.__init__(self)
self[<span class='pystring'>"name"</span>] = filename
<span class='pykeyword'>class</span><span class='pyclass'> MP3FileInfo</span>(FileInfo):
<span class='pystring'>"store ID3v1.0 MP3 tags"</span>
tagDataMap = {<span class='pystring'>"title"</span> : ( 3, 33, stripnulls),
<span class='pystring'>"artist"</span> : ( 33, 63, stripnulls),
<span class='pystring'>"album"</span> : ( 63, 93, stripnulls),
<span class='pystring'>"year"</span> : ( 93, 97, stripnulls),
<span class='pystring'>"comment"</span> : ( 97, 126, stripnulls),
<span class='pystring'>"genre"</span> : (127, 128, ord)}
<span class='pykeyword'>def</span><span class='pyclass'> __parse</span>(self, filename):
<span class='pystring'>"parse ID3v1.0 tags from MP3 file"</span>
self.clear()
<span class='pykeyword'>try</span>:
fsock = open(filename, <span class='pystring'>"rb"</span>, 0)
<span class='pykeyword'>try</span>:
fsock.seek(-128, 2)
tagdata = fsock.read(128)
<span class='pykeyword'>finally</span>:
fsock.close()
<span class='pykeyword'>if</span> tagdata[:3] == <span class='pystring'>"TAG"</span>:
<span class='pykeyword'>for</span> tag, (start, end, parseFunc) <span class='pykeyword'>in</span> self.tagDataMap.items():
self[tag] = parseFunc(tagdata[start:end])
<span class='pykeyword'>except</span> IOError:
<span class='pykeyword'>pass</span>
<span class='pykeyword'>def</span><span class='pyclass'> __setitem__</span>(self, key, item):
<span class='pykeyword'>if</span> key == <span class='pystring'>"name"</span> <span class='pykeyword'>and</span> item:
self.__parse(item)
FileInfo.__setitem__(self, key, item)
<span class='pykeyword'>def</span><span class='pyclass'> listDirectory</span>(directory, fileExtList):
<span class='pystring'>"get list of file info objects for files of particular extensions"</span>
fileList = [os.path.normcase(f)
<span class='pykeyword'>for</span> f <span class='pykeyword'>in</span> os.listdir(directory)]
fileList = [os.path.join(directory, f)
<span class='pykeyword'>for</span> f <span class='pykeyword'>in</span> fileList
<span class='pykeyword'>if</span> os.path.splitext(f)[1] <span class='pykeyword'>in</span> fileExtList]
<span class='pykeyword'>def</span><span class='pyclass'> getFileInfoClass</span>(filename, module=sys.modules[FileInfo.__module__]):
<span class='pystring'>"get file info class from filename extension"</span>
subclass = <span class='pystring'>"%sFileInfo"</span> % os.path.splitext(filename)[1].upper()[1:]
<span class='pykeyword'>return</span> hasattr(module, subclass) <span class='pykeyword'>and</span> getattr(module, subclass) <span class='pykeyword'>or</span> FileInfo
<span class='pykeyword'>return</span> [getFileInfoClass(f)(f) <span class='pykeyword'>for</span> f <span class='pykeyword'>in</span> fileList]
<span class='pykeyword'>if</span> __name__ == <span class='pystring'>"__main__"</span>:
<span class='pykeyword'>for</span> info <span class='pykeyword'>in</span> listDirectory(<span class='pystring'>"/music/_singles/"</span>, [<span class='pystring'>".mp3"</span>]): <a name="fileinfo_divein.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class='pykeyword'>print</span> <span class='pystring'>"\n"</span>.join([<span class='pystring'>"%s=%s"</span> % (k, v) <span class='pykeyword'>for</span> k, v <span class='pykeyword'>in</span> info.items()])
print</pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#fileinfo_divein.1.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">This program's output depends on the files on your hard drive. To get meaningful output, you'll need to change the directory
path to point to a directory of MP3 files on your own machine.
</td>
</tr>
</table>
</div>
</div>
<div class="informalexample">
<p>This is the output I got on my machine. Your output will be different, unless, by some startling coincidence, you share my
exact taste in music.
</p><pre class="screen"><span class="computeroutput">album=
artist=Ghost in the Machine
title=A Time Long Forgotten (Concept
genre=31
name=/music/_singles/a_time_long_forgotten_con.mp3
year=1999
comment=http://mp3.com/ghostmachine
album=Rave Mix
artist=***DJ MARY-JANE***
title=HELLRAISER****Trance from Hell
genre=31
name=/music/_singles/hellraiser.mp3
year=2000
comment=http://mp3.com/DJMARYJANE
album=Rave Mix
artist=***DJ MARY-JANE***
title=KAIRO****THE BEST GOA
genre=31
name=/music/_singles/kairo.mp3
year=2000
comment=http://mp3.com/DJMARYJANE
album=Journeys
artist=Masters of Balance
title=Long Way Home
genre=31
name=/music/_singles/long_way_home1.mp3
year=2000
comment=http://mp3.com/MastersofBalan
album=
artist=The Cynic Project
title=Sidewinder
genre=18
name=/music/_singles/sidewinder.mp3
year=2000
comment=http://mp3.com/cynicproject
album=Digitosis@128k
artist=VXpanded
title=Spinning
genre=255
name=/music/_singles/spinning.mp3
year=2000
comment=http://mp3.com/artists/95/vxp</span></pre></div>
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<p class="copyright">Copyright © 2000, 2001, 2002, 2003, 2004 <a href="mailto:mark@diveintopython.org">Mark Pilgrim</a></p>
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