projects / OpenSPARC-T2-DV / blame
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| 53 | ||
| 54 | <H2><A NAME="SECTION0051470000000000000000"></A><A NAME="pickle-example"></A> | |
| 55 | <BR> | |
| 56 | 3.14.7 Example | |
| 57 | </H2> | |
| 58 | ||
| 59 | <P> | |
| 60 | Here's a simple example of how to modify pickling behavior for a | |
| 61 | class. The <tt class="class">TextReader</tt> class opens a text file, and returns | |
| 62 | the line number and line contents each time its <tt class="method">readline()</tt> | |
| 63 | method is called. If a <tt class="class">TextReader</tt> instance is pickled, all | |
| 64 | attributes <em>except</em> the file object member are saved. When the | |
| 65 | instance is unpickled, the file is reopened, and reading resumes from | |
| 66 | the last location. The <tt class="method">__setstate__()</tt> and | |
| 67 | <tt class="method">__getstate__()</tt> methods are used to implement this behavior. | |
| 68 | ||
| 69 | <P> | |
| 70 | <div class="verbatim"><pre> | |
| 71 | class TextReader: | |
| 72 | """Print and number lines in a text file.""" | |
| 73 | def __init__(self, file): | |
| 74 | self.file = file | |
| 75 | self.fh = open(file) | |
| 76 | self.lineno = 0 | |
| 77 | ||
| 78 | def readline(self): | |
| 79 | self.lineno = self.lineno + 1 | |
| 80 | line = self.fh.readline() | |
| 81 | if not line: | |
| 82 | return None | |
| 83 | if line.endswith("\n"): | |
| 84 | line = line[:-1] | |
| 85 | return "%d: %s" % (self.lineno, line) | |
| 86 | ||
| 87 | def __getstate__(self): | |
| 88 | odict = self.__dict__.copy() # copy the dict since we change it | |
| 89 | del odict['fh'] # remove filehandle entry | |
| 90 | return odict | |
| 91 | ||
| 92 | def __setstate__(self,dict): | |
| 93 | fh = open(dict['file']) # reopen file | |
| 94 | count = dict['lineno'] # read from file... | |
| 95 | while count: # until line count is restored | |
| 96 | fh.readline() | |
| 97 | count = count - 1 | |
| 98 | self.__dict__.update(dict) # update attributes | |
| 99 | self.fh = fh # save the file object | |
| 100 | </pre></div> | |
| 101 | ||
| 102 | <P> | |
| 103 | A sample usage might be something like this: | |
| 104 | ||
| 105 | <P> | |
| 106 | <div class="verbatim"><pre> | |
| 107 | >>> import TextReader | |
| 108 | >>> obj = TextReader.TextReader("TextReader.py") | |
| 109 | >>> obj.readline() | |
| 110 | '1: #!/usr/local/bin/python' | |
| 111 | >>> # (more invocations of obj.readline() here) | |
| 112 | ... obj.readline() | |
| 113 | '7: class TextReader:' | |
| 114 | >>> import pickle | |
| 115 | >>> pickle.dump(obj,open('save.p','w')) | |
| 116 | </pre></div> | |
| 117 | ||
| 118 | <P> | |
| 119 | If you want to see that <tt class="module"><a href="module-pickle.html">pickle</a></tt> works across Python | |
| 120 | processes, start another Python session, before continuing. What | |
| 121 | follows can happen from either the same process or a new process. | |
| 122 | ||
| 123 | <P> | |
| 124 | <div class="verbatim"><pre> | |
| 125 | >>> import pickle | |
| 126 | >>> reader = pickle.load(open('save.p')) | |
| 127 | >>> reader.readline() | |
| 128 | '8: "Print and number lines in a text file."' | |
| 129 | </pre></div> | |
| 130 | ||
| 131 | <P> | |
| 132 | <div class="seealso"> | |
| 133 | <p class="heading">See Also:</p> | |
| 134 | ||
| 135 | <dl compact="compact" class="seemodule"> | |
| 136 | <dt>Module <b><tt class="module"><a href="module-copyreg.html">copy_reg</a></tt>:</b> | |
| 137 | <dd>Pickle interface constructor | |
| 138 | registration for extension types. | |
| 139 | </dl> | |
| 140 | ||
| 141 | <P> | |
| 142 | <dl compact="compact" class="seemodule"> | |
| 143 | <dt>Module <b><tt class="module"><a href="module-shelve.html">shelve</a></tt>:</b> | |
| 144 | <dd>Indexed databases of objects; uses <tt class="module">pickle</tt>. | |
| 145 | </dl> | |
| 146 | ||
| 147 | <P> | |
| 148 | <dl compact="compact" class="seemodule"> | |
| 149 | <dt>Module <b><tt class="module"><a href="module-copy.html">copy</a></tt>:</b> | |
| 150 | <dd>Shallow and deep object copying. | |
| 151 | </dl> | |
| 152 | ||
| 153 | <P> | |
| 154 | <dl compact="compact" class="seemodule"> | |
| 155 | <dt>Module <b><tt class="module"><a href="module-marshal.html">marshal</a></tt>:</b> | |
| 156 | <dd>High-performance serialization of built-in types. | |
| 157 | </dl> | |
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| 187 | <b class="navlabel">Previous:</b> | |
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| 196 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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