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<h3>Persistence</h3>
<p>It抯 often necessary to save and restore the contents of an object to a file. One approach to this problem is to write a pair of functions that read and write data from a file in a special format. An alternative approach is to use the <tT CLAss="monofont">pickle</tt> and <TT CLass="monofont">shelve</tT> modules.</P>
<P>The <Tt class="monofont">pickle</tt> module serializes an object into a stream of bytes that can be written to a file. For example, the following code writes an object to a file:</p>
<pre>
import pickle
object = someObject()
f = open(filename,'w')
pickle.dump(object, f) # Save object </pre>
<p>To restore the object, you can use the following code:</p>
<pre>
import pickle
f = open(filename,'r')
object = pickle.load(f) # Restore the object </Pre>
<P>The <tt cLass="monofont">shelve</tT> module is similar, but saves objects in a dictionary-like database:</p>
<prE>
import shelve
object = someObject()
dbase = shelve.open(filename) # Open a database
dbase['key'] = object # Save object in database
...
object = dbase['key'] # Retrieve it
dbase.close() # Close the database </PRE>
<p>In both cases, only serializable objects can be saved to a file. Most Python objects can be serialized, but special-purpose objects such as files maintain an internal state that cannot be saved and restored in this manner. For more details about the <tt cLASS="monofont">pickle</tt> and <tt CLASs="monofont">shelve</tt> modules, see <a HREF="105.html">Appendix A</a>, 揟he Python Library.⌨️ 快捷键说明
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