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finished #load and #dumps sections

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Mark Pilgrim
2009-08-18 00:43:49 -04:00
parent cf721a18c4
commit 3ae2729afd
1 changed files with 43 additions and 26 deletions
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serializing.html
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@@ -116,39 +116,55 @@ NameError: name 'entry' is not defined</samp>
'published_date': time.struct_time(tm_year=2009, tm_mon=3, tm_mday=27, tm_hour=22, tm_min=20, tm_sec=42, tm_wday=4, tm_yday=86, tm_isdst=-1),
'published': True}</samp></pre>
<ol>
<li>FIXME
<li>FIXME
<li>FIXME
<li>FIXME
<li>FIXME
<li>This is Python Shell #2.
<li>There is no <var>entry</var> variable defined here. You defined an <var>entry</var> variable in Python Shell #1, but that&#8217;s a completely different environment with its own state.
<li>Open the <code>entry.pickle</code> file you created in Python Shell #1. The <code>pickle</code> module uses a binary data format, so you should always open pickle files in binary mode.
<li>The <code>pickle.load()</code> function takes a <a href=files.html#file-objects>stream object</a>, reads the serialized data from the stream, creates a new Python object, recreates the serialized data in the new Python object, and returns the new Python object.
<li>Now the <var>entry</var> variable is a dictionary with familiar-looking keys and values.
</ol>
<p>FIXME
<p>The <code>pickle.dump() / pickle.load()</code> cycle results in an identical copy of the original data structure.
<pre class=screen>
<a><samp class=p>>>> </samp><kbd class=pp>shell</kbd> <span class=u>&#x2460;</span></a>
<samp class=pp>1</samp>
<a><samp class=p>>>> </samp><kbd class=pp>with open('entry.pickle', 'rb') as f:</kbd> <span class=u>&#x2461;</span></a>
<a><samp class=p>... </samp><kbd class=pp> entry2 = pickle.load(f)</kbd> <span class=u>&#x2462;</span></a>
<samp class=p>... </samp>
<a><samp class=p>>>> </samp><kbd class=pp>entry2 == entry</kbd> <span class=u>&#x2463;</span></a>
<samp class=pp>True</samp>
<a><samp class=p>>>> </samp><kbd class=pp>entry2['tags']</kbd> <span class=u>&#x2464;</span></a>
<samp class=pp>('diveintopython', 'docbook', 'html')</samp>
<samp class=p>>>> </samp><kbd class=pp>entry2['internal_id']</kbd>
<samp class=pp>b'\xde\xd5\xb4\xf8'</samp></pre>
<ol>
<li>Switch back to Python Shell #1.
<li>Open the <code>entry.pickle</code> file.
<li>Load the serialized data into a new variable, <var>entry2</var>.
<li>Python confirms that the two dictionaries, <var>entry</var> and <var>entry2</var>, are identical. In this shell, you built <var>entry</var> from the ground up, starting with an empty dictionary and manually assigning values to specific keys. You serialized this dictionary and stored it in the <code>entry.pickle</code> file. Now you&#8217;ve read the serialized data from that file and created a perfect replica of the original data structure.
<li>For reasons that will become clear later in this chapter, I want to point out that the value of the <code>'tags'</code> key is a tuple, and the value of the <code>'internal_id'</code> key is a <code>bytes</code> object.
</ol>
<h3 id=dumps>Pickling Without a File</h3>
<p>The examples in the previous section showed how to serialize a Python object directly to a file on disk. But what if you don&#8217;t want or need a file? You can also serialize to a <code>bytes</code> object in memory.
<pre class=screen>
<samp class=p>>>> </samp><kbd class=pp>shell</kbd>
<samp class=pp>1</samp>
<a><samp class=p>>>> </samp><kbd class=pp>with open('entry.pickle', 'rb') as f:</kbd> <span class=u>&#x2460;</span></a>
<a><samp class=p>... </samp><kbd class=pp> entry2 = pickle.load(f)</kbd> <span class=u>&#x2461;</span></a>
<samp class=p>... </samp>
<a><samp class=p>>>> </samp><kbd class=pp>entry2 == entry</kbd> <span class=u>&#x2462;</span></a>
<samp class=pp>True</samp>
<a><samp class=p>>>> </samp><kbd class=pp>entry2['tags']</kbd> <span class=u>&#x2463;</span></a>
<samp class=pp>('diveintopython', 'docbook', 'html')</samp>
<a><samp class=p>>>> </samp><kbd class=pp>entry2['internal_id']</kbd> <span class=u>&#x2464;</span></a>
<samp class=pp>b'\xde\xd5\xb4\xf8'</samp></pre>
<a><samp class=p>>>> </samp><kbd class=pp>b = pickle.dumps(entry)</kbd> <span class=u>&#x2460;</span></a>
<a><samp class=p>>>> </samp><kbd class=pp>type(b)</kbd> <span class=u>&#x2461;</span></a>
<samp class=pp>&lt;class 'bytes'></samp>
<a><samp class=p>>>> </samp><kbd class=pp>entry3 = pickle.loads(b)</kbd> <span class=u>&#x2462;</span></a>
<a><samp class=p>>>> </samp><kbd class=pp>entry3 == entry</kbd> <span class=u>&#x2463;</span></a>
<samp class=pp>True</samp></pre>
<ol>
<li>FIXME
<li>
<li>
<li>
<li>
<li>The <code>pickle.dumps()</code> function (note the <code>'s'</code> at the end of the function name) performs the same serialization as the <code>pickle.dump()</code> function. Instead of taking a stream object and writing the serialized data to a file on disk, it simply returns the serialized data.
<li>Since the pickle protocol uses a binary data format, the <code>pickle.dumps()</code> function returns a <code>bytes</code> object.
<li>The <code>pickle.loads()</code> function (again, note the <code>'s'</code> at the end of the function name) performs the same deserialization as the <code>pickle.load()</code> function. Instead of taking a stream object and reading the serialized data from a file, it takes a <code>bytes</code> object containing serialized data, such as the one returned by the <code>pickle.dumps()</code> function.
<li>The end result is the same: a perfect replica of the original dictionary.
</ol>
<h3 id=dumps>Saving to (and Loading from) an Object in Memory</h3>
<p>FIXME
<h3 id=protocol-versions>Bytes and Strings Rear Their Ugly Heads (Again!)</h3>
<p>FIXME - discussion of pickle protocol versions, backward incompatibility of protocol version 3 due to bytes/strings separation in Python 3, link to http://docs.python.org/3.1/library/pickle.html#data-stream-format
@@ -293,12 +309,12 @@ highest protocol among opcodes = 3</samp></pre>
<h3 id=json-unknown-types>Serializing Datatypes Unsupported by <abbr>JSON</abbr></h3>
<pre class=screen>
<a><samp class=p>>>> </samp><kbd class=pp>shell</kbd>
<a><samp class=p>>>> </samp><kbd class=pp>shell</kbd> <span class=u>&#x2460;</span></a>
<samp class=pp>1</samp>
<samp class=p>>>> </samp><kbd class=pp>entry</kbd>
<samp class=pp>FIXME</samp>
<samp class=p>>>> </samp><kbd class=pp>import json</kbd>
<a><samp class=p>>>> </samp><kbd class=pp>with open('entry.json', 'w', encoding='utf-8') as f:</kbd>
<a><samp class=p>>>> </samp><kbd class=pp>with open('entry.json', 'w', encoding='utf-8') as f:</kbd> <span class=u>&#x2461;</span></a>
<samp class=p>... </samp><kbd class=pp> json.dump(entry, f)</kbd>
<samp class=p>... </samp>
<samp class=traceback>Traceback (most recent call last):
@@ -316,6 +332,7 @@ highest protocol among opcodes = 3</samp></pre>
TypeError: b'\xde\xd5\xb4\xf8' is not JSON serializable</samp></pre>
<ol>
<li>FIXME
<li>FIXME
</ol>
<p>FIXME