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Mark Pilgrim
99 lines
7.3 KiB
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99 lines
7.3 KiB
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<title>Comprehensions - Dive into Python 3</title>
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<p>You are here: <a href=index.html>Home</a> <span class=u>‣</span> <a href=table-of-contents.html#comprehensions>Dive Into Python 3</a> <span class=u>‣</span>
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<p id=level>Difficulty level: <span class=u title=beginner>♦♦♢♢♢</span>
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<h1>Comprehensions</h1>
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<blockquote class=q>
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<p><span class=u>❝</span> FIXME <span class=u>❞</span><br>— FIXME
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</blockquote>
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<p id=toc>
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<h2 id=divingin>Diving In</h2>
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<p class=f>FIXME
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<h2 id=list-comprehensions>List Comprehensions</h2>
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<p>FIXME
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<p>One of the most powerful features of Python is the list comprehension, which provides a compact way of mapping a list into another list by applying a function to each
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of the elements of the list.
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<div class=example><h3>Example 3.24. Introducing List Comprehensions</h3><pre class=screen><samp class=p>>>> </samp><kbd>li = [1, 9, 8, 4]</kbd>
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<samp class=p>>>> </samp><kbd>[elem*2 for elem in li]</kbd> <span>①</span>
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[2, 18, 16, 8]
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<samp class=p>>>> </samp><kbd>li</kbd> <span>②</span>
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[1, 9, 8, 4]
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<samp class=p>>>> </samp><kbd>li = [elem*2 for elem in li]</kbd> <span>③</span>
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<samp class=p>>>> </samp><kbd>li</kbd>
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[2, 18, 16, 8]</pre>
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<ol>
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<li>To make sense of this, look at it from right to left. <var>li</var> is the list you're mapping. Python loops through <var>li</var> one element at a time, temporarily assigning the value of each element to the variable <var>elem</var>. Python then applies the function <code><var>elem</var>*2</code> and appends that result to the returned list.
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<li>Note that list comprehensions do not change the original list.
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<li>It is safe to assign the result of a list comprehension to the variable that you're mapping. Python constructs the new list in memory, and when the list comprehension is complete, it assigns the result to the variable.
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<p>Here are the list comprehensions in the <code>buildConnectionString</code> function that you declared in <a href="#odbchelper">Chapter 2</a>:<pre><code>
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["%s=%s" % (k, v) for k, v in params.items()]</pre><p>First, notice that you're calling the <code>items</code> function of the <var>params</var> dictionary. This function returns a list of tuples of all the data in the dictionary.
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<div class=example><h3 id="odbchelper.items">Example 3.25. The <code>keys</code>, <code>values</code>, and <code>items</code> Functions</h3><pre class=screen><samp class=p>>>> </samp><kbd>params = {"server":"mpilgrim", "database":"master", "uid":"sa", "pwd":"secret"}</kbd>
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<samp class=p>>>> </samp><kbd>params.keys()</kbd> <span>①</span>
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['server', 'uid', 'database', 'pwd']
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<samp class=p>>>> </samp><kbd>params.values()</kbd> <span>②</span>
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['mpilgrim', 'sa', 'master', 'secret']
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<samp class=p>>>> </samp><kbd>params.items()</kbd> <span>③</span>
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[('server', 'mpilgrim'), ('uid', 'sa'), ('database', 'master'), ('pwd', 'secret')]</pre>
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<ol>
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<li>The <code>keys</code> method of a dictionary returns a list of all the keys. The list is not in the order in which the dictionary was defined
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(remember that elements in a dictionary are unordered), but it is a list.
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<li>The <code>values</code> method returns a list of all the values. The list is in the same order as the list returned by <code>keys</code>, so <code>params.values()[n] == params[params.keys()[n]]</code> for all values of <var>n</var>.
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<li>The <code>items</code> method returns a list of tuples of the form <code>(<var>key</var>, <var>value</var>)</code>. The list contains all the data in the dictionary.
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<p>Now let's see what <code>buildConnectionString</code> does. It takes a list, <code><var>params</var>.<code>items</code>()</code>, and maps it to a new list by applying string formatting to each element. The new list will have the same number of elements
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as <code><var>params</var>.<code>items</code>()</code>, but each element in the new list will be a string that contains both a key and its associated value from the <var>params</var> dictionary.
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<div class=example><h3>Example 3.26. List Comprehensions in <code>buildConnectionString</code>, Step by Step</h3><pre class=screen><samp class=p>>>> </samp><kbd>params = {"server":"mpilgrim", "database":"master", "uid":"sa", "pwd":"secret"}</kbd>
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<samp class=p>>>> </samp><kbd>params.items()</kbd>
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[('server', 'mpilgrim'), ('uid', 'sa'), ('database', 'master'), ('pwd', 'secret')]
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<samp class=p>>>> </samp><kbd>[k for k, v in params.items()]</kbd> <span>①</span>
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['server', 'uid', 'database', 'pwd']
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<samp class=p>>>> </samp><kbd>[v for k, v in params.items()]</kbd> <span>②</span>
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['mpilgrim', 'sa', 'master', 'secret']
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<samp class=p>>>> </samp><kbd>["%s=%s" % (k, v) for k, v in params.items()]</kbd> <span>③</span>
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['server=mpilgrim', 'uid=sa', 'database=master', 'pwd=secret']</pre>
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<ol>
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<li>Note that you're using two variables to iterate through the <code>params.items()</code> list. This is another use of <a href="#odbchelper.multiassign" title="3.4.2. Assigning Multiple Values at Once">multi-variable assignment</a>. The first element of <code>params.items()</code> is <code>('server', 'mpilgrim')</code>, so in the first iteration of the list comprehension, <var>k</var> will get <code>'server'</code> and <var>v</var> will get <code>'mpilgrim'</code>. In this case, you're ignoring the value of <var>v</var> and only including the value of <var>k</var> in the returned list, so this list comprehension ends up being equivalent to <code><var>params</var>.<code>keys</code>()</code>.
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<li>Here you're doing the same thing, but ignoring the value of <var>k</var>, so this list comprehension ends up being equivalent to <code><var>params</var>.<code>values</code>()</code>.
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<li>Combining the previous two examples with some simple <a href="#odbchelper.stringformatting" title="3.5. Formatting Strings">string formatting</a>, you get a list of strings that include both the key and value of each element of the dictionary. This looks suspiciously
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like the <a href="#odbchelper.output">output</a> of the program. All that remains is to join the elements in this list into a single string.
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-->
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<p class=a>⁂
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<h2 id=set-comprehensions>Set Comprehensions</h2>
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<p>FIXME
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<p class=a>⁂
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<h2 id=dictionary-comprehensions>Dictionary Comprehensions</h2>
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<p>FIXME
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<p class=a>⁂
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<h2 id=furtherreading>Further Reading</h2>
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<ul>
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<li>FIXME
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</ul>
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<p class=v><a href=native-datatypes.html rel=prev title='back to “Native Datatypes”'><span class=u>☜</span></a> <a href=strings.html rel=next title='onward to “Strings”'><span class=u>☞</span></a>
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<p class=c>© 2001–9 <a href=about.html>Mark Pilgrim</a>
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