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<h2 class="title"><a name="plural.stage4"></a>17.5. <tt class="filename">plural.py</tt>, stage 4
</h2>
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<div></div>
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<div class="abstract">
<p>Let's factor out the duplication in the code so that defining new rules can be easier.</p>
</div>
<div class="example"><a name="plural.stage4.example.1"></a><h3 class="title">Example 17.9. <tt class="filename">plural4.py</tt></h3><pre class="programlisting"><span class='pykeyword'>
import</span> re
<span class='pykeyword'>def</span><span class='pyclass'> buildMatchAndApplyFunctions</span>((pattern, search, replace)):
matchFunction = <span class='pykeyword'>lambda</span> word: re.search(pattern, word) <a name="plural.stage4.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
applyFunction = <span class='pykeyword'>lambda</span> word: re.sub(search, replace, word) <a name="plural.stage4.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class='pykeyword'>return</span> (matchFunction, applyFunction) <a name="plural.stage4.1.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
</pre><div class="calloutlist">
<table border="0" summary="Callout list">
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<td width="12" valign="top" align="left"><a href="#plural.stage4.1.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="function">buildMatchAndApplyFunctions</tt> is a function that builds other functions dynamically. It takes <tt class="varname">pattern</tt>, <tt class="varname">search</tt> and <tt class="varname">replace</tt> (actually it takes a tuple, but more on that in a minute), and you can build the match function using the <tt class="literal">lambda</tt> syntax to be a function that takes one parameter (<tt class="varname">word</tt>) and calls <tt class="function">re.search</tt> with the <tt class="varname">pattern</tt> that was passed to the <tt class="function">buildMatchAndApplyFunctions</tt> function, and the <tt class="varname">word</tt> that was passed to the match function you're building. Whoa.
</td>
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<tr>
<td width="12" valign="top" align="left"><a href="#plural.stage4.1.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Building the apply function works the same way. The apply function is a function that takes one parameter, and calls <tt class="function">re.sub</tt> with the <tt class="varname">search</tt> and <tt class="varname">replace</tt> parameters that were passed to the <tt class="function">buildMatchAndApplyFunctions</tt> function, and the <tt class="varname">word</tt> that was passed to the apply function you're building. This technique of using the values of outside parameters within a
dynamic function is called <span class="emphasis"><em>closures</em></span>. You're essentially defining constants within the apply function you're building: it takes one parameter (<tt class="varname">word</tt>), but it then acts on that plus two other values (<tt class="varname">search</tt> and <tt class="varname">replace</tt>) which were set when you defined the apply function.
</td>
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<td width="12" valign="top" align="left"><a href="#plural.stage4.1.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Finally, the <tt class="function">buildMatchAndApplyFunctions</tt> function returns a tuple of two values: the two functions you just created. The constants you defined within those functions
(<tt class="varname">pattern</tt> within <tt class="varname">matchFunction</tt>, and <tt class="varname">search</tt> and <tt class="varname">replace</tt> within <tt class="varname">applyFunction</tt>) stay with those functions, even after you return from <tt class="function">buildMatchAndApplyFunctions</tt>. That's insanely cool.
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</div>
<p>If this is incredibly confusing (and it should be, this is weird stuff), it may become clearer when you see how to use it.</p>
<div class="example"><a name="d0e37935"></a><h3 class="title">Example 17.10. <tt class="filename">plural4.py</tt> continued
</h3><pre class="programlisting">
patterns = \
(
(<span class='pystring'>'[sxz]$'</span>, <span class='pystring'>'$'</span>, <span class='pystring'>'es'</span>),
(<span class='pystring'>'[^aeioudgkprt]h$'</span>, <span class='pystring'>'$'</span>, <span class='pystring'>'es'</span>),
(<span class='pystring'>'(qu|[^aeiou])y$'</span>, <span class='pystring'>'y$'</span>, <span class='pystring'>'ies'</span>),
(<span class='pystring'>'$'</span>, <span class='pystring'>'$'</span>, <span class='pystring'>'s'</span>)
) <a name="plural.stage4.2.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
rules = map(buildMatchAndApplyFunctions, patterns) <a name="plural.stage4.2.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
</pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#plural.stage4.2.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Our pluralization rules are now defined as a series of strings (not functions). The first string is the regular expression
that you would use in <tt class="function">re.search</tt> to see if this rule matches; the second and third are the search and replace expressions you would use in <tt class="function">re.sub</tt> to actually apply the rule to turn a noun into its plural.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#plural.stage4.2.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">This line is magic. It takes the list of strings in <tt class="varname">patterns</tt> and turns them into a list of functions. How? By mapping the strings to the <tt class="function">buildMatchAndApplyFunctions</tt> function, which just happens to take three strings as parameters and return a tuple of two functions. This means that <tt class="varname">rules</tt> ends up being exactly the same as the previous example: a list of tuples, where each tuple is a pair of functions, where
the first function is the match function that calls <tt class="function">re.search</tt>, and the second function is the apply function that calls <tt class="function">re.sub</tt>.
</td>
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</table>
</div>
</div>
<p>I swear I am not making this up: <tt class="varname">rules</tt> ends up with exactly the same list of functions as the previous example. Unroll the <tt class="varname">rules</tt> definition, and you'll get this:
</p>
<div class="example"><a name="d0e37982"></a><h3 class="title">Example 17.11. Unrolling the rules definition</h3><pre class="programlisting">
rules = \
(
(
<span class='pykeyword'>lambda</span> word: re.search(<span class='pystring'>'[sxz]$'</span>, word),
<span class='pykeyword'>lambda</span> word: re.sub(<span class='pystring'>'$'</span>, <span class='pystring'>'es'</span>, word)
),
(
<span class='pykeyword'>lambda</span> word: re.search(<span class='pystring'>'[^aeioudgkprt]h$'</span>, word),
<span class='pykeyword'>lambda</span> word: re.sub(<span class='pystring'>'$'</span>, <span class='pystring'>'es'</span>, word)
),
(
<span class='pykeyword'>lambda</span> word: re.search(<span class='pystring'>'[^aeiou]y$'</span>, word),
<span class='pykeyword'>lambda</span> word: re.sub(<span class='pystring'>'y$'</span>, <span class='pystring'>'ies'</span>, word)
),
(
<span class='pykeyword'>lambda</span> word: re.search(<span class='pystring'>'$'</span>, word),
<span class='pykeyword'>lambda</span> word: re.sub(<span class='pystring'>'$'</span>, <span class='pystring'>'s'</span>, word)
)
)
</pre></div>
<div class="example"><a name="plural.finishing.up"></a><h3 class="title">Example 17.12. <tt class="filename">plural4.py</tt>, finishing up
</h3><pre class="programlisting"><span class='pykeyword'>
def</span> plural(noun):
<span class='pykeyword'>for</span> matchesRule, applyRule <span class='pykeyword'>in</span> rules: <a name="plural.stage4.3.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class='pykeyword'>if</span> matchesRule(noun):
<span class='pykeyword'>return</span> applyRule(noun)
</pre><div class="calloutlist">
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<tr>
<td width="12" valign="top" align="left"><a href="#plural.stage4.3.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Since the <tt class="varname">rules</tt> list is the same as the previous example, it should come as no surprise that the <tt class="function">plural</tt> function hasn't changed. Remember, it's completely generic; it takes a list of rule functions and calls them in order.
It doesn't care how the rules are defined. In <a href="stage2.html" title="17.3. plural.py, stage 2">stage 2</a>, they were defined as seperate named functions. In <a href="stage3.html" title="17.4. plural.py, stage 3">stage 3</a>, they were defined as anonymous <tt class="literal">lambda</tt> functions. Now in stage 4, they are built dynamically by mapping the <tt class="function">buildMatchAndApplyFunctions</tt> function onto a list of raw strings. Doesn't matter; the <tt class="function">plural</tt> function still works the same way.
</td>
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</table>
</div>
</div>
<p>Just in case that wasn't mind-blowing enough, I must confess that there was a subtlety in the definition of <tt class="function">buildMatchAndApplyFunctions</tt> that I skipped over. Let's go back and take another look.
</p>
<div class="example"><a name="d0e38026"></a><h3 class="title">Example 17.13. Another look at <tt class="function">buildMatchAndApplyFunctions</tt></h3><pre class="programlisting"><span class='pykeyword'>
def</span> buildMatchAndApplyFunctions((pattern, search, replace)): <a name="plural.stage4.4.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
</pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#plural.stage4.4.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Notice the double parentheses? This function doesn't actually take three parameters; it actually takes one parameter, a tuple
of three elements. But the tuple is expanded when the function is called, and the three elements of the tuple are each assigned
to different variables: <tt class="varname">pattern</tt>, <tt class="varname">search</tt>, and <tt class="varname">replace</tt>. Confused yet? Let's see it in action.
</td>
</tr>
</table>
</div>
</div>
<div class="example"><a name="d0e38048"></a><h3 class="title">Example 17.14. Expanding tuples when calling functions</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>def</span><span class='pyclass'> foo</span>((a, b, c)):</span>
<tt class="prompt">... </tt><span class="userinput"><span class='pykeyword'>print</span> c</span>
<tt class="prompt">... </tt><span class="userinput"><span class='pykeyword'>print</span> b</span>
<tt class="prompt">... </tt><span class="userinput"><span class='pykeyword'>print</span> a</span>
<tt class="prompt">>>> </tt><span class="userinput">parameters = (<span class='pystring'>'apple'</span>, <span class='pystring'>'bear'</span>, <span class='pystring'>'catnap'</span>)</span>
<tt class="prompt">>>> </tt><span class="userinput">foo(parameters)</span> <a name="plural.stage4.5.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput">catnap</span>
<span class="computeroutput">bear</span>
<span class="computeroutput">apple</span>
</pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#plural.stage4.5.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The proper way to call the function <tt class="function">foo</tt> is with a tuple of three elements. When the function is called, the elements are assigned to different local variables within
<tt class="function">foo</tt>.
</td>
</tr>
</table>
</div>
<p>Now let's go back and see why this auto-tuple-expansion trick was necessary. <tt class="varname">patterns</tt> was a list of tuples, and each tuple had three elements. When you called <tt class="literal">map(buildMatchAndApplyFunctions, patterns)</tt>, that means that <tt class="function">buildMatchAndApplyFunctions</tt> is <span class="emphasis"><em>not</em></span> getting called with three parameters. Using <tt class="function">map</tt> to map a single list onto a function always calls the function with a single parameter: each element of the list. In the
case of <tt class="varname">patterns</tt>, each element of the list is a tuple, so <tt class="function">buildMatchAndApplyFunctions</tt> always gets called with the tuple, and you use the auto-tuple-expansion trick in the definition of <tt class="function">buildMatchAndApplyFunctions</tt> to assign the elements of that tuple to named variables that you can work with.
</p>
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