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<h2 class="title"><a name="apihelper.alltogether"></a>4.8. Putting It All Together
</h2>
</div>
</div>
<div></div>
</div>
<div class="abstract">
<p>The last line of code, the only one you haven't deconstructed yet, is the one that does all the work. But by now the work
is easy, because everything you need is already set up just the way you need it. All the dominoes are in place; it's time
to knock them down.
</p>
</div>
<div class="informalexample">
<p>This is the meat of <tt class="filename">apihelper.py</tt>:
</p><pre class="programlisting">
<span class='pykeyword'>print</span> <span class='pystring'>"\n"</span>.join([<span class='pystring'>"%s %s"</span> %
(method.ljust(spacing),
processFunc(str(getattr(object, method).__doc__)))
<span class='pykeyword'>for</span> method <span class='pykeyword'>in</span> methodList])</pre></div>
<p>Note that this is one command, split over multiple lines, but it doesn't use the line continuation character (<tt class="literal">\</tt>). Remember when I said that <a href="../native_data_types/declaring_variables.html#tip.implicitmultiline">some expressions can be split into multiple lines</a> without using a backslash? A list comprehension is one of those expressions, since the entire expression is contained in
square brackets.
</p>
<p>Now, let's take it from the end and work backwards. The </p><pre class="programlisting"><span class='pykeyword'>
for</span> method <span class='pykeyword'>in</span> methodList</pre><p>shows that this is a <a href="../native_data_types/mapping_lists.html" title="3.6. Mapping Lists">list comprehension</a>. As you know, <tt class="varname">methodList</tt> is a list of <a href="filtering_lists.html#apihelper.filter.care">all the methods you care about</a> in <tt class="varname">object</tt>. So you're looping through that list with <tt class="varname">method</tt>.
</p>
<div class="example"><a name="d0e10681"></a><h3 class="title">Example 4.22. Getting a <tt class="literal">doc string</tt> Dynamically
</h3><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>import</span> odbchelper</span>
<tt class="prompt">>>> </tt><span class="userinput">object = odbchelper</span> <a name="apihelper.alltogether.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">method = <span class='pystring'>'buildConnectionString'</span></span> <a name="apihelper.alltogether.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">getattr(object, method)</span> <a name="apihelper.alltogether.1.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="computeroutput"><function buildConnectionString at 010D6D74></span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> getattr(object, method).__doc__</span> <a name="apihelper.alltogether.1.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12">
<span class="computeroutput">Build a connection string from a dictionary of parameters.
Returns string.</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.alltogether.1.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">In the <tt class="function">info</tt> function, <tt class="varname">object</tt> is the object you're getting help on, passed in as an argument.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.alltogether.1.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">As you're looping through <tt class="varname">methodList</tt>, <tt class="varname">method</tt> is the name of the current method.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.alltogether.1.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Using the <a href="getattr.html" title="4.4. Getting Object References With getattr"><tt class="function">getattr</tt></a> function, you're getting a reference to the <i class="replaceable"><tt>method</tt></i> function in the <i class="replaceable"><tt>object</tt></i> module.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.alltogether.1.4"><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Now, printing the actual <tt class="literal">doc string</tt> of the method is easy.
</td>
</tr>
</table>
</div>
</div>
<p>The next piece of the puzzle is the use of <tt class="function">str</tt> around the <tt class="literal">doc string</tt>. As you may recall, <tt class="function">str</tt> is a built-in function that <a href="built_in_functions.html" title="4.3. Using type, str, dir, and Other Built-In Functions">coerces data into a string</a>. But a <tt class="literal">doc string</tt> is always a string, so why bother with the <tt class="function">str</tt> function? The answer is that not every function has a <tt class="literal">doc string</tt>, and if it doesn't, its <tt class="literal">__doc__</tt> attribute is <tt class="literal">None</tt>.
</p>
<div class="example"><a name="d0e10793"></a><h3 class="title">Example 4.23. Why Use <tt class="function">str</tt> on a <tt class="literal">doc string</tt>?
</h3><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput">>>> <span class='pykeyword'>def</span><span class='pyclass'> foo</span>(): <span class='pykeyword'>print</span> 2</span>
<tt class="prompt">>>> </tt><span class="userinput">>>> foo()</span>
<span class="computeroutput">2</span>
<tt class="prompt">>>> </tt><span class="userinput">>>> foo.__doc__</span> <a name="apihelper.alltogether.2.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">foo.__doc__ == None</span> <a name="apihelper.alltogether.2.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">True</span>
<tt class="prompt">>>> </tt><span class="userinput">str(foo.__doc__)</span> <a name="apihelper.alltogether.2.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="computeroutput">'None'</span>
</pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.alltogether.2.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">You can easily define a function that has no <tt class="literal">doc string</tt>, so its <tt class="literal">__doc__</tt> attribute is <tt class="literal">None</tt>. Confusingly, if you evaluate the <tt class="literal">__doc__</tt> attribute directly, the <span class="application">Python</span> <span class="acronym">IDE</span> prints nothing at all, which makes sense if you think about it, but is still unhelpful.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.alltogether.2.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">You can verify that the value of the <tt class="literal">__doc__</tt> attribute is actually <tt class="literal">None</tt> by comparing it directly.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.alltogether.2.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The <tt class="function">str</tt> function takes the null value and returns a string representation of it, <tt class="literal">'None'</tt>.
</td>
</tr>
</table>
</div>
</div><a name="compare.isnull.sql"></a><table class="note" border="0" summary="">
<tr>
<td rowspan="2" align="center" valign="top" width="1%"><img src="../images/note.png" alt="Note" title="" width="24" height="24"></td>
</tr>
<tr>
<td colspan="2" align="left" valign="top" width="99%">In <span class="acronym">SQL</span>, you must use <tt class="literal">IS NULL</tt> instead of <tt class="literal">= NULL</tt> to compare a null value. In <span class="application">Python</span>, you can use either <tt class="literal">== None</tt> or <tt class="literal">is None</tt>, but <tt class="literal">is None</tt> is faster.
</td>
</tr>
</table>
<p>Now that you are guaranteed to have a string, you can pass the string to <tt class="varname">processFunc</tt>, which you have <a href="lambda_functions.html" title="4.7. Using lambda Functions">already defined</a> as a function that either does or doesn't collapse whitespace. Now you see why it was important to use <tt class="function">str</tt> to convert a <tt class="literal">None</tt> value into a string representation. <tt class="varname">processFunc</tt> is assuming a string argument and calling its <tt class="function">split</tt> method, which would crash if you passed it <tt class="literal">None</tt> because <tt class="literal">None</tt> doesn't have a <tt class="function">split</tt> method.
</p>
<p>Stepping back even further, you see that you're using string formatting again to concatenate the return value of <tt class="varname">processFunc</tt> with the return value of <tt class="varname">method</tt>'s <tt class="function">ljust</tt> method. This is a new string method that you haven't seen before.
</p>
<div class="example"><a name="d0e10958"></a><h3 class="title">Example 4.24. Introducing <tt class="function">ljust</tt></h3><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput">s = <span class='pystring'>'buildConnectionString'</span></span>
<tt class="prompt">>>> </tt><span class="userinput">s.ljust(30)</span> <a name="apihelper.alltogether.3.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput">'buildConnectionString '</span>
<tt class="prompt">>>> </tt><span class="userinput">s.ljust(20)</span> <a name="apihelper.alltogether.3.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">'buildConnectionString'</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.alltogether.3.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="function">ljust</tt> pads the string with spaces to the given length. This is what the <tt class="function">info</tt> function uses to make two columns of output and line up all the <tt class="literal">doc string</tt>s in the second column.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.alltogether.3.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">If the given length is smaller than the length of the string, <tt class="function">ljust</tt> will simply return the string unchanged. It never truncates the string.
</td>
</tr>
</table>
</div>
</div>
<p>You're almost finished. Given the padded method name from the <tt class="function">ljust</tt> method and the (possibly collapsed) <tt class="literal">doc string</tt> from the call to <tt class="varname">processFunc</tt>, you concatenate the two and get a single string. Since you're mapping <tt class="varname">methodList</tt>, you end up with a list of strings. Using the <tt class="function">join</tt> method of the string <tt class="literal">"\n"</tt>, you join this list into a single string, with each element of the list on a separate line, and print the result.
</p>
<div class="example"><a name="d0e11026"></a><h3 class="title">Example 4.25. Printing a List</h3><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput">li = [<span class='pystring'>'a'</span>, <span class='pystring'>'b'</span>, <span class='pystring'>'c'</span>]</span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> <span class='pystring'>"\n"</span>.join(li)</span> <a name="apihelper.alltogether.4.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput">a
b
c</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.alltogether.4.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">This is also a useful debugging trick when you're working with lists. And in <span class="application">Python</span>, you're always working with lists.
</td>
</tr>
</table>
</div>
</div>
<p>That's the last piece of the puzzle. You should now understand this code.</p>
<div class="informalexample"><pre class="programlisting">
<span class='pykeyword'>print</span> <span class='pystring'>"\n"</span>.join([<span class='pystring'>"%s %s"</span> %
(method.ljust(spacing),
processFunc(str(getattr(object, method).__doc__)))
<span class='pykeyword'>for</span> method <span class='pykeyword'>in</span> methodList])</pre></div>
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