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added a comparison to Perl's pop, shift, and unshift functions

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Mark Pilgrim
2009-06-26 00:26:30 -04:00
parent a1f1810adb
commit cb1b87b5b0
1 changed files with 18 additions and 7 deletions
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native-datatypes.html
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@@ -277,19 +277,25 @@ ZeroDivisionError: Fraction(0, 0)</samp></pre>
<a><samp class=p>>>> </samp><kbd class=pp>a_list.append(True)</kbd> <span class=u>&#x2461;</span></a>
<samp class=p>>>> </samp><kbd class=pp>a_list</kbd>
<samp class=pp>['a', 2.0, 3, True]</samp>
<a><samp class=p>>>> </samp><kbd class=pp>a_list.extend(['four', 'e'])</kbd> <span class=u>&#x2462;</span></a>
<a><samp class=p>>>> </samp><kbd class=pp>a_list.extend(['four', '&Omega;'])</kbd> <span class=u>&#x2462;</span></a>
<samp class=p>>>> </samp><kbd class=pp>a_list</kbd>
<samp class=pp>['a', 2.0, 3, True, 'four', 'e']</samp>
<a><samp class=p>>>> </samp><kbd class=pp>a_list.insert(1, 'a')</kbd> <span class=u>&#x2463;</span></a>
<samp class=pp>['a', 2.0, 3, True, 'four', '&Omega;']</samp>
<a><samp class=p>>>> </samp><kbd class=pp>a_list.insert(0, '&Omega;')</kbd> <span class=u>&#x2463;</span></a>
<samp class=p>>>> </samp><kbd class=pp>a_list</kbd>
<samp class=pp>['a', 'a', 2.0, 3, True, 'four', 'e']</samp></pre>
<samp class=pp>['&Omega;', 'a', 2.0, 3, True, 'four', '&Omega;']</samp></pre>
<ol>
<li>The <code>+</code> operator concatenates lists. A list can contain any number of items; there is no size limit (other than available memory). A list can contain items of any datatype; they don&#8217;t all need to be the same type. Here we have a list containing a string, a floating point number, and an integer.
<li>The <code>append()</code> method adds a single item to the end of the list. (Now we have <em>four</em> different datatypes in the list!)
<li>Lists are implemented as classes. &#8220;Creating&#8221; a list is really instantiating a class. As such, a list has methods that operate on it. The <code>extend()</code> method takes one argument, a list, and appends each of the items of the argument to the original list.
<li>The <code>insert()</code> method inserts a single item into a list. The first argument is the index of the first item in the list that will get bumped out of position. List items do not need to be unique; for example, there are now two separate items with the value <code>'a'</code>, <code>a_list[0]</code> and <code>a_list[1]</code>.
<li>The <code>insert()</code> method inserts a single item into a list. The first argument is the index of the first item in the list that will get bumped out of position. List items do not need to be unique; for example, there are now two separate items with the value <code>'&Omega;'</code>: the first item, <code>a_list[0]</code>, and the last item, <code>a_list[6]</code>.
</ol>
<blockquote class='note compare perl'>
<p><span class=u>&#x261E;</span><code><var>a_list</var>.insert(0, <var>value</var>)</code> is like the <code>shift()</code> function in Perl. It adds an item to the beginning of the list, and all the other items have their positional index bumped up to make room.
</blockquote>
<p>Let&#8217;s look closer at the difference between <code>append()</code> and <code>extend()</code>.
<pre class=screen>
<samp class=p>>>> </samp><kbd class=pp>a_list = ['a', 'b', 'c']</kbd>
<a><samp class=p>>>> </samp><kbd class=pp>a_list.extend(['d', 'e', 'f'])</kbd> <span class=u>&#x2460;</span></a>
@@ -378,7 +384,7 @@ ValueError: list.remove(x): x not in list</samp></pre>
<p>Another interesting list method is <code>pop()</code>. The <code>pop()</code> method is yet another way to <a href=#removingfromlists>remove items from a list</a>, but with a twist.
<pre class=screen>
<samp class=p>>>> </samp><kbd class=pp>a_list = ['a', 'b', 'new', 'mpilgrim']</samp>
<samp class=p>>>> </samp><kbd class=pp>a_list = ['a', 'b', 'new', 'mpilgrim']</kbd>
<a><samp class=p>>>> </samp><kbd class=pp>a_list.pop()</kbd> <span class=u>&#x2460;</span></a>
<samp class=pp>'mpilgrim'</samp>
<samp class=p>>>> </samp><kbd class=pp>a_list</kbd>
@@ -395,11 +401,16 @@ ValueError: list.remove(x): x not in list</samp></pre>
<samp class=traceback>Traceback (most recent call last):
File "&lt;stdin>", line 1, in &lt;module>
IndexError: pop from empty list</samp></pre>
<ol>
<li>When called without arguments, the <code>pop()</code> list method removes the last item in the list <em>and returns the value it removed</em>.
<li>You can pop arbitrary items from a list. Just pass a positional index to the <code>pop()</code> method. It will remove that item, shift all the items after it to &#8220;fill the gap,&#8221; and return the value it removed.
<li>All around the mulberry bush&hellip; calling <code>pop()</code> on an empty list raises an exception.
<li>Calling <code>pop()</code> on an empty list raises an exception.
</ol>
<blockquote class='note compare perl'>
<p><span class=u>&#x261E;</span>Calling the <code>pop()</code> list method without an argument is like the <code>pop()</code> function in Perl. It removes the last item from the list and returns the value of the removed item. Perl has another function, <code>shift()</code>, which removes the first item and returns its value; in Python, this is equivalent to <code><var>a_list</var>.pop(0)</code>.
</blockquote>
<h3 id=lists-in-a-boolean-context>Lists In A Boolean Context</h3>
<aside>Empty lists are false; all other lists are true.</aside>
<p>You can also use a list in <a href=#booleans>a boolean context</a>, such as an <code>if</code> statement.