kennethreitz/dive-into-python3
1
0
Fork 0
mirror of https://github.com/kennethreitz/dive-into-python3.git synced 2026-06-05 23:10:17 +00:00
Code Issues Packages Projects Releases Wiki Activity

you wouldn't believe me if I told you

Browse Source
This commit is contained in:
Mark Pilgrim
2009-06-05 23:39:50 -04:00
parent cbdb346531
commit 654b102d74
64 changed files with 724 additions and 764 deletions
Download Patch File Download Diff File Expand all files Collapse all files
native-datatypes.html
+25 -25
View File
@@ -93,7 +93,7 @@ body{counter-reset:h1 2}
<li>Floating point numbers are accurate to 15 decimal places.
<li>Integers can be arbitrarily large.
</ol>
<blockquote class="note compare python2">
<blockquote class='note compare python2'>
<p><span>&#x261E;</span>Python 2 had separate types for <code>int</code> and <code>long</code>. The <code>int</code> datatype was limited by <code>sys.maxint</code>, which varied by platform but was usually <code>2<sup>32</sup>-1</code>. Python 3 has just one integer type, which behaves mostly like the old <code>long</code> type from Python 2. See <a href=http://www.python.org/dev/peps/pep-0237><abbr>PEP</abbr> 237</a> for details.
</blockquote>
<h3 id=common-numerical-operations>Common Numerical Operations</h3>
@@ -120,7 +120,7 @@ body{counter-reset:h1 2}
<li>The <code>**</code> operator means &#8220;raised to the power of.&#8221; <code>11<sup>2</sup></code> is <code>121</code>.
<li>The <code>%</code> operator gives the remainder after performing integer division. <code>11</code> divided by <code>2</code> is <code>5</code> with a remainder of <code>1</code>, so the result here is <code>1</code>.
</ol>
<blockquote class="note compare python2">
<blockquote class='note compare python2'>
<p><span>&#x261E;</span>In Python 2, the <code>/</code> operator usually meant integer division, but you could make it behave like floating point division by including a special directive in your code. In Python 3, the <code>/</code> operator always means floating point division. See <a href=http://www.python.org/dev/peps/pep-0238/><abbr>PEP</abbr> 238</a> for details.
</blockquote>
<h3 id=fractions>Fractions</h3>
@@ -161,9 +161,9 @@ body{counter-reset:h1 2}
<pre class=screen>
<a><samp class=p>>>> </samp><kbd>def is_it_true(anything):</kbd> <span>&#x2460;</span></a>
<samp class=p>... </samp><kbd> if anything:</kbd>
<samp class=p>... </samp><kbd> print("yes, it's true")</kbd>
<samp class=p>... </samp><kbd> print('yes, it's true')</kbd>
<samp class=p>... </samp><kbd> else:</kbd>
<samp class=p>... </samp><kbd> print("no, it's false")</kbd>
<samp class=p>... </samp><kbd> print('no, it's false')</kbd>
<samp class=p>...</samp>
<a><samp class=p>>>> </samp><kbd>is_it_true(1)</kbd> <span>&#x2461;</span></a>
<samp>yes, it's true</samp>
@@ -190,10 +190,10 @@ body{counter-reset:h1 2}
<h2 id=lists>Lists</h2>
<p>Lists are Python&#8217;s workhorse datatype. When I say &#8220;list,&#8221; you might be thinking &#8220;array whose size I have to declare in advance, that can only contain items of the same type, <i class=baa>&amp;</i>c.&#8221; Don&#8217;t think that. Lists are much cooler than that.
<blockquote class="note compare perl5">
<blockquote class='note compare perl5'>
<p><span>&#x261E;</span>A list in Python is like an array in Perl 5. In Perl 5, variables that store arrays always start with the <code>@</code> character; in Python, variables can be named anything, and Python keeps track of the datatype internally.
</blockquote>
<blockquote class="note compare java">
<blockquote class='note compare java'>
<p><span>&#x261E;</span>A list in Python is much more than an array in Java (although it can be used as one if that&#8217;s really all you want out of life). A better analogy would be to the <code>ArrayList</code> class, which can hold arbitrary objects and can expand dynamically as new items are added.
</blockquote>
<h3 id=creatinglists>Creating A List</h3>
@@ -316,9 +316,9 @@ ValueError: list.index(x): x not in list</samp></pre>
<pre class=screen>
<samp class=p>>>> </samp><kbd>def is_it_true(anything):</kbd>
<samp class=p>... </samp><kbd> if anything:</kbd>
<samp class=p>... </samp><kbd> print("yes, it's true")</kbd>
<samp class=p>... </samp><kbd> print('yes, it's true')</kbd>
<samp class=p>... </samp><kbd> else:</kbd>
<samp class=p>... </samp><kbd> print("no, it's false")</kbd>
<samp class=p>... </samp><kbd> print('no, it's false')</kbd>
<samp class=p>...</samp>
<a><samp class=p>>>> </samp><kbd>is_it_true([])</kbd> <span>&#x2461;</span></a>
<samp>no, it's false</samp>
@@ -341,44 +341,44 @@ ValueError: list.index(x): x not in list</samp></pre>
<h2 id=dictionaries>Dictionaries</h2>
<p>One of Python&#8217;s most important datatypes is the dictionary, which defines one-to-one relationships between keys and values.
<blockquote class="note compare perl5">
<blockquote class='note compare perl5'>
<p><span>&#x261E;</span>A dictionary in Python is like a hash in Perl 5. In Perl 5, variables that store hashes always start with a <code>%</code> character. In Python, variables can be named anything, and Python keeps track of the datatype internally.
</blockquote>
<h3 id=creating-dictionaries>Creating A Dictionary</h3>
<p>Creating a dictionary is easy. The syntax is similar to <a href=#sets>sets</a>, but instead of values, you have key-value pairs. Once you have a dictionary, you can look up values by their key.
<pre class=screen>
<a><samp class=p>>>> </samp><kbd>a_dict = {"server":"db.diveintopython3.org", "database":"mysql"}</kbd> <span>&#x2460;</span></a>
<a><samp class=p>>>> </samp><kbd>a_dict = {'server':'db.diveintopython3.org', 'database':'mysql'}</kbd> <span>&#x2460;</span></a>
<samp class=p>>>> </samp><kbd>a_dict</kbd>
<samp>{'server': 'db.diveintopython3.org', 'database': 'mysql'}</samp>
<a><samp class=p>>>> </samp><kbd>a_dict["server"]</kbd> <span>&#x2461;</span></a>
<a><samp class=p>>>> </samp><kbd>a_dict['server']</kbd> <span>&#x2461;</span></a>
'db.diveintopython3.org'
<a><samp class=p>>>> </samp><kbd>a_dict["database"]</kbd> <span>&#x2462;</span></a>
<a><samp class=p>>>> </samp><kbd>a_dict['database']</kbd> <span>&#x2462;</span></a>
'mysql'
<a><samp class=p>>>> </samp><kbd>a_dict["db.diveintopython3.org"]</kbd> <span>&#x2463;</span></a>
<a><samp class=p>>>> </samp><kbd>a_dict['db.diveintopython3.org']</kbd> <span>&#x2463;</span></a>
<samp class=traceback>Traceback (most recent call last):
File "&lt;stdin>", line 1, in &lt;module>
KeyError: 'db.diveintopython3.org'</samp></pre>
<ol>
<li>First, you create a new dictionary with two items and assign it to the variable <var>a_dict</var>. Each item is a key-value pair, and the whole set of items is enclosed in curly braces.
<li><code>'server'</code> is a key, and its associated value, referenced by <code>a_dict["server"]</code>, is <code>'db.diveintopython3.org'</code>.
<li><code>'database'</code> is a key, and its associated value, referenced by <code>a_dict["database"]</code>, is <code>'mysql'</code>.
<li>You can get values by key, but you can&#8217;t get keys by value. So <code>a_dict["server"]</code> is <code>'db.diveintopython3.org'</code>, but <code>a_dict["db.diveintopython3.org"]</code> raises an exception, because <code>'db.diveintopython3.org'</code> is not a key.
<li><code>'server'</code> is a key, and its associated value, referenced by <code>a_dict['server']</code>, is <code>'db.diveintopython3.org'</code>.
<li><code>'database'</code> is a key, and its associated value, referenced by <code>a_dict['database']</code>, is <code>'mysql'</code>.
<li>You can get values by key, but you can&#8217;t get keys by value. So <code>a_dict['server']</code> is <code>'db.diveintopython3.org'</code>, but <code>a_dict['db.diveintopython3.org']</code> raises an exception, because <code>'db.diveintopython3.org'</code> is not a key.
</ol>
<h3 id=modifying-dictionaries>Modifying A Dictionary</h3>
<p>Dictionaries do not have any predefined size limit. You can add new key-value pairs to a dictionary at any time, or you can modify the value of an existing key. Continuing from the previous example:
<pre class=screen>
<samp class=p>>>> </samp><kbd>a_dict</kbd>
<samp>{'server': 'db.diveintopython3.org', 'database': 'mysql'}</samp>
<a><samp class=p>>>> </samp><kbd>a_dict["database"] = "blog"</kbd> <span>&#x2460;</span></a>
<a><samp class=p>>>> </samp><kbd>a_dict['database'] = 'blog'</kbd> <span>&#x2460;</span></a>
<samp class=p>>>> </samp><kbd>a_dict</kbd>
<samp>{'server': 'db.diveintopython3.org', 'database': 'blog'}</samp>
<a><samp class=p>>>> </samp><kbd>a_dict["user"] = "mark"</kbd> <span>&#x2461;</span></a>
<a><samp class=p>>>> </samp><kbd>a_dict['user'] = 'mark'</kbd> <span>&#x2461;</span></a>
<a><samp class=p>>>> </samp><kbd>a_dict</kbd> <span>&#x2462;</span></a>
<samp>{'server': 'db.diveintopython3.org', 'user': 'mark', 'database': 'blog'}</samp>
<a><samp class=p>>>> </samp><kbd>a_dict["user"] = "dora"</kbd> <span>&#x2463;</span></a>
<a><samp class=p>>>> </samp><kbd>a_dict['user'] = 'dora'</kbd> <span>&#x2463;</span></a>
<samp class=p>>>> </samp><kbd>a_dict</kbd>
<samp>{'server': 'db.diveintopython3.org', 'user': 'dora', 'database': 'blog'}</samp>
<a><samp class=p>>>> </samp><kbd>a_dict["User"] = "mark"</kbd> <span>&#x2464;</span></a>
<a><samp class=p>>>> </samp><kbd>a_dict['User'] = 'mark'</kbd> <span>&#x2464;</span></a>
<samp class=p>>>> </samp><kbd>a_dict</kbd>
<samp>{'User': 'mark', 'server': 'db.diveintopython3.org', 'user': 'dora', 'database': 'blog'}</samp></pre>
<ol>
@@ -417,9 +417,9 @@ KeyError: 'db.diveintopython3.org'</samp></pre>
<pre class=screen>
<samp class=p>>>> </samp><kbd>def is_it_true(anything):</kbd>
<samp class=p>... </samp><kbd> if anything:</kbd>
<samp class=p>... </samp><kbd> print("yes, it's true")</kbd>
<samp class=p>... </samp><kbd> print('yes, it's true')</kbd>
<samp class=p>... </samp><kbd> else:</kbd>
<samp class=p>... </samp><kbd> print("no, it's false")</kbd>
<samp class=p>... </samp><kbd> print('no, it's false')</kbd>
<samp class=p>...</samp>
<a><samp class=p>>>> </samp><kbd>is_it_true({})</kbd> <span>&#x2460;</span></a>
<samp>no, it's false</samp>
@@ -457,9 +457,9 @@ KeyError: 'db.diveintopython3.org'</samp></pre>
<pre class=screen>
<samp class=p>>>> </samp><kbd>def is_it_true(anything):</kbd>
<samp class=p>... </samp><kbd> if anything:</kbd>
<samp class=p>... </samp><kbd> print("yes, it's true")</kbd>
<samp class=p>... </samp><kbd> print('yes, it's true')</kbd>
<samp class=p>... </samp><kbd> else:</kbd>
<samp class=p>... </samp><kbd> print("no, it's false")</kbd>
<samp class=p>... </samp><kbd> print('no, it's false')</kbd>
<samp class=p>...</samp>
<samp class=p>>>> </samp><kbd>is_it_true(None)</kbd>
<samp>no, it's false</samp>
@@ -474,7 +474,7 @@ KeyError: 'db.diveintopython3.org'</samp></pre>
<li><a href=http://www.python.org/dev/peps/pep-0237/><abbr>PEP</abbr> 237: Unifying Long Integers and Integers</a>
<li><a href=http://www.python.org/dev/peps/pep-0238/><abbr>PEP</abbr> 238: Changing the Division Operator</a>
</ul>
<p class=nav><a rel=prev href=your-first-python-program.html title="back to &#8220;Your First Python Program&#8221;"><span>&#x261C;</span></a> <a rel=next href=strings.html title="onward to &#8220;Strings&#8221;"><span>&#x261E;</span></a>
<p class=v><a href=your-first-python-program.html rel=prev title='back to &#8220;Your First Python Program&#8221;'><span>&#x261C;</span></a> <a href=strings.html rel=next title='onward to &#8220;Strings&#8221;'><span>&#x261E;</span></a>
<p class=c>&copy; 2001&ndash;9 <a href=about.html>Mark Pilgrim</a>
<script src=j/jquery.js></script>
<script src=j/dip3.js></script>