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Mark Pilgrim
294 lines
19 KiB
HTML
294 lines
19 KiB
HTML
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<title>Native datatypes - Dive into Python 3</title>
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<p class="skip"><a href="#divingin">skip to main content</a>
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<p class="nav">You are here: <a href="/">Home</a> <span>‣</span> <a href="table-of-contents.html">Dive Into Python 3</a> <span>‣</span>
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<h1>Native datatypes</h1>
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<blockquote class="q">
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<p><span>❝</span> Wonder is the foundation of all philosophy, research its progress, ignorance its end. <span>❞</span><br>— <cite>Michel de Montaigne</cite>
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</blockquote>
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<ol>
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<li><a href="#divingin">Diving in</a>
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<li><a href="#booleans">Booleans</a>
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<li><a href="#numbers">Numbers</a>
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<ol>
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<li><a href="#integers">Integers</a>
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<li><a href="#floats">Floating point numbers</a>
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<li><a href="#fractions">Fractions</a>
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<li><a href="#complexnumbers">Complex numbers</a>
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<li><a href="#numberoperations">Common operations on numbers</a>
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<li><a href="#math">The <code>math</code> module</a>
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</ol>
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<li><a href="#lists">Lists</a>
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<li><a href="#sets">Sets</a>
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<li><a href="#dictionaries">Dictionaries
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<li><a href="#none"><code>None</code></a>
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</ol>
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<h2 id="divingin">Diving in</h2>
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<p class="fancy">A short digression is in order. Put aside <a href="your-first-python-program.html">your first Python program</a> for just a minute, and let's talk about Python's native datatypes. These types are the foundation on which you will build all your future Python programs.
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<p>Python has many native datatypes. Here are the important ones:
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<ol>
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<li><b>Booleans</b> are either <code>True</code> or <code>False</code>.
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<li><b>Numbers</b> can be integers (<code>1</code> and <code>2</code>), floats (<code>1.1</code> and <code>1.2</code>), fractions (<code>1/2</code> and <code>2/3</code>), or even complex numbers (<code><var>i</var></code>, the square root of <code>-1</code>).
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<li><b>Strings</b> are sequences of Unicode characters, <i>e.g.</i> an <abbr>HTML</abbr> document.
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<li><b>Bytes</b> and <b>byte arrays</b>, <i>e.g.</i> a <abbr>JPEG</abbr> image file.
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<li><b>Lists</b> are ordered sequences of values.
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<li><b>Sets</b> are unordered bags of values.
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<li><b>Dictionaries</b> are unordered bags of key-value pairs.
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</ol>
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<p>Of course, there are a lot more types than these seven. <a href="your-first-python-program.html#everythingisanobject">Everything is an object</a> in Python, so there are types like <i>module</i>, <i>function</i>, <i>class</i>, <i>method</i>, <i>file</i>, and even <i>compiled code</i>. You've already seen some of these: <a href="your-first-python-program.html#runningscripts">modules have names</a>, <a href="your-first-python-program.html#docstrings">functions have <code>docstrings</code></a>, <i class="baa">&</i>c. You'll learn about classes in [FIXME xref] and files in [FIXME xref].
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<p>Strings and bytes are important enough — and complicated enough — that they get their own chapter. Let's look at the others first.
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<h2 id="booleans">Booleans</h2>
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<p>Booleans are either true or false. Python has two constants, <code>True</code> and <code>False</code>, which can be used to assign boolean values directly. Expressions can also evaluate to a boolean value. In certain places (like <code>if</code> statements), Python expects an expression to evaluate to a boolean value. These places are called <i>boolean contexts</i>. You can use virtually any expression in a boolean context, and Python will try to determine its truth value. Different datatypes have different rules about which values are true or false in a boolean context. (This will make more sense once you see some concrete examples later in this chapter.)
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<p>For example, take this snippet from <a href="your-first-python-program.html#divingin"><code>humansize.py</code></a>:
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<pre><code>if size < 0:
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raise ValueError('number must be non-negative')</code></pre>
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<p><var>size</var> is an integer, <code>0</code> is an integer, and <code><</code> is a numerical operator. The result of the expression <code>size < 0</code> is always a boolean. You can test this yourself in the Python interactive shell:
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<pre class="screen">
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<samp class="prompt">>>> </samp><kbd>size = 1</kbd>
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<samp class="prompt">>>> </samp><kbd>size < 0</kbd>
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<samp>False</samp>
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<samp class="prompt">>>> </samp><kbd>size = 0</kbd>
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<samp class="prompt">>>> </samp><kbd>size < 0</kbd>
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<samp>False</samp>
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<samp class="prompt">>>> </samp><kbd>size = -1</kbd>
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<samp class="prompt">>>> </samp><kbd>size < 0</kbd>
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<samp>True</samp></pre>
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<h2 id="numbers">Numbers</h2>
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<h3 id="integers">Integers</h3>
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<h3 id="floats">Floating point numbers</h3>
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<h3 id="fractions">Fractions</h3>
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<h3 id="complexnumbers">Complex numbers</h3>
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<h3 id="numberoperations">Common operations on numbers</h3>
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<h3 id="math">The <code>math</code> module</h3>
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<h2 id="none"><code>None</code></h2>
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<p><code>None</code> is a special constant in Python. It is a null value. <code>None</code> is not <code>False</code>; it is not <code>0</code>; it is not an empty string. Comparing <code>None</code> to anything other than <code>None</code> will always return <code>False</code>.
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<p><code>None</code> is the only null value. It has its own datatype (<code>NoneType</code>). You can assign <code>None</code> to any variable, but you can not create other <code>NoneType</code> objects. All variables whose value is <code>None</code> are equal to each other.
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<pre class="screen">
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<samp class="prompt">>>> </samp><kbd>type(None)</kbd>
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<samp><class 'NoneType'></samp>
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<samp class="prompt">>>> </samp><kbd>None == False</kbd>
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<samp>False</samp>
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<samp class="prompt">>>> </samp><kbd>None == 0</kbd>
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<samp>False</samp>
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<samp class="prompt">>>> </samp><kbd>None == ''</kbd>
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<samp>False</samp>
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<samp class="prompt">>>> </samp><kbd>None == None</kbd>
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<samp>True</samp>
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<samp class="prompt">>>> </samp><kbd>x = None</kbd>
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<samp class="prompt">>>> </samp><kbd>x == None</kbd>
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<samp>True</samp>
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<samp class="prompt">>>> </samp><kbd>y = None</kbd>
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<samp class="prompt">>>> </samp><kbd>x == y</kbd>
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<samp>True</samp>
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</pre>
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<!--
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<h2 id="odbchelper.dict">3.1. Introducing Dictionaries</h2>
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<p>One of Python's built-in datatypes is the dictionary, which defines one-to-one relationships between keys and values.<table id="compare.dict.perl" class="note" border="0" summary="">
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<tr>
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<td rowspan="2" align="center" valign="top" width="1%"><img src="images/note.png" alt="Note" title="" width="24" height="24"></td>
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</tr>
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<tr>
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<td colspan="2" align="left" valign="top" width="99%">A dictionary in Python is like a hash in Perl. In Perl, 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.
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</td>
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</tr>
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</table><table id="compare.dict.java" class="note" border="0" summary="">
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<tr>
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<td rowspan="2" align="center" valign="top" width="1%"><img src="images/note.png" alt="Note" title="" width="24" height="24"></td>
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</tr>
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<tr>
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<td colspan="2" align="left" valign="top" width="99%">A dictionary in Python is like an instance of the <code>Hashtable</code> class in Java.
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</td>
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</tr>
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</table><table id="compare.dict.vb" class="note" border="0" summary="">
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<tr>
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<td rowspan="2" align="center" valign="top" width="1%"><img src="images/note.png" alt="Note" title="" width="24" height="24"></td>
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</tr>
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<tr>
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<td colspan="2" align="left" valign="top" width="99%">A dictionary in Python is like an instance of the <code>Scripting.Dictionary</code> object in Visual Basic.
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</td>
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</tr>
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</table>
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<h3>3.1.1. Defining Dictionaries</h3>
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<div class="example"><h3 id="odbchelper.dict.define">Example 3.1. Defining a Dictionary</h3><pre class="screen"><samp class="prompt">>>> </samp>d = {"server":"mpilgrim", "database":"master"} <img id="odbchelper.dict.1.1" src="images/callouts/1.png" alt="1" border="0" width="12" height="12">
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<samp class="prompt">>>> </samp>d
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{'server': 'mpilgrim', 'database': 'master'}
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<samp class="prompt">>>> </samp>d["server"]<img id="odbchelper.dict.1.2" src="images/callouts/2.png" alt="2" border="0" width="12" height="12">
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'mpilgrim'
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<samp class="prompt">>>> </samp>d["database"] <img id="odbchelper.dict.1.3" src="images/callouts/3.png" alt="3" border="0" width="12" height="12">
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'master'
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<samp class="prompt">>>> </samp>d["mpilgrim"] <img id="odbchelper.dict.1.4" src="images/callouts/4.png" alt="4" border="0" width="12" height="12">
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<samp class="traceback">Traceback (innermost last):
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File "<interactive input>", line 1, in ?
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KeyError: mpilgrim</span></pre><div class="calloutlist">
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<table border="0" summary="Callout list">
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.1.1"><img src="images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left">First, you create a new dictionary with two elements and assign it to the variable <var>d</var>. Each element is a key-value pair, and the whole set of elements is enclosed in curly braces.
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</td>
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</tr>
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.1.2"><img src="images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left"><code>'server'</code> is a key, and its associated value, referenced by <code>d["server"]</code>, is <code>'mpilgrim'</code>.
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</td>
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</tr>
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.1.3"><img src="images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left"><code>'database'</code> is a key, and its associated value, referenced by <code>d["database"]</code>, is <code>'master'</code>.
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</td>
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</tr>
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.1.4"><img src="images/callouts/4.png" alt="4" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left">You can get values by key, but you can't get keys by value. So <code>d["server"]</code> is <code>'mpilgrim'</code>, but <code>d["mpilgrim"]</code> raises an exception, because <code>'mpilgrim'</code> is not a key.
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</td>
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</tr>
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</table>
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<h3>3.1.2. Modifying Dictionaries</h3>
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<div class="example"><h3 id="odbchelper.dict.modify">Example 3.2. Modifying a Dictionary</h3><pre class="screen"><samp class="prompt">>>> </samp>d
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{'server': 'mpilgrim', 'database': 'master'}
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<samp class="prompt">>>> </samp>d["database"] = "pubs" <img id="odbchelper.dict.2.1" src="images/callouts/1.png" alt="1" border="0" width="12" height="12">
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<samp class="prompt">>>> </samp>d
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{'server': 'mpilgrim', 'database': 'pubs'}
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<samp class="prompt">>>> </samp>d["uid"] = "sa" <img id="odbchelper.dict.2.2" src="images/callouts/2.png" alt="2" border="0" width="12" height="12">
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<samp class="prompt">>>> </samp>d
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{'server': 'mpilgrim', 'uid': 'sa', 'database': 'pubs'}</pre><div class="calloutlist">
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<table border="0" summary="Callout list">
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.2.1"><img src="images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left">You can not have duplicate keys in a dictionary. Assigning a value to an existing key will wipe out the old value.</td>
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</tr>
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.2.2"><img src="images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left">You can add new key-value pairs at any time. This syntax is identical to modifying existing values. (Yes, this will annoy
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you someday when you think you are adding new values but are actually just modifying the same value over and over because
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your key isn't changing the way you think it is.)
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</td>
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</tr>
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</table>
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<p>Note that the new element (key <code>'uid'</code>, value <code>'sa'</code>) appears to be in the middle. In fact, it was just a coincidence that the elements appeared to be in order in the first
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example; it is just as much a coincidence that they appear to be out of order now.<table id="tip.dictorder" class="note" border="0" summary="">
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<tr>
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<td rowspan="2" align="center" valign="top" width="1%"><img src="images/note.png" alt="Note" title="" width="24" height="24"></td>
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</tr>
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<tr>
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<td colspan="2" align="left" valign="top" width="99%">Dictionaries have no concept of order among elements. It is incorrect to say that the elements are “out of order”; they are simply unordered. This is an important distinction that will annoy you when you want to access the elements of
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a dictionary in a specific, repeatable order (like alphabetical order by key). There are ways of doing this, but they're
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not built into the dictionary.
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</td>
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</tr>
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</table>
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<p>When working with dictionaries, you need to be aware that dictionary keys are case-sensitive.
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<div class="example"><h3 id="odbchelper.dict.case">Example 3.3. Dictionary Keys Are Case-Sensitive</h3><pre class="screen">
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<samp class="prompt">>>> </samp>d = {}
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<samp class="prompt">>>> </samp>d["key"] = "value"
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<samp class="prompt">>>> </samp>d["key"] = "other value" <img id="odbchelper.dict.5.1" src="images/callouts/1.png" alt="1" border="0" width="12" height="12">
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<samp class="prompt">>>> </samp>d
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{'key': 'other value'}
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<samp class="prompt">>>> </samp>d["Key"] = "third value" <img id="odbchelper.dict.5.2" src="images/callouts/2.png" alt="2" border="0" width="12" height="12">
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<samp class="prompt">>>> </samp>d
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{'Key': 'third value', 'key': 'other value'}
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</pre><div class="calloutlist">
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<table border="0" summary="Callout list">
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.5.1"><img src="images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left">Assigning a value to an existing dictionary key simply replaces the old value with a new one.</td>
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</tr>
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.5.2"><img src="images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left">This is not assigning a value to an existing dictionary key, because strings in Python are case-sensitive, so <code>'key'</code> is not the same as <code>'Key'</code>. This creates a new key/value pair in the dictionary; it may look similar to you, but as far as Python is concerned, it's completely different.
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</td>
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</tr>
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</table>
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<div class="example"><h3 id="odbchelper.dictionarytypes">Example 3.4. Mixing Datatypes in a Dictionary</h3><pre class="screen"><samp class="prompt">>>> </samp>d
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{'server': 'mpilgrim', 'uid': 'sa', 'database': 'pubs'}
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<samp class="prompt">>>> </samp>d["retrycount"] = 3 <img id="odbchelper.dict.3.1" src="images/callouts/1.png" alt="1" border="0" width="12" height="12">
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<samp class="prompt">>>> </samp>d
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{'server': 'mpilgrim', 'uid': 'sa', 'database': 'master', 'retrycount': 3}
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<samp class="prompt">>>> </samp>d[42] = "douglas" <img id="odbchelper.dict.3.2" src="images/callouts/2.png" alt="2" border="0" width="12" height="12">
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<samp class="prompt">>>> </samp>d
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<samp class="computeroutput">{'server': 'mpilgrim', 'uid': 'sa', 'database': 'master',
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42: 'douglas', 'retrycount': 3}</span></pre><div class="calloutlist">
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<table border="0" summary="Callout list">
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.3.1"><img src="images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left">Dictionaries aren't just for strings. Dictionary values can be any datatype, including strings, integers, objects, or even
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other dictionaries. And within a single dictionary, the values don't all need to be the same type; you can mix and match
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as needed.
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</td>
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</tr>
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.3.2"><img src="images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left">Dictionary keys are more restricted, but they can be strings, integers, and a few other types. You can also mix and match
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key datatypes within a dictionary.
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</td>
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</tr>
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</table>
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<h3>3.1.3. Deleting Items From Dictionaries</h3>
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<div class="example"><h3 id="odbchelper.dict.del">Example 3.5. Deleting Items from a Dictionary</h3><pre class="screen"><samp class="prompt">>>> </samp>d
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<samp class="computeroutput">{'server': 'mpilgrim', 'uid': 'sa', 'database': 'master',
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42: 'douglas', 'retrycount': 3}</samp>
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<samp class="prompt">>>> </samp>del d[42] <img id="odbchelper.dict.4.1" src="images/callouts/1.png" alt="1" border="0" width="12" height="12">
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<samp class="prompt">>>> </samp>d
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{'server': 'mpilgrim', 'uid': 'sa', 'database': 'master', 'retrycount': 3}
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<samp class="prompt">>>> </samp>d.clear() <img id="odbchelper.dict.4.2" src="images/callouts/2.png" alt="2" border="0" width="12" height="12">
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<samp class="prompt">>>> </samp>d
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{}</pre><div class="calloutlist">
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<table border="0" summary="Callout list">
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.4.1"><img src="images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left"><code>del</code> lets you delete individual items from a dictionary by key.
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</td>
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</tr>
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<tr>
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<td width="12" valign="top" align="left"><a href="#odbchelper.dict.4.2"><img src="images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
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</td>
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<td valign="top" align="left"><code>clear</code> deletes all items from a dictionary. Note that the set of empty curly braces signifies a dictionary without any items.
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</td>
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</tr>
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</table>
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<div class="itemizedlist">
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<h3>Further Reading on Dictionaries</h3>
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<ul>
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<li><a href="http://www.ibiblio.org/obp/thinkCSpy/" title="Python book for computer science majors"><i class="citetitle">How to Think Like a Computer Scientist</i></a> teaches about dictionaries and shows how to <a href="http://www.ibiblio.org/obp/thinkCSpy/chap10.htm">use dictionaries to model sparse matrices</a>.
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<li><a href="http://www.faqts.com/knowledge-base/index.phtml/fid/199/">Python Knowledge Base</a> has a lot of <a href="http://www.faqts.com/knowledge-base/index.phtml/fid/541">example code using dictionaries</a>.
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<li><a href="http://www.activestate.com/ASPN/Python/Cookbook/" title="growing archive of annotated code samples">Python Cookbook</a> discusses <a href="http://www.activestate.com/ASPN/Python/Cookbook/Recipe/52306">how to sort the values of a dictionary by key</a>.
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<li><a href="http://www.python.org/doc/current/lib/"><i class="citetitle">Python Library Reference</i></a> summarizes <a href="http://www.python.org/doc/current/lib/typesmapping.html">all the dictionary methods</a>.
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</ul>
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<p class="c">© 2001-4, 2009 <span>ℳ</span>ark Pilgrim, <a rel="license" href="http://creativecommons.org/licenses/by/3.0/">CC-BY-3.0</a>
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