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Mark Pilgrim
2009-08-05 14:49:32 -07:00
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commit fb0aa874df
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@@ -40,7 +40,7 @@ body{counter-reset:h1 2}
<aside>You can use virtually any expression in a boolean context.</aside>
<p>Booleans are either true or false. Python has two constants, cleverly <code><dfn>True</dfn></code> and <code><dfn>False</dfn></code>, which can be used to assign <dfn>boolean</dfn> 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.)
<p>For example, take this snippet from <a href=your-first-python-program.html#divingin><code>humansize.py</code></a>:
<pre class=nd><code class=pp>if size &lt; 0:
<pre class='nd pp'><code>if size &lt; 0:
raise ValueError('number must be non-negative')</code></pre>
<p><var>size</var> is an integer, <code>0</code> is an integer, and <code>&lt;</code> is a numerical operator. The result of the expression <code>size &lt; 0</code> is always a boolean. You can test this yourself in the Python interactive shell:
<pre class='nd screen'>
@@ -865,7 +865,7 @@ KeyError: 'db.diveintopython3.org'</samp></pre>
<h3 id=mixed-value-dictionaries>Mixed-Value Dictionaries</h3>
<p>Dictionaries aren&#8217;t just for strings. Dictionary values can be any datatype, including integers, booleans, arbitrary objects, or even other dictionaries. And within a single dictionary, the values don&#8217;t all need to be the same type; you can mix and match as needed. Dictionary keys are more restricted, but they can be strings, integers, and a few other types. You can also mix and match key datatypes within a dictionary.
<p>In fact, you&#8217;ve already seen a dictionary with non-string keys and values, in <a href=your-first-python-program.html#divingin>your first Python program</a>.
<pre class=nd><code class=pp>SUFFIXES = {1000: ['KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'],
<pre class='nd pp'><code>SUFFIXES = {1000: ['KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'],
1024: ['KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB']}</code></pre>
<p>Let's tear that apart in the interactive shell.
<pre class=screen>