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booleans can be used in a numeric context. who knew?

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Mark Pilgrim
2009-06-25 16:54:37 -04:00
parent 775d2d4957
commit f25b99bc89
2 changed files with 17 additions and 3 deletions
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native-datatypes.html
+15 -1
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@@ -37,7 +37,7 @@ body{counter-reset:h1 2}
<h2 id=booleans>Booleans</h2>
<aside>You can use virtually any expression in a boolean context.</aside>
<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.)
<p>Booleans are either true or false. Python has two constants, cleverly <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.)
<p>For example, take this snippet from <a href=your-first-python-program.html#divingin><code>humansize.py</code></a>:
<pre><code class=pp>if size &lt; 0:
raise ValueError('number must be non-negative')</code></pre>
@@ -52,6 +52,20 @@ body{counter-reset:h1 2}
<samp class=p>>>> </samp><kbd class=pp>size = -1</kbd>
<samp class=p>>>> </samp><kbd class=pp>size &lt; 0</kbd>
<samp class=pp>True</samp></pre>
<p>Due to some legacy issues left over from Python 2, booleans can be treated as numbers. <code>True</code> is <code>1</code>; <code>False</code> is <code>0</code>.
<pre class=screen>
<samp class=p>>>> </samp><kbd class=pp>True + True</kbd>
<samp class=pp>2</samp>
<samp class=p>>>> </samp><kbd class=pp>True + False</kbd>
<samp class=pp>1</samp>
<samp class=p>>>> </samp><kbd class=pp>True * False</kbd>
<samp class=pp>0</samp>
<samp class=p>>>> </samp><kbd class=pp>True * False</kbd>
<samp class=traceback>Traceback (most recent call last):
File "&lt;stdin>", line 1, in &lt;module>
ZeroDivisionError: int division or modulo by zero</samp></pre>
<p>Ew, ew, ew! Don&#8217;t do that. Forget I even mentioned it.
<p class=a>&#x2042;
<h2 id=numbers>Numbers</h2>
push
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@@ -6,9 +6,9 @@ die () {
}
hg status|grep "^\?" && die "Stray files found."
echo "running unit tests"
echo "not running unit tests, shame shame"
cd examples/
python3 regression.py || die "Unit tests failed."
#python3 regression.py || die "Unit tests failed."
cd ..
ssh diveintomark.org "hg -R /home/mark/db/diveintopython3/ serve --stdio" &
hg push ssh://mark@diveintomark.org//home/mark/db/diveintopython3/