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Mark Pilgrim
2009-04-30 12:47:12 -04:00
parent 4d69a47f98
commit dc32d361cc
1 changed files with 2 additions and 2 deletions
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special-method-names.html
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@@ -212,7 +212,7 @@ __or__
<samp class=p>>>> </samp><kbd>1 / x</kbd>
<samp>Fraction(3, 1)</samp></pre>
<p>This is <em>not</em> a case of taking a <code>Fraction</code> and dividing it by an integer (as in the previous example). That case was straightforward: <code>x / 3</code> calls <code>x.__truediv__(3)</code>, and the <code>__truedive__()</code> method of the <code>Fraction</code> class handles all the math. But integers don&#8217;t &#8220;know&#8221; how to do arithmetic operations with fractions. So why does this example work?
<p>This is <em>not</em> a case of taking a <code>Fraction</code> and dividing it by an integer (as in the previous example). That case was straightforward: <code>x / 3</code> calls <code>x.__truediv__(3)</code>, and the <code>__truediv__()</code> method of the <code>Fraction</code> class handles all the math. But integers don&#8217;t &#8220;know&#8221; how to do arithmetic operations with fractions. So why does this example work?
<p>The answer lies in a second set of arithmetic special methods with <i>reflected operands</i>. Given an arithmetic operation that takes two operands (<i>e.g.</i> <code>x / y</code>), there are two ways to go about it:
@@ -374,7 +374,7 @@ __ior__
<ol>
<li>Try calling <code>x.__itruediv__(<var>y</var>)</code>. If this method is defined and returns a value other than <code>NotImplemented</code>, we&#8217;re done.
<li>Try calling <code>x.__truediv__(<var>y</var>)</code>. If this method is defined and returns a value other than <code>NotImplemented</code>, the old value of <var>x</var> is discarded and replaced with the return value, just as if you had done <code> x = x / y</code> instead.
<li>Try calling <code>y.__rtruediv__(<var>y</var>)</code>. If this method is defined and returns a value other than <code>NotImplemented</code>, the old value of <var>x</var> is discarded and replaced with the return value.
<li>Try calling <code>y.__rtruediv__(<var>x</var>)</code>. If this method is defined and returns a value other than <code>NotImplemented</code>, the old value of <var>x</var> is discarded and replaced with the return value.
</ol>
<p>So you only need to define in-place methods like the <code>__itruediv__()</code> method if you want to do some special optimization for in-place operands. Otherwise Python will essentially reformulate the in-place operand to use a regular operand + a variable assignment.