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<qhelp>

<overview>

<p>In order to conform to the object model, classes that define their own equality method should also

define their own hash method, or be unhashable. If the hash method is not defined then the <code>hash</code> of the

super class is used. This is unlikely to result in the expected behavior.</p>

<p>A class can be made unhashable by setting its <code>__hash__</code> attribute to <code>None</code>.</p>

<p>In Python 3, if you define a class-level equality method and omit a <code>__hash__</code> method

then the class is automatically marked as unhashable.</p>

</overview>

<recommendation>

<p>When you define an <code>__eq__</code> method for a class, remember to implement a <code>__hash__</code> method or set

<code>__hash__ = None</code>.</p>

</recommendation>

<example>

<p>In the following example the <code>Point</code> class defines an equality method but

no hash method. If hash is called on this class then the hash method defined for <code>object</code>

is used. This is unlikely to give the required behavior. The <code>PointUpdated</code> class

is better as it defines both an equality and a hash method.

If <code>Point</code> was not to be used in <code>dict</code>s or <code>set</code>s, then it could be defined as

<code>UnhashablePoint</code> below.

</p>

<p>

To comply fully with the object model this class should also define an inequality method (identified

by a separate rule).</p>

<sample src="EqualsOrHash.py" />

</example>

<references>

<li>Python Language Reference: <a href="http://docs.python.org/reference/datamodel.html#object.__hash__">object.__hash__</a>.</li>

<li>Python Glossary: <a href="http://docs.python.org/2/glossary.html#term-hashable">hashable</a>.</li>

</references>

</qhelp>