EqualsOrHash.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<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>