EqualsOrHash.qhelp

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

<overview>
<p>A hashable class has an <code>__eq__</code> method, and a <code>__hash__</code> method that agrees with equality. 
When a hash method is defined, an equality method should also be defined; otherwise object identity is used for equality comparisons 
which may not be intended.
 </p>

<p>Note that defining an <code>__eq__</code> method without defining a <code>__hash__</code> method automatically makes the class unhashable in Python 3.
(even if a superclass defines a hash method).</p>

</overview>
<recommendation>

<p>
If a <code>__hash__</code> method is defined, ensure a compatible <code>__eq__</code> method is also defined.
</p>

<p>
To explicitly declare a class as unhashable, set <code>__hash__ = None</code>, rather than defining a <code>__hash__</code> method that always
raises an exception. Otherwise, the class would be incorrectly identified as hashable by an <code>isinstance(obj, collections.abc.Hashable)</code> call.
</p>

</recommendation>
<example>
<p>In the following example, the <code>A</code> class defines an hash method but 
no equality method. Equality will be determined by object identity, which may not be the expected behaviour.
</p>

<sample src="examples/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/3/glossary.html#term-hashable">hashable</a>.</li>


</references>
</qhelp>