IncompleteOrdering.qhelp

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<qhelp>
<overview>
<p> A class that implements the rich comparison operators
(<code>__lt__</code>, <code>__gt__</code>, <code>__le__</code>, or <code>__ge__</code>) should ensure that all four 
comparison operations <code>&lt;</code>, <code>&lt;=</code>, <code>&gt;</code>, and <code>&gt;=</code> function as expected, consistent 
with expected mathematical rules.   
In Python 3, this is ensured by implementing one of <code>__lt__</code> or <code>__gt__</code>, and one of  <code>__le__</code> or <code>__ge__</code>.
If the ordering is not consistent with default equality, then <code>__eq__</code> should also be implemented.
</p>

</overview>
<recommendation>
<p>Ensure that at least one of <code>__lt__</code> or <code>__gt__</code> and at least one of <code>__le__</code> or <code>__ge__</code> is defined. 
  </p>

<p>
The <code>functools.total_ordering</code> class decorator can be used to automatically implement all four comparison methods from a 
single one,
which is typically the cleanest way to ensure all necessary comparison methods are implemented consistently.</p>

</recommendation>
<example>
<p>In the following example, only the <code>__lt__</code> operator has been implemented, which would lead to unexpected
errors if the <code>&lt;=</code> or <code>&gt;=</code> operators are used on <code>A</code> instances.
The <code>__le__</code> method should also be defined, as well as <code>__eq__</code> in this case.</p>
<sample src="examples/IncompleteOrdering.py" />

</example>
<references>

  <li>Python Language Reference: <a href="http://docs.python.org/3/reference/datamodel.html#object.__lt__">Rich comparisons in Python</a>.</li>


</references>
</qhelp>