NotImplementedIsNotAnException.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
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<qhelp>
<overview>

<p>
The constant <code>NotImplemented</code> is not an <code>Exception</code>, but is often confused for <code>NotImplementedError</code>.
If it is used as an exception, such as in <code>raise NotImplemented</code> or <code>raise NotImplemented("message")</code>,
a <code>TypeError</code> will be raised rather than the expected <code>NotImplemented</code>. This may make debugging more difficult.
</p>

<p><code>NotImplemented</code> should only be used as a special return value for implementing special methods such as <code>__lt__</code>.
Code that is not intended to be called should raise <code>NotImplementedError</code>.</p>

</overview>
<recommendation>
<p>If a <code>NotImplementedError</code> is intended to be raised, replace the use of <code>NotImplemented</code>
with that. If <code>NotImplemented</code> is intended to be returned rather than raised, replace the <code>raise</code> with <code>return NotImplemented</code>.
</p>
</recommendation>
<example>

<p>
In the following example, the method <code>wrong</code> will incorrectly raise a <code>TypeError</code> when called.
The method <code>right</code> will raise a <code>NotImplementedError</code>.
</p>

<sample src="NotImplemented.py" />


</example>

<references>

    <li>Python Language Reference: <a href="https://docs.python.org/library/exceptions.html#NotImplementedError">The NotImplementedError exception</a>.</li>
    <li>Python Language Reference: <a href="https://docs.python.org/3/library/constants.html#NotImplemented">The NotImplemented constant</a>.</li>
    <li>Python Language Reference: <a href="https://docs.python.org/3/reference/datamodel.html#emulating-numeric-types">Emulating numeric types</a>.</li>

</references>

</qhelp>