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<overview>

<p>

All exception classes in Python derive from <code>BaseException</code>. <code>BaseException</code> has three important subclasses,

<code>Exception</code> from which all errors and normal exceptions derive, <code>KeyboardInterrupt</code> which is raised when the

user interrupts the program from the keyboard and <code>SystemExit</code> which is raised by the <code>sys.exit()</code> function to

terminate the program.

</p>

<p>

Since <code>KeyboardInterrupt</code> and <code>SystemExit</code> are special they should not be grouped together with other

<code>Exception</code> classes.

</p>

<p>

Catching <code>BaseException</code>, rather than its subclasses may prevent proper handling of

<code>KeyboardInterrupt</code> or <code>SystemExit</code>. It is easy to catch <code>BaseException</code>

accidentally as it is caught implicitly by an empty <code>except:</code> statement.

</p>

</overview>

<recommendation>

<p>

Handle <code>Exception</code>, <code>KeyboardInterrupt</code> and <code>SystemExit</code> separately. Do not use the plain <code>except:</code> form.

</p>

</recommendation>

<example>

<p>

In these examples, a function <code>application.main()</code> is called that might raise <code>SystemExit</code>.

In the first two functions, <code>BaseException</code> is caught, but this will discard <code>KeyboardInterrupt</code>.

In the third function, <code>call_main_program_fixed</code> only <code>SystemExit</code> is caught,

leaving <code>KeyboardInterrupt</code> to propagate.

</p>

<p>In these examples <code>KeyboardInterrupt</code> is accidentally ignored.</p>

<sample src="CatchingBaseException.py" />

</example>

<references>

<li>Python Language Reference: <a href="http://docs.python.org/3/reference/compound_stmts.html#try">The try statement</a>,

<a href="http://docs.python.org/3/reference/executionmodel.html#exceptions">Exceptions</a>.</li>

<li>M. Lutz, Learning Python, Section 35.3: Exception Design Tips and Gotchas, O'Reilly Media, 2013.</li>

<li>Python Tutorial: <a href="https://docs.python.org/3/tutorial/errors.html">Errors and Exceptions</a>.</li>

</references>

</qhelp>