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<overview>

<p> When the signature of a method of a base class and a method of a subclass that overrides it don't match, a call to the base class method

may not be a valid call to the subclass method, and thus raise an exception if an instance of the subclass is passed instead.

If following the Liskov Substitution Principle, in which an instance of a subclass should be usable in every context as though it were an

instance of the base class, this behavior breaks the principle.

</p>

</overview>

<recommendation>

<p>Ensure that the overriding method in the subclass accepts the same parameters as the base method. </p>

</recommendation>

<example>

<p>In the following example, <code>Base.runsource</code> takes an optional <code>filename</code> argument. However, the overriding method

<code>Sub.runsource</code> does not. This means the <code>run</code> function will fail if passed an instance of <code>Sub</code>.

</p>

<sample src="SignatureOverriddenMethod.py" />

</example>

<references>

<li>Wikipedia: <a href="http://en.wikipedia.org/wiki/Liskov_substitution_principle">Liskov Substitution Principle</a>, <a href="http://en.wikipedia.org/wiki/Method_overriding#Python">Method overriding</a>.</li>

</references>

</qhelp>