<!DOCTYPE qhelp PUBLIC

"-//Semmle//qhelp//EN"

"qhelp.dtd">

<qhelp>

<overview>

<p>If an object is called, <code>obj()</code>, then that object must be a callable or

a <code>TypeError</code> will be raised. A callable object is any object whose class defines

the <code>__call__</code> special method.

Callable objects include functions, methods, classes.</p>

<p>The <code>callable(object)</code> builtin function determines if an object is callable or not.</p>

<p>

When the Python interpreter attempts to evaluate a call such as <code>func(arg)</code> it will

invoke the <code>__call__</code> special method on <code>func</code>.

Thus, <code>func(arg)</code> is roughly equivalent to <code>type(func).__call__(func, arg)</code>

which means that the <em>class</em> must define the attribute <code>__call__</code>,

merely adding it to the instance is not sufficient.

</p>

</overview>

<recommendation>

<p>Since this problem usually indicates a logical error, it is not possible to give a general recipe for fixing it.</p>

</recommendation>

<example>

<p><code>list</code>s are not callable. In this example, an attempt is made to call a <code>list</code>

which will fail with a <code>TypeError</code>.

</p>

<sample src="NonCallableCalled.py" />

</example>

<references>

<li>Python Standard Library: <a href="http://docs.python.org/2/library/functions.html#callable">callable</a>.</li>

<li>Python Language Reference: <a href="http://docs.python.org/2/reference/datamodel.html#object.__call__">object.__call__</a>.</li>

</references>

</qhelp>