what is __contains__ do, which one can call __contains__ function
<p>the next is my code:</p>

<pre><code>class a(object):
    d='ddd'
    def __contains__(self):
        if self.d:return true
b=a()
print b.contains('d')#error
print contains(b,'d')#error
</code></pre>

<p>thanks</p>
