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copied!<p>The reason why there is no <code>is Empty</code> is astoundingly simple once you understand what the <code>is</code> operator does.</p> <p>From the <a href="http://docs.python.org/2/reference/expressions.html#is">python manual</a>:</p> <blockquote> <p>The operators <code>is</code> and <code>is not</code> test for object identity: <code>x is y</code> is true if and only if <code>x</code> and <code>y</code> are the same object. <code>x is not y</code> yields the inverse truth value.</p> </blockquote> <p>That means <code>some_sequence is Empty</code> checks whether <code>some_sequence</code> is the same object as <code>Empty</code>. That cannot work the way you suggested.</p> <p>Consider the following example:</p> <pre><code>>>> a = [] >>> b = {} </code></pre> <p>Now let's pretend there is this <code>is Empty</code> construct in python:</p> <pre><code>>>> a is Empty True >>> b is Empty True </code></pre> <p>But since the <code>is</code> operator does identity check that means that <code>a</code> and <code>b</code> are identical to <code>Empty</code>. That in turn must mean that <code>a</code> and <code>b</code> are identical, but they are not:</p> <pre><code>>>> a is b False </code></pre> <p>So to answer your question "why is there no <code>is Empty</code> in python?": because <code>is</code> does identity check.</p> <p>In order to have the <code>is Empty</code> construct you must either hack the <code>is</code> operator to mean something else or create some magical <code>Empty</code> object which somehow detects empty collections and then be identical to them.</p> <p>Rather than asking why there is no <code>is Empty</code> you should ask why there is no builtin function <code>isempty()</code> which calls the special method <code>__isempty__()</code>.</p> <p>So instead of using implicit booleaness:</p> <pre><code>if saved: mess_up() </code></pre> <p>we have explicit empty check:</p> <pre><code>if not isempty(saved): mess_up() </code></pre> <p>where the class of <code>saved</code> has an <code>__isempty__()</code> method implemented to some sane logic.</p> <p>I find that far better than using implicit booleaness for emptyness check.</p> <p>Of course you can easily define your own <code>isempty()</code> function:</p> <pre><code>def isempty(collection): try: return collection.__isempty__() except AttributeError: # fall back to implicit booleaness but check for common pitfalls if collection is None: raise TypeError('None cannot be empty') if collection is False: raise TypeError('False cannot be empty') if collection == 0: raise TypeError('0 cannot be empty') return bool(collection) </code></pre> <p>and then define an <code>__isempty__()</code> method which returns a boolean for all your collection classes.</p>

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