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POWhy are Python decorators with arguments syntactically different from those without?
 

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  1. POWhy are Python decorators with arguments syntactically different from those without?
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    <p><a href="http://www.artima.com/weblogs/viewpost.jsp?thread=240845" rel="nofollow">This article</a>, linked to a number of times from various stackoverflow questions, describes how decorators with arguments are syntactically different from those without arguments.</p> <ul> <li>Decorators without arguments: "Notice that <code>__init__()</code> is the only method called to perform decoration, and <code>__call__()</code> is called every time you call the decorated <code>sayHello()</code>."</li> <li>Decorators with arguments: "Now the process of decoration calls the constructor and then immediately invokes <code>__call__()</code>, which can only take a single argument (the function object) and must return the decorated function object that replaces the original. Notice that <code>__call__()</code> is now only invoked once, during decoration, and after that the decorated function that you return from <code>__call__()</code> is used for the actual calls."</li> </ul> <p>The explanation given in the article doesn't tell me why the language is set up this way:</p> <blockquote> <p>Although this behavior makes sense -- the constructor is now used to capture the decorator arguments, but the object <code>__call__()</code> can no longer be used as the decorated function call, so you must instead use <code>__call__()</code> to perform the decoration -- it is nonetheless surprising the first time you see it</p> </blockquote> <p>There are two related features of this setup that are uncomfortable for me:</p> <ol> <li>Why provide a way to make no-argument decorators that's not just a special case of the more general method for specifying decorators? And why use <code>__init__</code> and <code>__call__</code> in both, but have them mean different things?</li> <li>Why use <code>__call__</code> in the case of decorators with arguments for a purpose other than calling the decorated function (as the name suggests, at least coming from the no argument case)? Given that <code>__call__</code> is only invoked right after <code>__init__</code>, why not just pass the function to be decorated as an argument to <code>__init__</code> and handle everything that would happen in <code>__call__</code> in <code>__init__</code> instead?</li> </ol>
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