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    <p>When you use a decorator, you're replacing one function with another. In other words, if you have a decorator</p> <pre><code>def logged(func): def with_logging(*args, **kwargs): print func.__name__ + " was called" return func(*args, **kwargs) return with_logging </code></pre> <p>then when you say</p> <pre><code>@logged def f(x): """does some math""" return x + x * x </code></pre> <p>it's exactly the same as saying</p> <pre><code>def f(x): """does some math""" return x + x * x f = logged(f) </code></pre> <p>and your function <code>f</code> is replaced with the function with_logging. Unfortunately, this means that if you then say</p> <pre><code>print f.__name__ </code></pre> <p>it will print <code>with_logging</code> because that's the name of your new function. In fact, if you look at the docstring for <code>f</code>, it will be blank because <code>with_logging</code> has no docstring, and so the docstring you wrote won't be there anymore. Also, if you look at the pydoc result for that function, it won't be listed as taking one argument <code>x</code>; instead it'll be listed as taking <code>*args</code> and <code>**kwargs</code> because that's what with_logging takes.</p> <p>If using a decorator always meant losing this information about a function, it would be a serious problem. That's why we have <code>functools.wraps</code>. This takes a function used in a decorator and adds the functionality of copying over the function name, docstring, arguments list, etc. And since <code>wraps</code> is itself a decorator, the following code does the correct thing:</p> <pre><code>from functools import wraps def logged(func): @wraps(func) def with_logging(*args, **kwargs): print func.__name__ + " was called" return func(*args, **kwargs) return with_logging @logged def f(x): """does some math""" return x + x * x print f.__name__ # prints 'f' print f.__doc__ # prints 'does some math' </code></pre>
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    1. POWhat does functools.wraps do?
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    1. USEli Courtwright
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    1. POWhat does functools.wraps do?
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    1. COThere's a solution to the help problem which uses the decorator module, and is mentioned in the comments on this answer: http://stackoverflow.com/questions/1782843/python-decorator-problem-with-docstrings/1782888#1782888
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      1. USScott Griffiths
    2. COYep, I prefer to avoid the decorator module since functools.wraps is part of the standard library and thus doesn't introduce another external dependency. But the decorator module does indeed solve the help problem, which hopefully functools.wraps someday will as well.
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      1. USEli Courtwright
    3. COhere's an example of what can happen if you don't use wraps: doctools tests can suddenly disappear. that's because doctools cannot find the tests in decorated functions unless something like wraps() has copied them across.
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      1. USandrew cooke
 

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