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POHow to decorate (monkeypatch...) a Python class with methods from another class?
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  1. POHow to decorate (monkeypatch...) a Python class with methods from another class?
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    copied!<p>Both <code>httplib.HTTPMessage</code> and <code>email.message.Message</code> classes[1] implements methods for RFC822 headers parsing. Unfortunately, they have different implementations[2] and they do not provide the same level of functionality.</p> <p>One example that is bugging me is that:</p> <ul> <li><p><code>httplib.HTTPMessage</code> is missing the <code>get_filename</code> method present in <code>email.Message</code>, that allows you to easily retrieve the filename from a <code>Content-disposition: attachment; filename="fghi.xyz"</code> header;</p></li> <li><p><code>httplib.HTTPMessage</code> has <code>getparam</code>, <code>getplist</code> and <code>parseplist</code> methods but AFAIK, they are not and cannot be used outside of the <code>content-type</code> header parsing;</p></li> <li><p><code>email.Message</code> has a generic <code>get_param</code> method to parse any RFC822 header with parameters, such as <code>content-disposition</code> or <code>content-type</code>.</p></li> </ul> <p>Thus, I want the <code>get_filename</code> or <code>get_param</code> methods of <code>email.message.Message</code> in <code>httplib.HTTPMessage</code> but, of course, I can't patch <code>httplib.HTTPMessage</code> as it's in the standard library... :-q</p> <p>And finally, here comes the decorator subject... :-)</p> <p>I succesfully created a <code>monkeypatch_http_message</code> function to decorate an <code>httplib.HTTPMessage</code> with my missing parsing methods:</p> <pre><code>def monkeypatch_http_message(obj): from email import utils from email.message import ( _parseparam, _unquotevalue, ) cls = obj.__class__ # methods **copied** from email.message.Message source code def _get_params_preserve(self, failobj, header): ... def get_params(self, failobj=None, header='content-type', unquote=True): ... def get_param(self, param, failobj=None, header='content-type', unquote=True): ... def get_filename(self, failobj=None): ... # monkeypatching httplib.Message cls._get_params_preserve = _get_params_preserve cls.get_params = get_params cls.get_param = get_param cls.get_filename = get_filename </code></pre> <p>Now I can do:</p> <pre><code>import mechanize from some.module import monkeypatch_http_message browser = mechanize.Browser() # in that form, browser.retrieve returns a temporary filename # and an httplib.HTTPMessage instance (tmp_filename, headers) = browser.retrieve(someurl) # monkeypatch the httplib.HTTPMessage instance monkeypatch_http_message(headers) # yeah... my original filename, finally filename = headers.get_filename() </code></pre> <p>The issue here is that I literally copied the decorating methods code from the source class, which I'd like to avoid.</p> <p>So, I tried decorating by referencing the source methods:</p> <pre><code>def monkeypatch_http_message(obj): from email import utils from email.message import ( _parseparam, _unquotevalue, Message # XXX added ) cls = obj.__class__ # monkeypatching httplib.Message cls._get_params_preserve = Message._get_params_preserve cls.get_params = Message.get_params cls.get_param = Message.get_param cls.get_filename = Message.get_filename </code></pre> <p>But that gives me:</p> <pre><code>Traceback (most recent call last): File "client.py", line 224, in &lt;module&gt; filename = headers.get_filename() TypeError: unbound method get_filename() must be called with Message instance as first argument (got nothing instead) </code></pre> <p>I'm scratching my head now... how can I decorate my class without literally copying the source methods ?</p> <p>Any suggestions ? :-)</p> <p>Regards,</p> <p>Georges Martin</p> <hr> <ol> <li><p>In Python 2.6. I can't use 2.7 nor 3.x in production.</p></li> <li><p><code>httplib.HTTPMessage</code> inherits from <code>mimetools.Message</code> and <code>rfc822.Message</code> while <code>email.Message</code> has its own implementation.</p></li> </ol>
 

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