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POUsing a decorator function that is a method in a class on other methods within the class in python
 

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  1. POUsing a decorator function that is a method in a class on other methods within the class in python
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    <p>I have been trying to learn more about decorators using a tweaked version as below from: <a href="http://code.activestate.com/recipes/440499/" rel="nofollow">here</a></p> <pre><code>def case(comparision): def __assign_case(f): f.__case = comparision return f return __assign_case class switch: def __init__(self): self.__case_map = {} def set_case(key,f): self.__case_map[key] = f a = [getattr(self,e) for e in dir(self) if getattr(self,e) is not None and hasattr(getattr(self,e),'__case')] for f in a: cases = getattr(f,'__case') if isinstance(cases,tuple) or isinstance(cases,list): for c in cases: set_case(c,f) else: set_case(cases,f) print(self.__case_map) def match(self,value): try: self.__case_map[value] except KeyError: return self.__case_map['_default'] return self.__case_map[value] class b(switch): @case((1,3)) def event_one(self): print('Event handler for 1,3 in b') @case(2) def event_two(self): print('Event handler for 2 in b') @case('_default') def default(self): print('No match was found, using default case') a = b() a.match(1)() a.match(2)() a.match(5)() </code></pre> <p>With results with run of:</p> <pre><code>$ ./switch_decor.py {1: &lt;bound method b.event_one of &lt;__main__.b object at 0x7f03374849d0&gt;&gt;, '_default': &lt;bound method b.default of &lt;__main__.b object at 0x7f03374849d0&gt;&gt;, 3: &lt;bound method b.event_one of &lt;__main__.b object at 0x7f03374849d0&gt;&gt;, 2: &lt;bound method b.event_two of &lt;__main__.b object at 0x7f03374849d0&gt;&gt;} Event handler for 1,3 in b Event handler for 2 in b No match was found, using default case </code></pre> <p>Notice the filled dictionary</p> <p>I like to keep my code contained, so was trying to move the <code>case</code> function into the <code>switch</code> class like so:</p> <pre><code>class switch: def __init__(self): self.__case_map = {} def set_case(key,f): self.__case_map[key] = f a = [getattr(self,e) for e in dir(self) if getattr(self,e) is not None and hasattr(getattr(self,e),'__case')] for f in a: cases = getattr(f,'__case') if isinstance(cases,tuple) or isinstance(cases,list): for c in cases: set_case(c,f) else: set_case(cases,f) print(self.__case_map) def match(self,value): try: self.__case_map[value] except KeyError: return self.__case_map['_default'] return self.__case_map[value] @staticmethod def case(comparision): def __assign_case(f): f.__case = comparision return f return __assign_case class b(switch): @switch.case((1,3)) def event_one(self): print('Event handler for 1,3 in b') @switch.case(2) def event_two(self): print('Event handler for 2 in b') @switch.case('_default') def default(self): print('No match was found, using default case') a = b() a.match(1)() a.match(2)() a.match(5)() </code></pre> <p>But I end up getting an empty <code>self.__case_map</code> dictionary resulting in this error:</p> <pre><code>$ ./switch_decor_contained.py {} Traceback (most recent call last): File "./switch_decor_contained.py", line 23, in match self.__case_map[value] KeyError: 1 During handling of the above exception, another exception occurred: Traceback (most recent call last): File "./switch_decor_contained.py", line 50, in &lt;module&gt; a.match(1)() File "./switch_decor_contained.py", line 25, in match return self.__case_map['_default'] KeyError: '_default' </code></pre> <p>Notice the blank dictionary at the beginning. It fails at</p> <pre><code>a.match(1) </code></pre> <p>before that return function is invoked with the second <code>()</code> which is a function of the dictionary being blank tripping the exception but there is no <code>_default</code> key to the function value to return.</p> <p>On the first set of code above, running this in a console gives:</p> <pre><code>&gt;&gt;&gt; hasattr(a.event_one, "__case") True </code></pre> <p>But on the second set of code above, running </p> <pre><code>&gt;&gt;&gt; hasattr(a.event_one, "__case") False </code></pre> <p>Despite the fact that <code>@switch.case</code> is being run as tests on that showed. There is some untoward side effect from invoking it within a class. What is the side effect?</p> <p>How can I move the <code>case</code> function into the <code>switch</code> class and use it as a method successfully to decorate the functions in class <code>b</code>?</p>
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    1. COon further working this through, there are no items in the dictionary because the class `b` methods don't have the attribute `__case` assigned despite the decorator being called thus the `__init__` in switch never finds the attribute in them so they may be added to the dictionary. Why is this failing?
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