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POBound Python method accessed with getattr throwing:" takes no arguments (1 given) "
 

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  1. POBound Python method accessed with getattr throwing:" takes no arguments (1 given) "
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    <p>Something rather odd is happening in an interaction between bound methods, inheritance, and getattr that I am failing to understand.</p> <p>I have a directory setup like:</p> <pre><code>/a __init__.py start_module.py /b __init__.py imported_module.py </code></pre> <p>imported_module.py contains a number of class objects one of which is of this form:</p> <pre><code>class Foo(some_parent_class): def bar(self): return [1,2,3] </code></pre> <p>A function in start_module.py uses inspect to get a list of strings representing the classes in imported_module.py. "Foo" is the first of those strings. The goal is to run bar in start_module.py using that string and getattr.*</p> <p>To do this I use code in start_module of the form:</p> <pre><code>for class_name in class_name_list: instance = getattr(b.imported_module, class_name)() function = getattr(instance, "bar") for doodad in [x for x in function()]: print doodad </code></pre> <p>Which does successfully start to iterate over the list comprehension, but on the first string, "bar", I get a bizarre error. Despite bar being a bound method, and so as far as I understand expecting an instance of Foo as an argument, I am told:</p> <p>TypeError: bar() takes no arguments (1 given)</p> <p>This makes it seem like my call to function() is passing the Foo instance, but the code is not expecting to receive it. </p> <p>I really have no idea what is going on here and couldn't parse out an explanation through looking on Google and Stack Overflow. Is the double getattr causing some weird interaction? Is my understanding of class objects in Python too hazy? I'd love to hear your thoughts.</p> <p>*To avoid the anti-pattern, the real end objective is to have start_module.py automatically have access to all methods of name bar across a variety of classes similar to Foo in imported_module.py. I am doing this in the hopes of avoiding making my successors maintain a list for what could be a very large number of Foo-resembling classes. </p> <p>Answered below: I think the biggest takeaways here are that inspect is very useful, and that if there is a common cause for the bug you are experiencing, make absolutely sure you've ruled that out before moving on to search for other possibilities. In this case I overlooked the fact that the module I was looking at that had correct code might not be the one being imported due to recent edits to the file structure.</p>
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    1. USMark Garcia
    1. USTim Wilder
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    1. CO`return = [1,2,3]` isn't valid Python. Please make sure you are posting the actual code you are using
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      1. POBound Python method accessed with getattr throwing:" takes no arguments (1 given) "
      1. USJohn La Rooy
    2. COProbably a better idea to make a metaclass for `Foo` that can "register" all the `bar` eg. store them in a list
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      1. POBound Python method accessed with getattr throwing:" takes no arguments (1 given) "
      1. USJohn La Rooy
    3. COOops. Sorry about that. Fixing it. I added the class description in a very simplified form so as to make it legible and accidentally slipped an extra character in.
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      1. POBound Python method accessed with getattr throwing:" takes no arguments (1 given) "
      1. USTim Wilder
 

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