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POHow do I instantiate a specific subclass based on arguments passed to __init__?
 

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  1. POHow do I instantiate a specific subclass based on arguments passed to __init__?
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    <p>I'm wrapping a remote XML-based API from python 2.7. The API throws errors by sending along a <code>&lt;statusCode&gt;</code> element as well as a <code>&lt;statusDescription&gt;</code> element. Right now, I catch this condition and raise a single exception type. Something like:</p> <pre><code>class ApiError(Exception): pass def process_response(response): if not response.success: raise ApiError(response.statusDescription) </code></pre> <p>This works fine, except I now want to handle errors in a more sophisticated fashion. Since I have the <code>statusCode</code> element, I would like to raise a specific subclass of ApiError based on the statusCode. Effectively, I want my wrapper to be extended like this:</p> <pre><code>class ApiError(Exception): def __init__(self, description, code): # How do I change self to be a different type? if code == 123: return NotFoundError(description, code) elif code == 456: return NotWorkingError(description, code) class NotFoundError(ApiError): pass class NotWorkingError(ApiError): pass def process_response(response): if not response.success: raise ApiError(response.statusDescription, response.statusCode) def uses_the_api(): try: response = call_remote_api() except NotFoundError, e: handle_not_found(e) except NotWorkingError, e: handle_not_working(e) </code></pre> <p>The machinery for tying specific <code>statusCode</code>'s to specific subclasses is straightforward. But what I want is for that to be buried inside of ApiError somewhere. Specifically, I don't want to change process_response except to pass in the value <code>statusCode</code>.</p> <p>I've looked at metaclasses, but not sure they help the situation, since <code>__new__</code> gets write-time arguments, not run-time arguments. Similarly unhelpful is hacking around <code>__init__</code> since it isn't intended to return an instance. So, how do I instantiate a specific subclass based on arguments passed to <code>__init__</code>?</p>
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    1. USDon Spaulding
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    1. COWhy do you want to use a class if you need a method? Use a function that returns a different instance based on the status code
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      1. POHow do I instantiate a specific subclass based on arguments passed to __init__?
      1. USGabi Purcaru
    2. CO@GabiPurcaru Good question. Because I don't want to significantly change the `raise ApiError()` line in `process_response`. I'm interested in how this could be done without disturbing existing `raise` statements.
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      1. POHow do I instantiate a specific subclass based on arguments passed to __init__?
      1. USDon Spaulding
 

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