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PODependency injection in (Python) Google App Engine
 

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  1. PODependency injection in (Python) Google App Engine
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    <p>I want to achieve maximum testability in my <a href="http://en.wikipedia.org/wiki/YAML" rel="nofollow">Google App Engine</a> app which I'm writing in Python.</p> <p>Basically what I'm doing is creating an all-purpose base handler which inherits the <code>google.appengine.ext.webapp.RequestHandler</code>. My base handler will expose common functionality in my app such as repository functions, a session object and the like.</p> <p>When the <code>WSGIApplication</code> receives a request it will find the handler class that has been registered for the requested URL, and call its constructor and after that it will call a method called <code>initialize</code> passing in the <code>request</code> and <code>response</code> objects. </p> <p>Now, for the sake of testability I want to be able to "<em>mock</em>" these objects (along with my own objects). So my question is how do I go about injecting these mocks? I can override the <code>initialize</code> method in my base handler and check for some global "test flag" and initialize some dummy <code>request</code> and <code>response</code> objects. But it seems wrong (in <em>my</em> mind at least). And how do I go about initializing my other objects (which may depend on the <code>request</code> and <code>response</code> objects)?</p> <p>As you can probably tell I'm a little new to Python so any recommendations would be most welcome.</p> <p><strong>EDIT:</strong></p> <p>It has been pointed out to me that this question was a little hard to answer without some code, so here goes:</p> <pre><code>from google.appengine.ext import webapp from ..utils import gmemsess from .. import errors _user_id_name = 'userid' class Handler(webapp.RequestHandler): ''' classdocs ''' def __init__(self): ''' Constructor ''' self.charset = 'utf8' self._session = None def _getsession(self): if not self._session: self._session = gmemsess.Session(self) return self._session def _get_is_logged_in(self): return self.session.has_key(_user_id_name) def _get_user_id(self): if not self.is_logged_in: raise errors.UserNotLoggedInError() return self.session[_user_id_name] session = property(_getsession) is_logged_in = property(_get_is_logged_in) user_id = property(_get_user_id) </code></pre> <p>As you can see, no dependency injection is going on here at all. The session object is created by calling <code>gmemsess.Session(self)</code>. The <code>Session</code> class expects a class which has a <code>request</code> object on it (it uses this to read a cookie value). In this case, <code>self</code> does have such a property since it inherits from <code>webapp.RequestHandler</code>. It also only has the object on it because after calling (the empty) constructor, <code>WSGIApplication</code> calls a method called <code>initialize</code> which sets this object (and the response object). The initialize method is declared on the base class (<code>webapp.RequestHandler</code>). It looks like this:</p> <pre><code>def initialize(self, request, response): """Initializes this request handler with the given Request and Response.""" self.request = request self.response = response </code></pre> <p>When a request is made, the <code>WSGIApplication</code> class does the following:</p> <pre><code>def __call__(self, environ, start_response): """Called by WSGI when a request comes in.""" request = self.REQUEST_CLASS(environ) response = self.RESPONSE_CLASS() WSGIApplication.active_instance = self handler = None groups = () for regexp, handler_class in self._url_mapping: match = regexp.match(request.path) if match: handler = handler_class() handler.initialize(request, response) groups = match.groups() break self.current_request_args = groups if handler: try: method = environ['REQUEST_METHOD'] if method == 'GET': handler.get(*groups) elif method == 'POST': handler.post(*groups) '''SNIP''' </code></pre> <p>The lines of interest are those that say:</p> <pre><code> handler = handler_class() handler.initialize(request, response) </code></pre> <p>As you can see, it calls the empty constructor on my handler class. And this is a problem for me, because what I think I would like to do is to inject, at runtime, the type of my session object, such that my class would look like this instead (fragment showed):</p> <pre><code> def __init__(self, session_type): ''' Constructor ''' self.charset = 'utf8' self._session = None self._session_type = session_type def _getsession(self): if not self._session: self._session = self._session_type(self) return self._session </code></pre> <p>However I can't get my head around how I would achieve this, since the <code>WSGIApplication</code> only calls the empty constructor. I guess I could register the <code>session_type</code> in some global variable, but that does not really follow the philosophy of dependency injection (as I understand it), but as stated I'm new to Python, so maybe I'm just thinking about it the wrong way. In any event I would rather pass in a session object instead of it's type, but this looks kind of impossible here.</p> <p>Any input is appreciated. </p>
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    1. USPeter Mortensen
    1. USKlaus Byskov Pedersen
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    1. COCould you provide some code demonstrating the steps you've outlined here? It would help us provide concrete answers.
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      1. PODependency injection in (Python) Google App Engine
      1. USNed Batchelder
    2. CO@Ned Batchelder, I have elaborated some more on my question.
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      1. PODependency injection in (Python) Google App Engine
      1. USKlaus Byskov Pedersen
    3. CO@Ned Batchelder it just dawned on me, while thinking of your response to my earlier question, that python will probably not prevent me from overriding/inheriting from the `WSGIApplication` which is the main entry point for all requests. So I should probably just do that, and inject all my dependencies there?
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      1. PODependency injection in (Python) Google App Engine
      1. USKlaus Byskov Pedersen
 

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