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POString construction using OOP and Proxy pattern
 

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  1. POString construction using OOP and Proxy pattern
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    <p>I find it very interesting the way how SQLAlchemy constructing query strings, eg:</p> <pre><code>(Session.query(model.User) .filter(model.User.age &gt; 18) .order_by(model.User.age) .all()) </code></pre> <p>As far as I can see, there applied some kind of <strong>Proxy Pattern</strong>. In my small project I need to make similar string construction using OOP approach. So, I tried to reconstitute this behavior.</p> <p>Firstly, some kind of object, one of plenty similar objects:</p> <pre><code>class SomeObject(object): items = None def __init__(self): self.items = [] def __call__(self): return ' '.join(self.items) if self.items is not None else '' def a(self): self.items.append('a') return self def b(self): self.items.append('b') return self </code></pre> <p>All methods of this object return <strong>self</strong>, so I can call them in any order and unlimited number of times.</p> <p>Secondly, proxy object, that will call subject's methods if it's not a <strong>perform</strong> method, which calls object to see the resulting string.</p> <pre><code>import operator class Proxy(object): def __init__(self, some_object): self.some_object = some_object def __getattr__(self, name): self.method = operator.methodcaller(name) return self def __call__(self, *args, **kw): self.some_object = self.method(self.some_object, *args, **kw) return self def perform(self): return self.some_object() </code></pre> <p>And finally:</p> <pre><code>&gt;&gt;&gt; obj = SomeObject() &gt;&gt;&gt; p = Proxy(obj) &gt;&gt;&gt; print p.a().a().b().perform() a a b </code></pre> <p>What can you say about this implementation? Is there better ways to make the desirable amount of classes that would make such a string cunstructing with <strong>the same syntax</strong>?</p> <p>PS: Sorry for my english, it's not my primary language.</p>
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    1. COWhy don't you look at the SQLAlchemy code to see how they do this?
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