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2012-10-02T23:32:21.600
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2017-05-23T12:24:07.900
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<ul> <li><code>print result</code> in my own class always prints None.</li> </ul> This is because constructor doesn't return anything :) <ul> <li>Why is the metaclass using <code>__init__</code> and not <code>__new__</code></li> </ul> I think it is because SQLAlchemy needs to store a reference of cls into the declarative class registry. In <code>__new__</code>, the class doesn't exist yet (see <a href="https://stackoverflow.com/a/1840466">https://stackoverflow.com/a/1840466</a>). When I subclassed <code>DeclarativeMeta</code>, I actually did everything in <code>__init__</code> following SQLAlchemy's code. In retrospect after reading your question, my code should use <code>__new__</code> instead. <ul> <li><code>declarative_base</code> returns an instance of this class. Shouldn't it return a class having an attribute <code>__metaclass__</code> having <code>MyDeclarativeMeta</code> as value?</li> </ul> I think Ben explained this very well. Anyway, if you want (not recommended), you can skip calling <code>declarative_base()</code> and create your own Base class, e.g. <pre><code># Almost the same as: # Base = declarative_base(cls=Entity, name='Base', metaclass=MyDeclarativeMeta) # minus the _declarative_constructor. class Base(Entity): __metaclass__ = MyDeclarativeMeta _decl_class_registry = dict() metadata = MetaData() </code></pre> In this case, the <code>__metaclass__</code> attribute will be there. I actually created my Base class like this to help PyCharm getting the auto-completion for things defined in <code>Entity</code>.
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