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copied!<p>Globals are rarely a good idea.</p> <p>Implicit variables are rarely a good idea.</p> <p>An implicitly-used global is easy to indict as also "rarely good".</p> <p>Additionally, you don't want <code>A.__init__()</code> doing anything "class-level" like updating some mysterious collection that exists for the class as a whole. That's often a bad idea. </p> <p>Rather than mess with implicit class-level collection, you want a <strong>Factory</strong> in <code>module_a</code> that (1) creates <code>A</code> or <code>B</code> instances and (b) updates an explicit collection.</p> <p>You can then use this factory in <code>module_b</code>, except with a different collection.</p> <p>This can promote testability by exposing an implicit dependency.</p> <p>module_a.py</p> <pre><code>class Factory( object ): def __init__( self, collection ): self.collection= collection def make( self, name, *args, **kw ): obj= eval( name )( *args, **kw ) self.collection.append( obj ) return obj module_collection = [] factory= Factory( module_collection ) </code></pre> <p>module_b.py</p> <pre><code>module_collection = [] factory = module_a.Factory( module_collection ) </code></pre> <p>Now a client can do this</p> <pre><code>import module_b a = module_b.factory.make( "A" ) b = module_b.factory.make( "B" ) print( module_b.module_collection ) </code></pre> <p>You can make the API a bit more fluent by making the factory "callable" (implementing <code>__call__</code> instead of <code>make</code>.</p> <p>The point is to make the collection <em>explicit</em> via a factory class.</p>

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