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POFactory method for objects - best practice?
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copied!<p>This is a question regarding the best practice for creating an instance of a class or type from different forms of the same data using python. Is it better to use a class method or is it better to use a separate function altogether? Let's say I have a class used to describe the size of a document. (Note: This is simply an example. I want to know the best way to create an instance of the class <strong>not</strong> the best way to describe the size of a document.)</p> <pre><code>class Size(object): """ Utility object used to describe the size of a document. """ BYTE = 8 KILO = 1024 def __init__(self, bits): self._bits = bits @property def bits(self): return float(self._bits) @property def bytes(self): return self.bits / self.BYTE @property def kilobits(self): return self.bits / self.KILO @property def kilobytes(self): return self.bytes / self.KILO @property def megabits(self): return self.kilobits / self.KILO @property def megabytes(self): return self.kilobytes / self.KILO </code></pre> <p>My <code>__init__</code> method takes a size value represented in bits (bits and only bits and I want to keep it that way) but lets say I have a size value in bytes and I want to create an instance of my class. Is it better to use a class method or is it better to use a separate function altogether?</p> <pre><code>class Size(object): """ Utility object used to describe the size of a document. """ BYTE = 8 KILO = 1024 @classmethod def from_bytes(cls, bytes): bits = bytes * cls.BYTE return cls(bits) </code></pre> <p>OR</p> <pre><code>def create_instance_from_bytes(bytes): bits = bytes * Size.BYTE return Size(bits) </code></pre> <p>This may not seem like an issue and perhaps both examples are valid but I think about it every time I need to implement something like this. For a long time I have preferred the class method approach because I like the organisational benefits of tying the class and the factory method together. Also, using a class method preserves the ability to create instances of any subclasses so it's more object orientated. On the other hand, a friend once said "When in doubt, do what the standard library does" and I am yet to find an example of this in the standard library.</p>

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