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copied!<p>First, most of the time you think you need something like this, you don't; it's a sign that you're trying to treat Python like Java, and the solution is to step back and ask why you need a factory.</p> <p>Often, the simplest thing to do is to just have a constructor with defaulted/optional/keyword arguments. Even cases that you'd never write that way in Java—even cases where overloaded constructors would feel wrong in C++ or ObjC—may look perfectly natural in Python. For example, <code>size = Size(bytes=20)</code>, or <code>size = Size(20, Size.BYTES)</code> look reasonable. For that matter, a <code>Bytes(20)</code> class that inherits from <code>Size</code> and adds absolutely nothing but an <code>__init__</code> overload looks reasonable. And these are trivial to define:</p> <pre><code>def __init__(self, *, bits=None, bytes=None, kilobits=None, kilobytes=None): </code></pre> <p>Or:</p> <pre><code>BITS, BYTES, KILOBITS, KILOBYTES = 1, 8, 1024, 8192 # or object(), object(), object(), object() def __init__(self, count, unit=Size.BITS): </code></pre> <p>But, sometimes you <em>do</em> need factory functions. So, what do you do then? Well, there are two kinds of things that are often lumped together into "factories".</p> <p>A <code>@classmethod</code> is the idiomatic way to do an "alternate constructor"—there are examples all over the stdlib—<code>itertools.chain.from_iterable</code>, <code>datetime.datetime.fromordinal</code>, etc.</p> <p>A function is the idiomatic way to do an "I don't care what the actual class is" factory. Look at, e.g., the built-in <code>open</code> function. Do you know what it returns in 3.3? Do you care? Nope. That's why it's a function, not <code>io.TextIOWrapper.open</code> or whatever.</p> <p>Your given example seems like a perfectly legitimate use case, and fits pretty clearly into the "alternate constructor" bin (if it doesn't fit into the "constructor with extra arguments" bin).</p>

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