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POPassing Python slice syntax around to functions
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copied!<p>In Python, is it possible to encapsulate exactly the common slice syntax and pass it around? I know that I can use <code>slice</code> or <code>__slice__</code> to emulate slicing. But I want to pass the exact same syntax that I would put in the square brackets that would get used with <code>__getitem__</code>.</p> <p>For example, suppose I wrote a function to return some slice of a list.</p> <pre><code>def get_important_values(some_list, some_condition, slice): elems = filter(some_condition, some_list) return elems[slice] </code></pre> <p>This works fine if I manually pass in a slice object:</p> <pre><code>In [233]: get_important_values([1,2,3,4], lambda x: (x%2) == 0, slice(0, None)) Out[233]: [2, 4] </code></pre> <p>But what I want to let the user pass is <em>exactly</em> the same slicing they would have used with <code>__getitem__</code>:</p> <pre><code>get_important_values([1,2,3,4], lambda x: (x%2) == 0, (0:-1) ) # or get_important_values([1,2,3,4], lambda x: (x%2) == 0, (0:) ) </code></pre> <p>Obviously this generates a syntax error. But is there any way to make this work, without writing my own mini parser for the <code>x:y:t</code> type slices, and forcing the user to pass them as strings?</p> <p><strong>Motivation</strong></p> <p>I could just make this example function return something directly sliceable, such as <code>filter(some_condition, some_list)</code>, which will be the whole result as a list. In my actual example, however, the internal function is much more complicated, and if I know the slice that the user wants ahead of time, I can greatly simplify the calculation. But I want the user to not have to do much extra to tell me the slice ahead of time.</p>

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