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    <p>The single star <code>*</code> unpacks the sequence/collection into positional arguments, so you can do this:</p> <pre><code>def sum(a, b): return a + b values = (1, 2) s = sum(*values) </code></pre> <p>This will unpack the tuple so that it actually executes as:</p> <pre><code>s = sum(1, 2) </code></pre> <p>The double star <code>**</code> does the same, only using a dictionary and thus named arguments:</p> <pre><code>values = { 'a': 1, 'b': 2 } s = sum(**values) </code></pre> <p>You can also combine:</p> <pre><code>def sum(a, b, c, d): return a + b + c + d values1 = (1, 2) values2 = { 'c': 10, 'd': 15 } s = sum(*values1, **values2) </code></pre> <p>will execute as: </p> <pre><code>s = sum(1, 2, c=10, d=15) </code></pre> <p>Also see section <a href="http://docs.python.org/tutorial/controlflow.html#unpacking-argument-lists" rel="noreferrer">4.7.4 - Unpacking Argument Lists</a> of the Python documentation.</p> <hr> <p>Additionally you can define functions to take <code>*x</code> and <code>**y</code> arguments, this allows a function to accept any number of positional and/or named arguments that aren't specifically named in the declaration.</p> <p>Example:</p> <pre><code>def sum(*values): s = 0 for v in values: s = s + v return s s = sum(1, 2, 3, 4, 5) </code></pre> <p>or with <code>**</code>:</p> <pre><code>def get_a(**values): return values['a'] s = get_a(a=1, b=2) # returns 1 </code></pre> <p>this can allow you to specify a large number of optional parameters without having to declare them.</p> <p>And again, you can combine:</p> <pre><code>def sum(*values, **options): s = 0 for i in values: s = s + i if "neg" in options: if options["neg"]: s = -s return s s = sum(1, 2, 3, 4, 5) # returns 15 s = sum(1, 2, 3, 4, 5, neg=True) # returns -15 s = sum(1, 2, 3, 4, 5, neg=False) # returns 15 </code></pre>
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    1. POWhat does the star operator mean?
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    1. USLasse Vågsæther Karlsen
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    1. POWhat does the star operator mean?
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    1. COwhy would you need this, couldn't the function just iterate over the supplied list without it being expanded?
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      1. USMartin Beckett
    2. COSure, but then you would have to call it: `s = sum((1, 2, 3, 4, 5))` or `s = sum([1, 2, 3, 4, 5])`, the `*values` option makes the call look like it takes a number of arguments, but they're packed up into a collection for the function code.
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      1. USLasse Vågsæther Karlsen
    3. CObest answer i have ever seen in SOF. thanks thans **thanks**
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