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    <p>Let's look at the <a href="http://docs.python.org/2.7/reference/expressions.html#calls" rel="nofollow noreferrer">language specification</a>:</p> <pre><code>call ::= primary "(" [argument_list [","] | expression genexpr_for] ")" argument_list ::= positional_arguments ["," keyword_arguments] ["," "*" expression] ["," keyword_arguments] ["," "**" expression] | keyword_arguments ["," "*" expression] ["," "**" expression] | "*" expression ["," "*" expression] ["," "**" expression] | "**" expression positional_arguments ::= expression ("," expression)* keyword_arguments ::= keyword_item ("," keyword_item)* keyword_item ::= identifier "=" expression </code></pre> <p>Let's sift down to the parts we care about:</p> <pre><code>call ::= primary "(" [argument_list [","]] ")" argument_list ::= positional_arguments ["," keyword_arguments] ["," "*" expression] ["," keyword_arguments] ["," "**" expression] positional_arguments ::= expression ("," expression)* keyword_arguments ::= keyword_item ("," keyword_item)* keyword_item ::= identifier "=" expression </code></pre> <p>So, it looks like after any arguments to a function call, we're allowed an extra <code>,</code>. So this looks like a bug in the cpython implementation.</p> <p>Something like: <code>f(1, *(2,3,4), )</code> should work according to this grammar, but doesn't in CPython.</p> <hr> <p>In an earlier answer, <a href="https://stackoverflow.com/users/102441/eric">Eric</a> linked to the <a href="http://docs.python.org/2/reference/grammar.html" rel="nofollow noreferrer">CPython grammar specification</a>, which includes the CPython implementation of the above grammar. Here it is below:</p> <pre><code>arglist: (argument ',')* ( argument [','] | '*' test (',' argument)* [',' '**' test] | '**' test ) </code></pre> <p>Note, that this grammar is <strong>not the same</strong> as the one proposed by the language specification. I'd consider this an implementation bug.</p> <hr> <p>Note that there are additional issues with the CPython implementation. This should also be supported: <code>f(*(1,2,3), *(4,5,6))</code></p> <p>Oddly though, the specification does not allow <code>f(*(1,2,3), *(4,5,6), *(7,8,9))</code></p> <p><strong>As I look at this more,</strong> I think this part of the specification needs some fixing. This is allowed: <code>f(x=1, *(2,3))</code>, but this isn't: <code>f(x=1, 2, 3)</code>.</p> <hr> <p>And to perhaps be helpful to the original question, in CPython, you can have a trailing comma if you don't use the <code>*args</code> or the <code>**kwargs</code> feature. I agree that this is lame.</p>
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    1. POWhy is a trailing comma a SyntaxError in an argument list that uses *args syntax?
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    1. USBill Lynch
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    1. POWhy is a trailing comma a SyntaxError in an argument list that uses *args syntax?
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    1. COGot to upvote this. This is the first time I've seen someone pull out Python's language specification to support an answer.
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      1. USlightalchemist
    2. COGood answer in general. But I'm not sure what you mean when you say that `f(*(1,2,3), *(4,5,6))` "should be supported" -- it looks to me like neither grammar (the language spec and the cpython grammar) would allow for that.
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      1. USEdward Loper
    3. CO@EdwardLoper Probably referring to the line of the spec grammar that says `| "*" expression ["," "*" expression] ["," "**" expression]`. The first clause in square brackets looks like it got there by mistake, the line should be `| "*" expression ["," "**" expression]`. As sharth says, the specification does need some work.
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