Pass in tuple to multi-argument function

Question

The issue here is I want to pass a tuple as an argument to a second function. Before there are outcries of "duplicate!" I have already looked at a post regarding a similar question Expanding tuples into arguments

Here is the code I am testing with:

def producer(): return ('a','b') def consumer(first, second, third): print first+second+third arg = producer() consumer(*arg, 'c') # expected abc

This outputs the error:

There's an error in your program *** only named arguments may follow *expression

This useful error message has led switch the order of arguments to consumer('c', *arg), but this does not quite solve the issue as it will output 'cab'.

So my question is, is there a better way to pass in a tuple to a multi argument function, and preserve the ordering?

Also for bonus points, what does the '*' operator do? (this was not explained in the previous post)

See What does ** (double star) and * (star) do for Python parameters? about what * does. It is not an operator. — Martijn Pieters
– Martijn Pieters, Commented Jul 10, 2014 at 21:19
While it's not legal now, the draft PEP 448 proposes to make func(*args, arg) and many other unpacking variations legal in some future Python version. See also issue 2292 which discusses attempts at implementing this behavior. Alas, there doesn't seem to have been much progress in the last year or so. — Blckknght
– Blckknght, Commented Jul 10, 2014 at 22:05

score 6 · Accepted Answer · 2014-07-10 21:29:13Z

As the error message states, Python does not allow you to have unnamed arguments after *arg.

Therefore, you need to explicitly name third:

>>> def producer(): ... return ('a','b') ... >>> def consumer(first, second, third): ... print first+second+third ... >>> arg = producer() >>> consumer(*arg, third='c') abc >>>

Martijn Pieters · Accepted Answer · 2014-07-10 21:25:53Z

If you need to add an argument, concatenate the tuple:

arg += ('c',) consumer(*arg)

Alternatively, you can name the argument explicitly by using a keyword parameter:

consumer(third='c', *arg)

You cannot put more positional arguments after *arg, these are always added to any explicit positional arguments.

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