pythonpython-3.xstring-formattingpython-2.xiterable-unpacking

SyntaxError with starred expression when unpacking a tuple on its own for string formatting


I tried the following using the REPL in Python 3.5.2:

>>> a = (1, 2)
>>> '%d %d %d' % (0, *a)
'0 1 2'
>>> '%d %d %d' % (*a, 3)
'1 2 3'
>>> '%d %d' % (*a)
  File "<stdin>", line 1
SyntaxError: can't use starred expression here
>>> 

My question, why?

In a more serious tone: I'd like an answer, or a reference, that details all the ins and outs of using a starred expression, as it happens that I am sometimes surprised from its behaviours...

Addendum

To reflect some of the enlightening comments that immediately followed my question I add the following code

>>> '%d %d' % (, *a)
  File "<stdin>", line 1
    '%d %d' % (, *a)
               ^
SyntaxError: invalid syntax
>>> '%d %d' % (*a,)
'1 2'
>>> 

(I had tried the (, a) part before posting the original question but I've omitted it 'cause the error was not related to the starring.)

There is a syntax, in python ≥ 3.5, that "just works" but nevertheless I would like some understanding.


Solution

  • The error occurs because (a) is just a value surrounded by parenthesis. It's not a new tuple object.

    Thus, '%d %d' % (*a) is equivalent to '%d %d' % * a, which is obviously wrong in terms of python syntax.

    To create a new tuple, with one expression as an initializer, use a comma after that expression:

    >>> '%d %d' % (*a,)
    '1 2'
    

    Of course, since a is already a tuple, we can use it directly:

    >>> '%d %d' % a
    '1 2'