On 25/05/13 04:53, Albert-Jan Roskam wrote:
Why do I need to use a trailing comma to create a singleton tuple? Without a comma it
seems to mean "parenthesized single object", ie the parentheses are basically
not there.
Because round brackets are also used for grouping. You can group any expression:
z = (x+1)*y
Even an expression consisting of a single element:
z = (x+1)*(y)
That might not be a sensible thing to do, but it's not worth special-casing the
parser to reject parentheses in this case. Now bring in tuples. Tuples are
created by the comma operator, NOT the parentheses:
a = 23, 42, None
creates a tuple. At least one comma is necessary to distinguish an element from
a tuple of one element:
a = 23 # a is the int 23
a = 23, # a is a one-item tuple containing 23
Sometimes you use round brackets to group the tuple item, either because you
need to change the precedence:
a = 23, (2, 4, 8), None
groups the three elements 2, 4, 8 into a tuple, which in turn is in a tuple:
print a
=> (23, (2, 4, 8), None)
Or we use round brackets to make a tuple just because it looks better, and
matches the display of them:
a = (23, 42, None)
print a
=> (23, 42, None)
But make no mistake: it is the comma, not the brackets, that makes the tuple.
So single-item tuples are not special. They just follow the rules for
multiple-item tuples, except that you only have item. That does leave the
question of how to specify an empty tuple. And that is special:
a = ()
however it does match the display of empty tuples, and looks rather similar to
empty lists [] and empty dicts {}.
--
Steven
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