Pre-PEP: Unpacking in Comprehensions

[blhsing (Ben Hsing)]

@adqm

Of course, there are alternative ways to do things like this. Taking the concatenation of a bunch of lists as an example, any of the following would result in the same output (though some are more efficient than others):

[x for it in its for x in it]
list(itertools.chain(*its))
sum((it for it in its), [])
functools.reduce(operator.concat, its, [])

To even better justify the new syntax over existing approaches, I think we can further generalize unpacking in comprehensions by allowing * and ** to unpack any arbitrary expression within a comprehension, not just the outermost expression.

So that as an example to flatten [1, 2, [3, 4]] into [1, 2, 3, 4]:

[*x if isinstance(x, Iterable) else x for x in its]

would be equivalent to:

[x for it in its for x in (it if isinstance(it, Iterable) else [it])]

This can potentially be done by making * and ** true unary operators that wrap the operand in a new Unpackable object for the yielder of a comprehension to unwrap and unpack.