python: when can I unpack a generator?

python args iterable-unpacking
robert king picture robert king · Jun 10, 2012 · Viewed 10k times · Source

How does it work under the hood? I don't understand the reason for the errors below:

>>> def f():
...     yield 1,2
...     yield 3,4
...
>>> *f()
  File "<stdin>", line 1
    *f()
    ^
SyntaxError: invalid syntax
>>> zip(*f())
[(1, 3), (2, 4)]
>>> zip(f())
[((1, 2),), ((3, 4),)]
>>> *args = *f()
File "<stdin>", line 1
  *args = *f()
    ^
SyntaxError: invalid syntax

Answer

Sven Marnach picture Sven Marnach · Jun 10, 2012

The *iterable syntax is only supported in an argument list of a function call (and in function definitions).

In Python 3.x, you can also use it on the left-hand side of an assignment, like this:

[*args] = [1, 2, 3]

Edit: Note that there are plans to support the remaining generalisations.

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