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Question

Is it possible to unpack a tuple in Python without creating unwanted variables?

Answer 1

+14 A:

In Python the _ is often used as an ignored placeholder.

(path, _) = self._treeView.get_cursor()

You could also avoid unpacking as a tuple is indexable.

def get_selected_index(self):
    return self._treeView.get_cursor()[0][0]

KennyTM 2010-09-27 09:16:24

It's worth noting that `_` is not any kind of syntactic magic, but a convention (of course, it's a convention recognised by IDEs and pylint, and Python conventions are pretty strong...)

detly 2010-09-27 09:18:30

Python would be nothing without conventions!

fredley 2010-09-27 09:19:33

Is this convention documented?

Nathan 2010-09-27 09:20:50

@fredley - I'm not disputing that at all :) but still worth mentioning. @Nathan - ...maybe?

detly 2010-09-27 09:46:57

The same is often used in Lua; I suspect it's a common theme in languages that support this sort of unpacking.

Glenn Maynard 2010-09-27 10:22:16

It is common in OCaml and Haskell.

Gaius 2010-09-27 13:58:35

And is also common in Prolog: in fact it is a little more than a convention, it is an anonymous variable (and can be repeated, without it meaning the same value) in a matching statement. IIRC Erlang handles it similarly.

Matthew Schinckel 2010-09-28 00:17:15

@Gaius: I believe in Haskell it explicitly means "discard this value"; in Python `_` is still an assigned variable that you could in theory use just like any other variable.

me_and 2010-09-30 12:52:19

The python command line interpreter "magically" assigns _ to the result of the last computation so if you run something but forget to store the return value, it will be captured for you in the `_` variable. It's a useful thing while doing interactive work. In the above case, I'd probably do indexing rather than unpacking though.

Noufal Ibrahim 2010-10-19 10:40:45

Answer 2

+2 A:

If you don't care about the second item, why not just extract the first one:

def get_selected_index(self):
    path = self._treeView.get_cursor()[0]
    return path[0]

Steven 2010-09-27 11:28:27

Note that the return should now be `return path`, not `return path[0]`

Michael Mior 2010-09-27 14:59:03

This is weaker, because it removes the assertion that there are exactly two items.

jleedev 2010-09-28 00:39:10

@Michael Mior: I've rolled back your edit: Based on the original question, it should still be `path[0]` (`path` seems to be a sequence itself)

Steven 2010-09-28 13:40:06

Sorry about that @Steven. I wasn't reading carefully enough. It was confusing because the definition of `path` in your code is different than that used by the OP.

Michael Mior 2010-09-28 14:26:41

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Is it possible to unpack a tuple in Python without creating unwanted variables?

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