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Comment by nerdponx | Hacker News Reader

Out of curiosity, what don't you like about Python's packing/unpacking?

I feel it's meant to be a part of the features that try to make Python more viable for functional programming, but when you compare it to the equivalent in Haskell, for example:

* You can't nest, like it was mentioned

  ((x1, y1), (x2, y2)) = makeLine ...

* You can't use aliases

  line@(point1@(x1, y1), point2@(x2, y2)) = makeLine ...

* You can't use it on all objects/structures/datatypes

  Line (Point x1 y1) (Point x2 y2) = makeLine ...

* You can't pattern match on it

  case nextShape of
    Circle centerPoint radius -> ...
    Square width height -> ...
    Triangle point1 point2 point3 -> ...

* You can't use it on functions/lambdas

  \ Line (point1@(Point x1 y1) _) -> ...
  drawLine (Line (point1@(Point x1 y1) _)) = ...

* The syntax in Python 2 doesn't support unpacking the first few elements of a list of variable length. Python 3 added support, though.

  x1:x2:xs = ...

Tuple/list unpacking has been in Python for a long time and likely has nothing to do with catering to the functional crowd, which Guido isn't known for to begin with. Anyway...

> * You can't nest, like it was mentioned

    ((x1, y1), (x2, y2)) = makeLine ...

This works fine in Python:

    ((x1,y1), (x2,y2)) = ((1,2),(3,4))

I don't know what "makeLine ..." returns, but if it doesn't exactly match the tuples on the left hand side, it won't fly in Haskell either.

> * You can't use aliases

    line@(point1@(x1, y1), point2@(x2, y2)) = makeLine ...

No, but this works:

    line = p1, p2 = ((x1,y1), (x2,y2)) = ((1,2), (3,4))

and is more readable IMHO.

Point 3, 4 and 5 don't make sense in Python. This is pattern matching, not tuple/list unpacking. (One could argue that tuple unpacking is a form of pattern matching, but that is a different story...)

Not GP, but it's _very_ limited.

Off the top of my head, things you can't do:

    # nested unpacking
    (a, b), c = ((1, 2), 3)

    # destructuring arguments
    (lambda (a, b): None)((1, 2))

First example works fine in both Python 2 and 3. Second example works in Python 2.7 and earlier, but tuple unpacking for arguments was removed in Python 3. [1]

[1] https://www.python.org/dev/peps/pep-3113/

For your second example, you need to unpack your parameters.

    (lambda x, y: None)(*(1, 2))

Unless you're saying you should be able to define the set size you pass in.

Yes, it was an example of something you _can't_ do.

Both of these were intentionally removed around Python 3, iirc: python 2.7 allows nested tuple/list destructuring as well as destructuring in function arg lists.

If I had to say anything I would guess that the number you unpack has to match the number of things returned rather than being a head and tail.

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