Python: Why can't I unpack a tuple into a dictionary?

Question

Why doesn't this work?:

d["a"], d["b"] = *("foo","bar")

Is there a better way to achieve what I'm trying to achieve?

I would have never even thought to try something like this ... — mgilson, Feb 13 '13 at 13:46

Rohit Jain · Accepted Answer · 2013-08-21T16:42:42.863

18

It would work if you define a dictionary d before hand, and remove the * from there:

>>> d = {}
>>> d["a"], d["b"] = ("foo","bar")

In fact, you don't need those parenthesis on the RHS, so this will also work:

>>> d['a'], d['b'] = 'foo', 'bar'

edited Aug 21 '13 at 16:42

answered Feb 13 '13 at 13:28

Rohit Jain

Ahh, you don't need the *. Thanks. – jsj Feb 13 '13 at 13:29
@trideceth12: Out of curiosity, where did you get the idea for the `*`? – David Robinson Feb 13 '13 at 13:32
3

This is definitely cool (+1) -- [But I still assert `d.update(a='foo',b='bar')` is nicer](http://stackoverflow.com/a/14854908/748858) :) – mgilson Feb 13 '13 at 13:42
You also don't need the `()` on the RHS: `d['a'], d['b'] = 'foo', 'bar'` – ecatmur Feb 13 '13 at 13:50
@DavidRobinson I'd guess it may come from (in)famous *splat* operator of Ruby :-) – David Unric Feb 13 '13 at 14:06
@DavidRobinson You need it to unpack tuples into function arguments – jsj Feb 13 '13 at 14:08

mgilson · Answer 2 · 2013-02-13T13:54:25.407

Others have showed how you can unpack into a dict. However, in answer to your question "is there a better way", I would argue that:

d.update(a='foo',b='bar')

much easier to parse. Admitedtly, this doesn't work if you have a and b which are variables, but then you could use:

d.update({a:'foo',b:'bar'})

and I think I still prefer that version for the following reasons:

It scales up to multiple (>2) values nicer as it can be broken onto multiple lines more cleanly
It makes it immediately clear which key is associated with which value

And if you start off with a 2-tuple of values, rather than it being static as you show, you could even use zip:

d.update( zip(("a","b"),("foo","bar")) )

which is admittedly not as nice as the other two options ...

... And we've just covered all 3 ways you can use dict.update :).

This is really nice. Forgot about it. At least this will not get ugly when number of value increases. +1 — Rohit Jain, Feb 13 '13 at 13:43

score 0 · Answer 3 · answered Feb 13 '13 at 13:32

0

It's just a typo (the *). This works (tested in Python 2.7.3):

d = dict()
d["a"], d["b"] = ("foo", "bar")

answered Feb 13 '13 at 13:32

Carsten

1

It's not a typo. He used an unpacking operator which is used with lists and args. There is no need to use this operator when working with tuples. :) – Marek Marczak Sep 26 '18 at 10:25

3 Answers3