Dict Unpacking in Python

Did not know that such things could be accomplished by registering a new file coding format. Reminds me of https://pypi.org/project/goto-statement/

In short, it runs a text preprocessor as the source text decoder (like you would decode from Latin-1 or Shift-JIS to Unicode).

This one is arguably even more of a hack; it's working at the source code level rather than the AST level.

The "coding" here is a bytes-to-text encoding. The Python lexer expects to see character data; you get to insert arbitrary code to convert the bytes to characters (or just use existing schemes the implement standards like UTF-8).

I found dictionary unpacking to be quite useful, when you don't want to mutate things. Code like:

    new_dict = {**old_dict, **update_keys_and_values_dict}

Or even complexer:

    new_dict = {
        **old_dict,
        **{
            key: val
            for key, val in update_keys_and_values_dict
            if key not in some_other_dict
        }
    }

It is quite flexible.

The author also has an accompanying video: https://youtu.be/eqiM0xRmFJg

  def u(**kwargs):
    return tuple(kwargs.values())

Am I missing something, is this effectively the same?

*I realize the tuple can be omitted here

Yours relies on ordering, OP's presumably does not.

You have to pull them out by key name, and not just get everything. Here's a working version, though with a totally different syntax (to avoid having to list the keys twice, once as keys and once as resulting variable names):

  >>> def u(locals, dct, keys):
  ...     for k in keys:
  ...         locals[k] = dct[k]
  ... 
  >>> dct = {'greeting': 'hello', 'thing': 'world', 'farewell': 'bye'}
  >>> u(locals(), dct, ['greeting', 'thing'])
  >>> greeting
  'hello'
  >>> thing
  'world'
  >>> farewell
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  NameError: name 'farewell' is not defined

Modifying locals() is generally frowned upon, as there's no guarantee it'll work. But it does for this example.

I love the union syntax in 3.9+:

  new_dict = old_dict | update_keys_and_values_dict

Don’t forget the in place variant!

  the_dict |= update_keys_and_values_dict

While not nearly as fun as the OP, I’d note that this sort of unpacking is very pleasant in the newish PEP636 match case statements:

https://peps.python.org/pep-0636/#matching-builtin-classes

I would donate $500 to the PSF tomorrow if they added this, the lack of it is daily pain

you can't do this consistently across all cases without compiler assistance (see https://doc.rust-lang.org/book/ch19-03-pattern-syntax.html or https://peps.python.org/pep-0636/#matching-builtin-classes linked below).

yeah that's the funny part here, would never have thought of this

Coming from lisp/haskell I always wanted destructuring but after using it quite a lot in ES6/Typescript, I found it's not always as ergonomic and readable as I thought.

perfect is enemy of good imo, dict destructuring is so valuable that I'm willing to bend some rules / add some rules to make it possible. can't we just copy whatever JS does?

If it's that valuable to you personally you can use that project to remove your "daily pain". No need to inflict the pain caused by such a thing being present in official Python. Some of us like for the language to remain highly readable.

Or use itemgetter:

  >>> from operator import itemgetter
  >>> dct = {'greeting': 'hello', 'thing': 'world', 'farewell': 'bye'}
  >>> thing, greeting = itemgetter("thing", "greeting")(dct)
  >>> thing
  'world'
  >>> greeting
  'hello'

No no, do forget about it: like += for lists, |= mutates “the dict”, which often makes for awkward bugs.

And like += over list.extend, |= over dict.update is very little gain, and restricts legal locations (augmented assignments are statements, method calls are expressions even if they return "nothing")

> perfect is enemy of good imo

You can't land a language feature that only sometimes works - that's absolutely horrid UX.

> can't we just copy whatever JS does?

I wasn't aware that js does this and I don't know it's implemented. So maybe I should retract my claim about compiler assistance.

TFA looks things up by key, and allows pulling a subset of the dict.

This confuses me a bit

  dct = {'a': [1, 2, 3]}
  {'a': [1, *rest]} = dct
  print(rest)  # [2, 3]

Does this mean that i can use?

  dct = {'a': [1, 2, 3]}
  {'b': [4, *rest]} = dct
  print(rest)  # [2, 3]

and more explicit

  dct = {'a': [1, 2, 3]}
  {'_': [_, *rest]} = dct
  print(rest)  # [2, 3]

You shouldn't be using dicts for data that you know the name of anyway - use dataclasses or named tuples. Dicts are best for things with keys that are not known at compile time.