Dict Unpacking in Python / hacker news

I use the Python package 'sorcery' [0] in all my production services.

It gives dict unpacking but also a shorthand dict creation like this:

    from sorcery import dict_of, unpack_keys
    a, b = unpack_keys({'a': 1, 'b': 42})
    assert a == 1
    assert b == 42
    assert dict_of(a, b) == {'a': 1, 'b': 42}

[0] https://github.com/alexmojaki/sorcery

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

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/

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 think there's a package to treat Jupyter notebooks as source code (so you can import them as modules).

While the OP package is obviously a joke, the one with notebooks is kind of useful. And, of course, obligatory quote about how languages that don't have meta-programming at the design level will reinvent it, but poorly.

You talking about this?

https://jupyter-notebook.readthedocs.io/en/stable/examples/N...

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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.

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

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")

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

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

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

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]

> Does this mean that i can use?

They'll both trigger a runtime error, since the key you're using in the pattern (LHS) does not match any key in the dict.

Note that `'_'` is an actual string, and thus key, it's not any sort of wildcard. Using a bare `_` as key yields a syntax error, I assume because it's too ambiguous for the author to want to support it.

None of the last two LHSes will match `dct`, so you'll get a runtime error.

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.

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

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.

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).

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.

> 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.

Now come on... for code golf? Why on Earth would anyone want extra syntax in a language with already tons of bloat in the syntax that contribute nothing to language's capabilities? It's, in Bill Gates words, like paying to make airplanes heavier...

This package is a funny gimmick, to illustrate, probably, unintended consequences of some of the aspects of Python's parser. Using this for anything other than another joke is harmful...

Python needs a better dictionary. Also, Python needs better names for things than dict.

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

Am I missing something, is this effectively the same?

*I realize the tuple can be omitted here

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.

Or use itemgetter:

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

There are so many things like this one in the standard library that it kinda pisses me off when I discover a new one.

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

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

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