[Python-ideas] Suggestions: dict.flow_update and dict.__add__

[Guido van Rossum]

If you have to tell such a long and convoluted story to explain a name that
you've picked out of the blue and that has no equivalent in other Python
data types, it's probably a bad idea. If you're proposing that other
mutating methods also gain a flow_XXX variant, please, no! That's like the
theory of supersymmetry (SUSY) in particle physics, where ever known
particle from the Standard Model would have to have a much heavier
"superpartner" just to make some esoteric idea work.

On Tue, Mar 5, 2019 at 12:54 AM Jonathan Fine <jfine2358 at gmail.com> wrote:

> SUMMARY
> Instead of using dict + dict, perhaps use dict.flow_update. Here,
> flow_update is just like update, except that it returns self.
>
> BACKGROUND
> There's a difference between a sorted copy of a list, and sorting the
> list in place.
>
>     >>> items = [2, 0, 1, 9]
>     >>> sorted(items), items
>     ([0, 1, 2, 9], [2, 0, 1, 9])
>     >>> items.sort(), items
>    (None, [0, 1, 2, 9])
>
> In Python, mutating methods generally return None. Here, this prevents
> beginners thinking their code has produced a sorted copy of a list,
> when in fact it has done an in-place sort on the list. If they write
>     >>> aaa = my_list.sort()
> they'll get a None error when they use aaa.
>
> The same goes for dict.update. This is a useful feature, particularly
> for beginners. It helps them think clearly, and express themselves
> clearly.
>
> THE PROBLEM
> This returning None can be a nuisance, sometimes. Suppose we have a
> dictionary of default values, and a dictionary of use supplied
> options. We wish to combine the two dictionaries, say into a new
> combined dictionary.
>
> One way to do this is:
>
>    combined = defaults.copy()
>    combined.update(options)
>
> But this is awkward when you're in the middle of calling a function:
>
>       call_big_method(
>           # lots of arguments, one to a line, with comments
>           arg = combined, # Look up to see what combined is.
>          # more arguments
>         )
>
> USING +
> There's a suggestion, that instead one extends Python so that this works:
>         arg = defaults + options # What does '+' mean here?
>
> USING flow_update
> Here's another suggestion. Instead write:
>         dict_arg = defaults.copy().flow_update(options) # Is this clearer?
>
> IMPLEMENTATION
> Here's an implementation, as a subclass of dict.
>
>     class mydict(dict):
>
>         def flow_update(self, *argv, **kwargs):
>             self.update(*argv, **kwargs)
>             return self
>
>         def copy(self):
>             return self.__class__(self)
>
> A DIRTY HACK
> Not tested, using an assignment expression.
>    dict_arg = (tmp := defaults.copy(), tmp.update(options))[0]
> Not recommend.
>
> --
> Jonathan
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>


-- 
--Guido van Rossum (python.org/~guido)
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