> On May 11, 2019, at 4:51 PM, Christopher Barker <python...@gmail.com> wrote: > >> On Sat, May 11, 2019 at 1:38 PM Brendan Barnwell <brenb...@brenbarn.net> >> wrote: > >> > protocol for something different -- I can't image what that would be. >> > Sure, on the general case, maybe, but for a class that has a "natural" >> > use case for dictifying, how else would you want to iterate over it?? >> >> One example would be. . . dicts. Calling dict on a dict gives you a >> dict, but iterating over the dict just gives you the keys. Certain >> other mapping types (such as Pandas Series and DataFrames) also have >> this behavior where iterating over the mapping gives you only the keys. > > well, there is that ... :-) And that IS the mapping protocol. I guess i was > thinking that you had an object that was NOT a Mapping already. > > If it IS a Mapping, then presumably it already can be passed into the dict > constructor, yes? > > So if you want your class "dictable", then you can implement either the > Mapping ABC, or the iteration protocol. I"m having an even harder time > imagining why you would want a Mapping interface that isn't the same as what > you'd want turned into a dict. > > So why again do we yet another way to do it? > > Anyway, I was playing with this, and write a prototype decorator that makes a > datacalss "dictable" -- that it, it can be passed in to the dict constructor > to make a dict. This is done by providing an iterator that returns (key, > value) pairs. > > The code is in this git rep, if anyone wants to comment on it or improve, it, > or... > > https://github.com/PythonCHB/PythonTopics/blob/master/Sources/code/dict_making/dictable_dataclass.py > (also here for your reading pleasure) > > I'm not sure how the dict constructor decides an input is a mapping or an > iterable -- maybe I'll play with that now.
It’s a design goal of dataclasses to not be iterable. https://www.python.org/dev/peps/pep-0557/#why-not-just-use-namedtuple Eric > > -CHB > > #!/usr/bin/env python > > """ > A prototype of a decorator that adds an iterator to a dataclass > so it can be passed in to the dict() constructor to make a dict. > > If this is thought to be useful it could be added to the dataclass > decorator itself, to give all decorators this functionality. > """ > > > from dataclasses import dataclass > > > class DataClassIterator: > """ > Iterator for dataclasses > > This used the class' __dataclass_fields__ dict to iterate through the > fields and their values > """ > > def __init__(self, dataclass_instance): > self.dataclass_instance = dataclass_instance > self.keys_iter = iter(dataclass_instance.__dataclass_fields__.keys()) > > def __iter__(self): > return self > > def __next__(self): > key = next(self.keys_iter) > return (key, getattr(self.dataclass_instance, key)) > > > def _dunder_iter(self): > """ > function used as the __iter__ method in the dictable_dataclass decorator > """ > return DataClassIterator(self) > > > def dictable_dataclass(the_dataclass): > """ > class decorator for making a dataclass iterable in a way that is > compatible > with the dict() constructor > """ > the_dataclass.__iter__ = _dunder_iter > > return the_dataclass > > > # Example from the dataclass docs: > @dictable_dataclass > @dataclass > class InventoryItem: > '''Class for keeping track of an item in inventory.''' > name: str > unit_price: float > quantity_on_hand: int = 0 > > def total_cost(self) -> float: > return self.unit_price * self.quantity_on_hand > > > if __name__ == "__main__": > # try it out: > inv_item = InventoryItem("sneakers", 50.0, 20) > > print("an InventoryItem:\n", inv_item) > print() > > print("And the dict you can make from it:") > print(dict(inv_item)) > > > > > > > > > > >> -- >> Brendan Barnwell >> "Do not follow where the path may lead. Go, instead, where there is no >> path, and leave a trail." >> --author unknown >> _______________________________________________ >> Python-ideas mailing list >> Python-ideas@python.org >> https://mail.python.org/mailman/listinfo/python-ideas >> Code of Conduct: http://python.org/psf/codeofconduct/ > > > -- > Christopher Barker, PhD > > Python Language Consulting > - Teaching > - Scientific Software Development > - Desktop GUI and Web Development > - wxPython, numpy, scipy, Cython > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/
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