4 ms·

Pretty much anywhere you're tempted to use a namedtuple, you should be using a dataclass[0] instead. And typing JSON-like data is possible with TypedDict[1].

by rcfox 2y ago

Pretty much anywhere you're tempted to use a namedtuple, you should be using a dataclass[0] instead. And typing JSON-like data is possible with TypedDict[1]. [0] https://docs.python.org/3/library/dataclasses.html https://docs.python.org/3/library/dataclasses.html [1] https://docs.python.org/3/library/typing.html#typing.TypedDict https://docs.python.org/3/library/typing.html#typing.TypedDi...

ansgri 2y ago
Why? I thought one should prefer immutability. As for typed dicts.. yes, I’m mostly stuck on old python versions, nice reminder.
- maleldil 2y ago
  You can use TypedDict from `typing_extensions` if your version doesn't have it. You can use a lot of the newer stuff from there, too, especially if you enable `__future__.annotations`. How old is your Python, though? TypedDict is from 3.8. That was 5 years ago.
- throwaway2037 2y ago
  You can use: > @dataclass(frozen=True) to create an immutable data class.
  - maleldil 2y ago
    While that works (and I use it extensively), it's a bit hacky. You have to use `object.__setattr__` to set attributes in `__init__` or `__post_init__`, which looks so wrong.
    - maxbond 2y ago
      I think the cleaner alternative would be to use a static or class method as an alternative constructor and use the init the dataclass decorator provides for you. Eg something like: @dataclass(frozen=True) class Foo: bar: int baz: str @classmethod def new(cls, bar: int) -> "Foo": baz = calculate_baz(bar) return cls(bar, baz) foo = Foo.new(10)
    - throwaway2037 2y ago
      What is wrong with a static factory method?
- int_19h 2y ago
  In general, preferring immutability is great. In Python specifically, it can be hard to pull off given that e.g. something as basic as dict does not have a standard immutable equivalent. You inevitably have to rely on conventions - basically saying "this is supposed to be immutable" rather than enforcing it.
PeterisP 2y ago
I don't get why I would choose a dataclass in cases where I've already decided that an ordinary tuple would be a better fit than a normal class (i.e. "anywhere you're tempted to use a namedtuple") To me, namedtuples are a convenience to give a nicer syntax than ordinary tuples in scenarios where I don't want the overhead of having to store a copy of all the keys with every object, like a dict would. Dataclass seems to be even more stuff on top of a class which is effectively even more stuff on top of a dict, but all the use cases of namedtuples are those where you want much less stuff than an ordinary class has. And I don't want to have to define a custom class just as I often don't define a custom namedtuple in my code but use the one the database driver generates based on the query, which is a very common use case for namedtuples as efficient temporary storage of data that then gets processed to something else.

by rcfox 2y ago

ansgri 2y ago

Why? I thought one should prefer immutability. As for typed dicts.. yes, I’m mostly stuck on old python versions, nice reminder.

maleldil 2y ago
You can use TypedDict from `typing_extensions` if your version doesn't have it. You can use a lot of the newer stuff from there, too, especially if you enable `__future__.annotations`. How old is your Python, though? TypedDict is from 3.8. That was 5 years ago.
throwaway2037 2y ago
You can use: > @dataclass(frozen=True) to create an immutable data class.
- maleldil 2y ago
  While that works (and I use it extensively), it's a bit hacky. You have to use `object.__setattr__` to set attributes in `__init__` or `__post_init__`, which looks so wrong.
  - maxbond 2y ago
    I think the cleaner alternative would be to use a static or class method as an alternative constructor and use the init the dataclass decorator provides for you. Eg something like: @dataclass(frozen=True) class Foo: bar: int baz: str @classmethod def new(cls, bar: int) -> "Foo": baz = calculate_baz(bar) return cls(bar, baz) foo = Foo.new(10)
  - throwaway2037 2y ago
    What is wrong with a static factory method?
int_19h 2y ago
In general, preferring immutability is great. In Python specifically, it can be hard to pull off given that e.g. something as basic as dict does not have a standard immutable equivalent. You inevitably have to rely on conventions - basically saying "this is supposed to be immutable" rather than enforcing it.

PeterisP 2y ago

I don't get why I would choose a dataclass in cases where I've already decided that an ordinary tuple would be a better fit than a normal class (i.e. "anywhere you're tempted to use a namedtuple") To me, namedtuples are a convenience to give a nicer syntax than ordinary tuples in scenarios where I don't want the overhead of having to store a copy of all the keys with every object, like a dict would. Dataclass seems to be even more stuff on top of a class which is effectively even more stuff on top of a dict, but all the use cases of namedtuples are those where you want much less stuff than an ordinary class has. And I don't want to have to define a custom class just as I often don't define a custom namedtuple in my code but use the one the database driver generates based on the query, which is a very common use case for namedtuples as efficient temporary storage of data that then gets processed to something else.