CODEWITHSUNDEEP
pythonsyntax-highlightingpython-dataclasses
Jonathanthesailor
Jonathanthesailor

Reputation: 181

understanding dataclass field usage

I am confused about how the usage of field() in dataclasses works. I have this (senseless) example:

from dataclasses import dataclass, field

@dataclass
class Person:
    name: str = field(init=False)

    def print_name(self):
        print(self.name)


dieter = Person()
dieter.print_name()

I would expect this to create an empty string. However, this only happens, when I add the parameter default="". Evenmore, in pycharm, no syntax highlight complains, when I write following code:

name: int = field(init=False, default="")

What is the point of type hints, if there is not syntax highlighting? I don't really have any problem, I'm just trying to grasp, what is actually happening.

The usage case is, that I am creating a class, that later contains matplotlib axes, plots etc. Since they don't exist in the beginning, I only want to initialize them, so they are connected to the class, and can later be created.

Upvotes: 0

Views: 2593

Answers (1)

chepner
chepner

Reputation: 531738

From the documentation (emphasis mine):

The dataclass() decorator examines the class to find fields. A field is defined as a class variable that has a type annotation. With two exceptions described below, nothing in dataclass() examines the type specified in the variable annotation.

So the type hint is mainly used as a flag to tell dataclass to pay attention to the name.

The two exceptions are if the annotation is ClassVar or InitVar, used to indicate that the name is not an instance attribute, but either a class attribute or an init-only parameter, respectively.

What I consider a third exception is the new KW_ONLY value introduced in Python 3.10, which indicates that the following fields cannot be passed as positional arguments to __init__. (The "field" so annotated doesn't define a field at all, but it's still a decorated name whose type dataclass pays attention to.)

Upvotes: 3

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