How to retrieve attributes?

Question

class character():

  class personalized:
    def __init__(self, name, age, height, width, hair_color, eye_color):

      # I want to do this
      for attr in personalized:
        personalized.attr = attr


      #instead of this
      personalized.name = name
      personalized.age = age
      personalized.height = height

If I am using classes with a lot of attributes I don't want to have to set it equal to a variable every time because it will take up a lot of space. Is there a way to write it like I did above but actually works. In essence I don't know how to retrieve attributes from the __init__ function.

Attributes don't exist until you add them. There's no way for the `__init__` method to know what attributes you want to use in the class. — Barmar, Sep 11 '20 at 22:44
You can't access parameter variables dynamically, unless you use `eval()`. — Barmar, Sep 11 '20 at 22:45
also consider refractoring the class so it doesn't have to own all of those argument, therefore having to set less variables in your class. — Arturo, Sep 11 '20 at 22:53
Pass a kwargs-style dict to `init()`, instead of individual arguments. That way you can loop over the dict and extract the variable names. — John Gordon, Sep 11 '20 at 23:14
@Barmar `setattr(self,'parameter',value)` will create an instance variable dynamically. — Mark Tolonen, Sep 12 '20 at 00:48
But just do it the long way. The code is clear. Or use `dataclass` if using Python 3.7+ if this is a data structure and not a class with other methods. — Mark Tolonen, Sep 12 '20 at 00:49
@MarkTolonen But how would it know which attributes to create? — Barmar, Sep 12 '20 at 00:58
@Barmar I didn’t say it was a solution, just a comment about your `eval` comment. — Mark Tolonen, Sep 12 '20 at 02:57
@MarkTolonen I was talking about accessing the parameter variables dynamically, not the attributes. E.g. `setattr(self, parameter, eval(parameter))` — Barmar, Sep 12 '20 at 03:55
@Barmar Still don't need `eval`. `vars()` can access the parameter names and values. I made an answer with that solution, although think dataclasses are a better way to go. — Mark Tolonen, Sep 12 '20 at 05:03

score 1 · Answer 1 · answered Sep 12 '20 at 00:36

I would recommend using dataclasses for this. In your case you would just add:

from dataclasses import dataclass

@dataclass
class personalized:
    name: str
    age: int
    height: int
    width: int
    hair_color: str
    eye_color: str

This will auto-construct a init for you with self-assigned attributes

Frank · Answer 2 · 2020-09-11T22:56:36.490

You could use the __set__(..) Function (https://docs.python.org/3/howto/descriptor.html), but I do not suggest this because:

From a readability perspective this will harder to maintain over the long term (code is more often read than written)
everytime you want to access such an entry you first of all have to check if the descriptor/attribute is availible, thus making your down the road code worse. see: How to know if an object has an attribute in Python

Mark Tolonen · Answer 3 · 2020-09-12T05:12:19.740

Using vars() and setattr() can do what you want, but I recommend dataclasses as in the other answer if using Python 3.7+.

vars() returns a dictionary of local variables. At the top of __init__ dictionary contains self and any parameters as keys and along with their values.

setattr() will set an attribute value on an object.

class Personalized:
    def __init__(self, name, age, height, width, hair_color, eye_color):
      for key,value in vars().items():
          if key != 'self':
              setattr(self, key, value)

p = Personalized('name',5,10,12,'black','blue')
print(p.name,p.age,p.height,p.width,p.hair_color,p.eye_color)

Output:

name 5 10 12 black blue

How to retrieve attributes?

3 Answers3