Class init in Python with custom list of parameters, set default if parameters is not explicitly passed

Question

I want to define in Python 3.9 a class which gets as a parameter a dictionary or a list of arguments and sets the class attributes in the following matter:

If the key name is passed in the argument it uses it to set the corresponding class attribute.
If the key name is not passed, it sets the class attribute with a default value.

One way to do this is:

class example():
   def __init__(self, x=None, y=None):
       if x is None:
          self.x = "default_x"
       else:
          self.x = x
       if y is None:
          self.y = "default_y"
       else:
          self.y = y

or shortly (thank to matszwecja):

class example():
   def __init__(self, x="default_x", y="default_y"):
      self.x = x
      self.y = y

Is there a more Pythonic way to do so without hard-coding if to each parameter? I want some flexibility so that if I add another attribute in the future, I won't be needed to hard-code another if for it.

I want to declare a class with

class example():
   def __init__(kwargs):
     ?

Such as if I pass example(y="some", z=1, w=2) it will generate

self.x = "default_x"
self.y = "some"
self.z = 1
self.w = 2

x = 'default_x' instead of x=None? That's literally what default is for. — matszwecja
– matszwecja, Commented Dec 21, 2022 at 9:08
Yes, but I want that I can pass the argument in a dynamic way, so I won't need to specify the entire list of arguments in the header. — Triceratops
– Triceratops, Commented Dec 21, 2022 at 9:12
I don't understand how that changes anything. You're already doing the exact same thing, just setting it to None for some reason instead of the actual default value you want. — matszwecja
– matszwecja, Commented Dec 21, 2022 at 9:13
This is how I do it now. Is there an automated way to generate the list of self.x from the arguments? — Triceratops
– Triceratops, Commented Dec 21, 2022 at 9:22
Something like this? pythontutorial.net/python-oop/python-dataclass — matszwecja
– matszwecja, Commented Dec 21, 2022 at 9:39

matszwecja · Accepted Answer · 2022-12-21 09:18:46Z

1

class example():
    def __init__(self, x="default_x", y="default_y"):
        self.x = x
        self.y = y

answered Dec 21, 2022 at 9:18

matszwecja

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Comments

chepner · Accepted Answer · 2022-12-21 14:00:38Z

0

Consider using a dataclass to generate the boilerplatey __init__ method for you.

from dataclasses import dataclass


@dataclass
class Example:
    x: str = "default_x"
    y: str = "default_y"

If you need to provide a mutable default (say, a new empty list if no explicit list is provided), you can use dataclasses.field to explicitly create a field with a default factory; the dataclass decorator uses the Field object to generate the necessary code for the instance attribute.

from dataclasses import dataclass, field


@dataclass
class Example:
    x: str = "default_x"
    # Bad!
    # y: list[str] = []
    y: list[str] = field(default_factory=list)  # Good

answered Dec 21, 2022 at 14:00

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chepner

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