How to avoid redefining default parameter with Python inheritance

Question

For simple inheritance such as

class Base:
  def __init__(self, env: str = 'prod'):
    #...

class Derived(Base):
  def __init__(self, env: str = 'prod'):
    super().__init__(str)

How do I avoid redefining default constructor parameters in both classes? If I instantiate Derived() I do want env to default to 'prod', I just don't want to redefine the value in multiple classes. If I later decide that the default value should be something else (e.g. 'dev'), then I would have to change all derived constructors.

I was thinking of something like

class Base:
  def __init__(self, env: str = 'prod'):
    #...

class Derived(Base):
  def __init__(self, env: str = None):
    if env is None:
      super().__init__()
    else:
      super().__init__(env)

or maybe defining the default value as a global constant in the base class? Either method seems overly complex. What's the Pythonic way?

If the __init__ in the derived class is the same as in the base class, just don't redefine it in Derived. — finswimmer
– finswimmer, Commented Aug 13, 2019 at 12:39

Valentin B. · Accepted Answer · 2019-08-13 12:48:23Z

Make a class variable.

class Base:
    DEFAULT_ENV = "prod"
    def __init__(self, env=None):
        if env is None:
            env = self.DEFAULT_ENV
        # ...

If you want to have a different default env in a subclass, just override DEFAULT_ENV in that class. If you want to change your default env for all classes that do not override it, changing DEFAULT_ENV in Base will work.

Also redifining __init__ in a subclass is optional in Python so if you don't want __init__ in your subclasses to implement different behavior from Base just ommit it in subclasses. Another advantage of having your default env in a class variable is that if you want to change only that in a subclass you can override the class variable and you do not have to redifine __init__ in the child !

EDIT: if you feel that env determination might require some funny business (like testing for environment variables, or anything else in your app) later on, you might want to wrap default_env in a property:

class Base:
    DEFAULT_ENV = "prod"
    def __init__(self, env=None):
        if env is None:
            env = self.default_env
        # ...
    @property
    def default_env(self):
        return self.DEFAULT_ENV

That way later on you can implement the expected behaviour in the property getter.

6761646f6e · Accepted Answer · 2021-12-13 14:12:47Z

1

Another option that has its pros and cons could be to use kwargs. Something like:

class Base:
  def __init__(self, env: str='prod'):
    #...

class Derived(Base):
  def __init__(self, **kwargs):
    super().__init__(**kwargs)

answered Dec 13, 2021 at 14:12

6761646f6e

1013 bronze badges

Comments

kdubs · Accepted Answer · 2025-05-21 02:56:54Z

0

tweaked the answer so you don't need the IF.

class Base:
    default = 123

    def __init__(self, what=default):
        self.what = what
        self.kind = 'base'

    def __call__(self):
        print('INFO', self.what, self.kind)

class Sub(Base):

    def __init__(self, what=Base.default):
        super().__init__(what)
        self.kind = 'sub'
        

b = Base()
s = Sub()
s1 = Sub(456)

b()
s()
s1()

and the output:

INFO 123 base
INFO 123 sub
INFO 456 sub

answered May 21 at 2:56

kdubs

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