Question

In the following code:

class A(object):
  VALUE = 1
  def __init__(self, value=VALUE):
    self.value = value

class B(A):
  VALUE = 2

i’d expect that B().value should be equal to 2, however:

B().value = 1

Is there an elegant way to define a class hierarchy where child classes can just declare class variables they want to override and have them be defaults for the instance variables? I still want to allow for these to be changed on a per-instance level, eg.

b = B(value=3)

Answer 1

This is another default arguments question. The point is that when you write

def foo(value=VALUE):

the code inside the function is compiled and made into a function object. It is at this time — not at call time! — that the default arguments are stored. So by the time you have defined B it is too late: the default value of foo is already set and changing VALUE won’t have any effect.

If this seems a strange thing to do, suppose foo was a global function:

default = 3
def foo(x=default): pass

Then any other code, anywhere, could screw up foo by doing

global default
default = 4

This is arguably just as confusing.

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To force the lookups to be done at runtime not compile time, you need to put them inside the function:

def foo(value=None):
    self.value = self.VALUE if value is None else value

or (not quite the same but prettier)

self.value = value or self.VALUE

(This is different because it will treat any ‘falsy’ value as a sentinel — that is, 0, [], {} etc will all be overwritten by VALUE.)

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EDIT: @mgilson pointed out another way of doing this:

def foo(**kwargs):
    self.value = kwargs.get("value", self.VALUE)

This is neater in that it doesn’t require you to make a sentinel value (like None or object(), but it does change the argument specification of foo quite fundamentally since now it will accept arbitrary keyword arguments. Your call.