CODEWITHSUNDEEP
pythonpython-3.x
Jean-Marc Volle
Jean-Marc Volle

Reputation: 3323

python class attributes as instance constructor default parameters

I would like to use class attributes as default parameters for instance constructor, and it seems python does not support it. Could someone explain why or correct the code below so that it can work?

OK but not explicit enough from my point of view

class B:
    class_size = 4
    def __init__(self):
        self.size = B.class_size

What I would like (does not work)

class A:
    class_size = 4
    def __init__(self,size = A.a_size):
        self.size = size

Using the above declaration python complains:

def __init__(self,size = A.a_size): NameError: name 'A' is not defined

Upvotes: 0

Views: 57

Answers (1)

AKX
AKX

Reputation: 168913

A is not a name while the class namespace is being built. Just un-qualify the name (A.class_size -> class_size):

class A:
    class_size = 4

    def __init__(self, size=class_size):
        self.size = size

Caveat

The above does not take inheritance into account, though, should you want to override the default in a subclass. For that you're going to need to just manually look up the default value in the constructor:

class A:
    default_size = 4

    def __init__(self, size=None):
        if size is None:
            size = self.default_size
        self.size = size

class B(A):
    default_size = 15


print(A().size)
print(B().size)
print(B("yes").size)

outputs

4
15
yes

If None would otherwise be a valid value for your argument, you may opt for a sentinel value instead:

NOTSET = object()
# ...
def __init__(self, size=NOTSET):
    if size is NOTSET:
        # ...

Upvotes: 2

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