Python class's init function takes instance of own class as default parameter

Question

I'm trying to define a class which takes in a default parameter for its init function. I've defined a class as follows:

class Node:
    def __init__(self,name,visited=False,distance=math.inf,path=Node('-')):
        self.name = name
        self.visited = visited
        self.distance = distance
        self.path = path

and I get the following error:

NameError: name 'Node' is not defined

I was able to get around this problem by "pre-defining" the parts of the class that I needed, like so:

class Node:
    def __init__(self,name):
        self.name = name

class Node:
    def __init__(self,name,visited=False,distance=math.inf,path=Node('-')):
        self.name = name
        self.visited = visited
        self.distance = distance
        self.path = path

but I can't shake the feeling there's a better, more pythonic way.

What should the path be for the Node object you'd be setting as the default? — Blckknght
– Blckknght, Commented May 2, 2020 at 2:26
Sure, but does the dummy node point to itself, or to some other Node object? What does the first Node to ever exist point to, when there don't exist any other Nodes yet? — Blckknght
– Blckknght, Commented May 2, 2020 at 2:56
I see what you're getting at after reading through juanpa's answer. I've accepted his 'default-path' solution. — funkopops
– funkopops, Commented May 2, 2020 at 3:03
Another alternative is just to use a sentinel object as the default value for that Node, even just SENTINEL = object() — juanpa.arrivillaga
– juanpa.arrivillaga, Commented May 2, 2020 at 3:21

wjandrea · Accepted Answer · 2022-01-16 18:50:38Z

2

You should do this:

class Node:
    def __init__(self, name, visited=False, distance=math.inf, path=None):
        self.name = name
        self.visited = visited
        self.distance = distance
        if path is None:
            self.path = Node('-', path="default-path")
        else:
            self.path = path

This is the idiom you should be following with mutable default arguments to begin with.

However, you need the default to have a path, or it will recurse without stopping.

edited Jan 16, 2022 at 18:50

wjandrea

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answered May 2, 2020 at 2:27

juanpa.arrivillaga

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2 Comments

Blckknght Over a year ago

I'm pretty sure this recurses forever, since the new node you create needs a path value of its own.

juanpa.arrivillaga Over a year ago

@Blckknght yes, yes

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