Dynamically adding attributes to init method in python

Question

Is there any way you can define a function that can add,at some later point in the program, new attributes to an already existing __init__ method? For example, below I created a Class for a Family Tree. Each instance would create a root.

class FamilyTree:
    def __init__(self,rootObj):
        self.key = rootObj

I want each new root instance to have the ability to have a specific number of children attributes: For example:

self.child1 = 'B'
self.child2 = 'C'
self.child3 = 'D'

Since every new root instance can have various number of children, how can I variably add new attributes to the __init__ method?

how do you know how many children does an instance have? a new variable used when initalizing maybe? — lesingerouge
– lesingerouge, Commented May 12, 2016 at 6:44
Your wording is confusing and misleading. You are not adding to the __init__ method, you are adding to the class instance. Consider rewording your question slightly to clear up this ambiguity. — Paul Rooney
– Paul Rooney, Commented May 12, 2016 at 9:45

Byte Commander · Accepted Answer · 2016-05-12 06:50:58Z

3

I guess what you really (should) want is a list of children:

class FamilyTree:

    def __init__(self, rootObj):
        self.key = rootObj
        self.children = []

    def add_children(*new_children)
        self.children.extend(new_children)

Now you can use the add_children method to add any number of children to the list at once, or you could simply directly access the children instance member list as well:

tree = FamilyTree("whatever a rootObj is...")

tree.add_children("Alice", "Bob", "Claudia", "Dave")
tree.children.append("Eva")
tree.children += ["Fred", "Gina", "Herbert"]

print(tree.children)
# Output: ["Alice", "Bob", "Claudia", "Dave", "Eva", "Fred", "Gina", "Herbert"]

answered May 12, 2016 at 6:50

Byte Commander

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Comments

Dhia · Accepted Answer · 2016-05-12 07:07:14Z

2

A possible way to automate this is as follow:

class FamilyTree:
    def __init__(self,rootObj, child_names=None):
        self.key = rootObj
        if child_names is not None:
            # child_names is a list of child attributes values
            for i, child in enumerate(child_names):
                setattr(self, 'child{0}'.format(i), child)
            setattr(self, 'child_nbr', len(child_names))

    def add_child(self, *child_names)
        for name in child_names:
            self.child_nbr += 1
            setattr(self, 'child{0}'.format(self.child_nbr), name)

usage:

>>> f=FamilyTree('Family1', ['B', 'C', 'D'])
>>> print(f.child1, f.child2, f.child3)
>>> i = 0
>>> while i < f.child_nbr:
>>>     print getattr(f, 'child{0}'.format(i+1))
>>> f.add_child('E')
>>> print(f.child4)
>>> f.add_child('F', 'G')
>>> print(f.child5, f.child6)

edited May 12, 2016 at 7:07

answered May 12, 2016 at 7:00

Dhia

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1 Comment

NoPayn Over a year ago

Thank you. setattr is exactly what I was looking for.

Abhra · Accepted Answer · 2016-05-12 04:57:56Z

0

class FamilyTree:
    def __init__(self,rootObj):
        self.key = rootObj



child1='B'
child2='C'
child3='D'

objold=FamilyTree('keyold')

FamilyTree.child1=child1
FamilyTree.child2=child2
FamilyTree.child3=child3


objnew=FamilyTree('keynew')

print objnew.key,objnew.child1,objnew.child2,objnew.child3
print objold.key,objold.child1,objold.child2,objold.child3
'''
keynew B C D
keyold B C D
'''

answered May 12, 2016 at 4:57

Abhra

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Comments

Matthias Schreiber · Accepted Answer · 2016-05-12 06:34:28Z

Maybe I understand this wrong, but you don't add attributes to the 'init' method, you add attributes to the instance of the class. 'self' refers to the instance of the class. To add attributes during runtime to a class, look at Abhra's example. To add attributes to a specific instance of a class, look here (sorry Abhra, stole your example):

class FamilyTree:
    def __init__(self,rootObj):
        self.key = rootObj



child1='B'
child2='C'
child3='D'

objold=FamilyTree('keyold')

objold.child1=child1
objold.child2=child2
objold.child3=child3


objnew=FamilyTree('keynew')

print objnew.key, objnew.child1, objnew.child2, objnew.child3
print objold.key, objold.child1, objold.child2, objold.child3
'''
keynew AttributeError: 'FamilyTree' object has no attribute 'child1' (same error for the others)
keyold B C D
'''

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