Python: how parse this dict recursively?

Question

I've a flat dict with entities. Each entity can have a parent. I'd like to recursively build each entity, considering the parent values.

Logic:

Each entity inherits defaults from its parent (e.g. is_mammal)
Each entity can overwrite the defaults of its parent (e.g. age)
Each entity can add new attributes (e.g. hobby)

I'm struggling to get it done. Help is appreciated, thanks!

entities = {
    'human': {
        'is_mammal': True,
        'age': None,
    },
    'man': {
        'parent': 'human', 
        'gender': 'male',
    },
    'john': {
        'parent': 'man',
        'age': 20,
        'hobby': 'football',
    }
};

def get_character(key):
    # ... recursive magic with entities ...
    return entity
    
john = get_character('john')
print(john)

Expected output:

{
    'is_mammal': True,  # inherited from human
    'gender': 'male'  # inherited from man
    'parent': 'man', 
    'age': 20, # overwritten
    'hobby': 'football', # added
}

Can entity have more than 1 parent? Can you have more than 2 levels of dictionaries? — matszwecja, Nov 03 '22 at 12:50
@matszwecja each entity has one parent but the parent can have another parent: john -> man (parent) -> human (grand-parent). Yes, it can be infinite levels. It must be recursive. Thanks for your help. — Mr. B., Nov 03 '22 at 12:53
"Yes, it can be infinite levels": Can you clarify if you mean: 1. infinite? 2. any number, but finite? 3. Any finite small number (within default stack depth) — Lesiak, Nov 03 '22 at 13:02
I was mostly asking about if "parent" can contain a dict itself instead of another key. — matszwecja, Nov 03 '22 at 13:06

matszwecja · Accepted Answer · 2022-11-03T13:05:23.780

2

def get_character(entities, key):
    try:
        entity = get_character(entities, entities[key]['parent'])
    except KeyError:
        entity = {}
    entity.update(entities[key])
    return entity

edited Nov 03 '22 at 13:05

answered Nov 03 '22 at 12:55

matszwecja

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btw Can the last two lines be just an `update()`? – quamrana Nov 03 '22 at 12:58
@quamrana yes, yes they can. – matszwecja Nov 03 '22 at 13:05
Good, that's what I thought. – quamrana Nov 03 '22 at 13:06

Nathan Furnal · Answer 2 · 2022-11-03T13:46:20.467

1

This solution is using recursion and a Python quirk where mutables (here it's a dictionary {}), are shared among function calls. See the discussion below for why this is somewhat surprising, though useful for accumulating recursion results.

def get_character(d, key, entity = {}):
    if d.get(key) is None:
        return entity
    return get_character(d, d.get(key).get('parent'), d.get(key) | entity)

get_character(entities, 'john')

{'is_mammal': True,
 'age': 20,
 'parent': 'man',
 'gender': 'male',
 'hobby': 'football'}

edited Nov 03 '22 at 13:46

answered Nov 03 '22 at 13:23

Nathan Furnal

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Don't use dictionaries as default argumetns - ["Least Astonishment" and the Mutable Default Argument](https://stackoverflow.com/q/1132941/9296093) – matszwecja Nov 03 '22 at 13:24
But you're not actually accumulating results in this mutable. In fact, it never is anything else other than empty dictionary. – matszwecja Nov 03 '22 at 13:32
So while the code works, I think not using this argument would make the code more explicit and also not teach bad code practice. – matszwecja Nov 03 '22 at 13:34
It is getting updated at each iteration though? – Nathan Furnal Nov 03 '22 at 13:36
Ok, my bad, although I'd still replace that with `get_character(d, key, entity = None)` and `if not entity: entity = {}` just to be 100% sure. – matszwecja Nov 03 '22 at 13:40
1

I won't because I don't see the point but I'll add a disclaimer. – Nathan Furnal Nov 03 '22 at 13:44

Python: how parse this dict recursively?

2 Answers2