Python - Create object from JSON with optional key/value parameters

Question

Trying to create objects based on name/value pairs obtained in JSON form

e.g.

json = {'name': 'ruby',
        'type': 'rare',
        'size': {'weight': '5', 'height': '2'}
       }

however the name/values can be optional. So it could also be of the form

json = {'name': 'saph',
        'size': {'height': '4'}
        #'type' and 'height' have been omitted
       }

I attempted

class Gem(object):
    def __init__(self, **params):
        self.name = params['name']
        self.type = params['type']
        self.size = params['size']

gem = Gem(**json)

but it is expecting all parameters to be available.

I want to eventually be able to access properties of the object, e.g.

print(gem.name) #prints 'ruby'
print(gem.type) #prints 'rare'

Do you expect gem.name to be defined if json has no name entry? If so, what default value? — Patrick Haugh
– Patrick Haugh, Commented Aug 23, 2017 at 0:22

Egal · Accepted Answer · 2019-07-02 17:25:51Z

Use get() with a default value:

self.type = params.get('type', '')

Update

A more elegant way of achieving this would be to use the combination of the packages attrs and cattrs. Install them with pip (the minimal version of cattrs is important):

pip install attrs
pip install 'cattrs>=1.0.0rc0'

Then define your class as an attrs dataclass (cattrs doesn't support Python 3.7's standard dataclasses yet, which is why we use attrs):

In [1]: import attr

In [2]: import cattr

In [3]: @attr.s(auto_attribs=True) 
   ...: class Gem: 
   ...:     name: str = None 
   ...:     type: str = None  # Not a good name since it's a keyword, but I'm building upon the structure in the question
   ...:     size: str = None

And finally to construct an instance from a dictionary (which you possibly parsed from a JSON):

In [4]: cattr.structure({'name': 'saph', 
   ...:         'size': {'height': '4'} 
   ...:         #'type' and 'height' have been omitted 
   ...:        }, Gem)
Out[4]: Gem(name='saph', type=None, size="{'height': '4'}")

Vasif · Accepted Answer · 2017-08-23 00:25:28Z

1

You are trying to access undefined key type when you are doing this self.type = params['type'].

This will throw KeyError.

You might want to use:

self.type = params.get('type', '') #sets self.type to '' empty string
self.type = params.get('type') #sets self.type to None

answered Aug 23, 2017 at 0:25

Vasif

1,40311 silver badges26 bronze badges

Comments

audeos · Accepted Answer · 2017-08-23 00:26:02Z

0

class Gem(object):
    def __init__(self, **params):
        self.name = params['name'] if 'name' in params else None
        self.type = params['type'] if 'type' in params else None
        self.size = params['size'] if 'size' in params else None

gem = Gem(**json)

I would first check if the key is there if not leave it as None

answered Aug 23, 2017 at 0:26

audeos

3362 silver badges6 bronze badges

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