Python conditional get value from dict?

Question

Given this json string, how can I pull out the value of id if code equals 4003?

error_json = '''{
    'error': {
        'meta': {
            'code': 4003,
            'message': 'Tracking already exists.',
            'type': 'BadRequest'
        },
        'data': {
            'tracking': {
                'slug': 'fedex',
                'tracking_number': '442783308929',
                'id': '5b59ea69a9335baf0b5befcf',
                'created_at': '2018-07-26T15:36:09+00:00'
            }
        }
    }
}'''

I can't assume that anything other than the error element exists at the beginning, so the meta and code elements may or may not be there. The data, tracking, and id may or may not be there either.

The question is how to extract the value of id if the elements are all there. If any of the elements are missing, the value of id should be None

Answer 1

A python dictionary has a get(key, default) method that supports returning a default value if a key is not found. You can chain empty dictionaries to reach nested elements.

# use get method to access keys without raising exceptions 
# see https://docs.quantifiedcode.com/python-anti-patterns/correctness/not_using_get_to_return_a_default_value_from_a_dictionary.html
code = error_json.get('error', {}).get('meta', {}).get('code', None)

if code == 4003:
    # change id with _id to avoid overriding builtin methods
    # see https://docs.quantifiedcode.com/python-anti-patterns/correctness/assigning_to_builtin.html
    _id = error_json.get('error', {}).get('data', {}).get('tracking', {}).get('id', None)

Now, given a string that looks like a JSON you can parse it into a dictionary using json.loads(), as shown in Parse JSON in Python