Is there an exception free way to access values from a dictionary containing lists. For example, if I have:
data = { "object_1": { "object_2": { "list": [ { "property": "hello" } ] } } }
How do I access the path data['object_1']['object_2']['list'][0]['property']
safely(i.e. return some default value if not possible to access without throwing error)? I am trying to avoid wrapping these in try-except
‘s. I have seen the reduce based approach but it doesn’t take into account having lists inside the dictionary.
In JS, I can write something like:
data.object_1?.object_2?.list[0]?.property ?? 'nothing_found'
Is there something similar in Python?
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Answer
For dict
you can use the get
method. For lists you can just be careful with the index:
data.get('object_1', {}).get('object_2', {}).get('list', [{}])[0].get('property', default)
This is a bit awkward because it makes a new temporary dict or lost for each call get
. It’s also not super safe for lists, which don’t have an equivalent method.
You can wrap the getter in a small routine to support lists too, but it’s not really worth it. You’re better off writing a one-off utility function that uses either exception handling or preliminary checking to handle the cases you want to react to:
def get(obj, *keys, default=None): for key in keys: try: obj = obj[key] except KeyError, IndexError: return default return obj
Exception handing has a couple of huge advantages over doing it the other way. For one thing, you don’t have to do separate checks on the key depending on whether the object is a dict or list. For another, you can support almost any other reasonable type that supports __getitem__
indexing. To show what I mean, here is the asking for permission rather than forgiveness approach:
from collections.abc import Mapping, Sequence from operator import index def get(obj, *keys, default=None): for key in keys: if isinstance(obj, Mapping): if key not in obj: return default elif isinstance(obj, Sequence): try: idx = index(key) except TypeError: return default if len(obj) <= idx or len(obj) < -idx: return default obj = obj[key] return obj
Observe how awkward and error-prone the checking is. Try passing in a custom object instead of a list
, or a key that’s not an integer. In Python, carefully used exceptions are your friend, and there’s a reason it’s pythonic to ask for forgiveness rather than for permission.