Elegant way to check if a nested key exists in a dict?

StackOverflow

Is there are more readable way to check if a key buried in a dict exists without checking each level independently?

Lets say I need to get this value in a object buried (example taken from Wikidata):

x = s["mainsnak"]["datavalue"]["value"]["numeric-id"]

To make sure that this does not end with a runtime error it is necessary to either check every level like so:

if "mainsnak" in s and "datavalue" in s["mainsnak"] and "value" in s["mainsnak"]["datavalue"] and "nurmeric-id" in s["mainsnak"]["datavalue"]["value"]:
    x = s["mainsnak"]["datavalue"]["value"]["numeric-id"]

The other way I can think of to solve this is wrap this into a try catch construct which I feel is also rather awkward for such a simple task.

I am looking for something like:

x = exists(s["mainsnak"]["datavalue"]["value"]["numeric-id"])

which returns True if all levels exists.

Answer rating: 159

To be brief, with Python you must trust it is easier to ask for forgiveness than permission

try:
    x = s["mainsnak"]["datavalue"]["value"]["numeric-id"]
except KeyError:
    pass




The answer

Here is how I deal with nested dict keys:

def keys_exists(element, *keys):
    """
    Check if *keys (nested) exists in `element` (dict).
    """
    if not isinstance(element, dict):
        raise AttributeError("keys_exists() expects dict as first argument.")
    if len(keys) == 0:
        raise AttributeError("keys_exists() expects at least two arguments, one given.")

    _element = element
    for key in keys:
        try:
            _element = _element[key]
        except KeyError:
            return False
    return True

Example:

data = {
    "spam": {
        "egg": {
            "bacon": "Well..",
            "sausages": "Spam egg sausages and spam",
            "spam": "does not have much spam in it"
        }
    }
}

print "spam (exists): {}".format(keys_exists(data, "spam"))
print "spam > bacon (do not exists): {}".format(keys_exists(data, "spam", "bacon"))
print "spam > egg (exists): {}".format(keys_exists(data, "spam", "egg"))
print "spam > egg > bacon (exists): {}".format(keys_exists(data, "spam", "egg", "bacon"))

Output:

spam (exists): True
spam > bacon (do not exists): False
spam > egg (exists): True
spam > egg > bacon (exists): True

It loop in given element testing each key in given order.

I prefere this to all variable.get("key", {}) methods I found because it follows EAFP.

Function except to be called like: keys_exists(dict_element_to_test, "key_level_0", "key_level_1", "key_level_n", ..). At least two arguments are required, the element and one key, but you can add how many keys you want.

If you need to use kind of map, you can do something like:

expected_keys = ["spam", "egg", "bacon"]
keys_exists(data, *expected_keys)




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