Optional chaining in Python

user_6094042 0
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In JavaScript, if I'm not sure whether every element of the chain exists/is not undefined, I can do foo?.bar, and if bar does not exist on foo, the interpreter will silently short circuit it and not throw an error.

Is there anything similar in Python? For now, I've been doing it like this:

if foo and foo.bar and foo.bar.baz:
    # do something

My intuition tells me that this isn't the best way to check whether every element of the chain exists. Is there a more elegant/Pythonic way to do this?

python-3.x python
7 Answers

If it's a dictionary you can use get(keyname, value)

{'foo': {'bar': 'baz'}}.get('foo', {}).get('bar')
user_76004920

Most pythonic way is:

try:
    # do something
    ...
except (NameError, AttributeError) as e:
    # do something else
    ...
user_21225020

You can use getattr:

getattr(getattr(foo, 'bar', None), 'baz', None)
user_48106390

You can use the Glom.

from glom import glom

target = {'a': {'b': {'c': 'd'}}}
glom(target, 'a.b.c', default=None)  # returns 'd'

https://github.com/mahmoud/glom

user_132411820

Combining a few things I see here.

from functools import reduce


def optional_chain(obj, keys):
    try:
        return reduce(getattr, keys.split('.'), obj)
    except AttributeError:
        return None

optional_chain(foo, 'bar.baz')

Or instead extend getattr so you can also use it as a drop-in replacement for getattr

from functools import reduce


def rgetattr(obj, attr, *args):
    def _getattr(obj, attr):
        return getattr(obj, attr, *args)
    return reduce(_getattr, attr.split('.'), obj)

With rgetattr it can still raise an AttributeError if the path does not exist, and you can specify your own default instead of None.

user_84770660

Combining some of the other answers into a function gives us something that's easily readable and something that can be used with objects and dictionaries.

def optional_chain(root, *keys):
    result = root
    for k in keys:
        if isinstance(result, dict):
            result = result.get(k, None)
        else:
            result = getattr(result, k, None)
        if result is None:
            break
    return result

Using this function you'd just add the keys/attributes after the first argument.

obj = {'a': {'b': {'c': {'d': 1}}}}
print(optional_chain(obj, 'a', 'b'), optional_chain(obj, 'a', 'z'))

Gives us:

{'c': {'d': 1}} None
user_50068430
user_23105220
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