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?
Most pythonic way is:
try:
# do something
...
except (NameError, AttributeError) as e:
# do something else
...
If it's a dictionary you can use get(keyname, value)
{'foo': {'bar': 'baz'}}.get('foo', {}).get('bar')
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
You can use the Glom.
from glom import glom
target = {'a': {'b': {'c': 'd'}}}
glom(target, 'a.b.c', default=None) # returns 'd'
Combining a few things I see here.
from functools import reduce
def optional_chain(obj, keys):
try:
reduce(getattr, keys.split('.'), root)
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.
I like modern languages like Kotlin which allow this:
foo?.bar?.baz
Recently I had fun trying to implement something similar in python: https://gist.github.com/karbachinsky/cc5164b77b09170edce7e67e57f1636c
Unfortunately, the question mark is not a valid symbol in attribute names in python, thus I used a similar mark from Unicode :)