In Python what is the most efficient way to do this:
my_var = some_var['my_key'] | None
ie. assign some_var['my_key'] to my_var if some_var contains 'my_key', otherwise make my_var be None.
Python will throw a KeyError if the key doesn't exist in the dictionary so you can't write your code in quite the same way as your JavaScript. However, if you are operating specifically with dicts as in your example, there is a very nice function mydict.get('key', default) which attempts to get the key from the dictionary and returns the default value if the key doesn't exist.
If you just want to default to be None you don't need to explicitly pass the second argument.
Depending on what your dict contains and how often you expect to access unset keys, you may also be interested in using the defaultdict from the collections package. This takes a factory and uses it to return new values from the __missing__ magic method whenever you access a key that hasn't otherwise been explicitly set. It's particularly useful if your dict is expected to contain only one type.
from collections import defaultdict
foo = defaultdict(list)
bar = foo["unset"]
# bar is now a new empty list
N.B. the docs (for 2.7.13) claim that if you don't pass an argument to defaultdict it'll return None for unset keys. When I tried it (on 2.7.10, it's just what I happened to have installed), that didn't work and I received a KeyError. YMMV. Alternatively, you can just use a lambda: defaultdict(lambda: None)