创建或更新python词典条目

Question

Perhaps I've been spoiled by Brand X programming languages, but is there a better pythonic idiom for the following?

thing_dict = {}

def find_or_create_thing(key):
    if (thing_dict.has_key(key)):
        thing = thing_dict[key]
    else:
        thing = create_new_thing(key)
        thing_dict[key] = thing
    return thing

It seems like something like this could be done in one or two lines. I considered using a Conditional Expression , but Python's odd syntax simply didn't lend itself to legibility.

I also considered a try: ... except KeyError: , but that was just about as much text and probably considerably more execution overhead.

PS I know that asking programming style questions on SO is problematic, but I'll take my chances...

Answer 1

Using in is more Pythonic

thing_dict = {}

def find_or_create_thing(key):
    if not key in thing_dict:
        thing_dict[key] = create_new_thing(key)
    return thing_dict[key]

If you absolutely need the function on two lines:

thing_dict = {}

def find_or_create_thing(key):
    if not key in thing_dict: thing_dict[key] = create_new_thing(key)
    return thing_dict[key]

Answer 2

Not much shorter but maybe "prettier" (depending on the use case):

class ThingDict(dict):
    def __missing__(self, key):
        self[key] = create_new_thing(key)
        return self[key]

thing_dict = ThingDict()

def find_or_create_thing(key):
    return thing_dict[key]