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Comment by avip | Hacker News Reader

The pythonic example I've seen not once:

{"some": "dict"}.setdefault("some", foo())

while expecting foo() to be short-circuited.

This is why I've started to think about () as the Invoke operator. Seeing it should always make the programmer assume that whatever that operator is to the right of will likely be invoked with possible side effects.

I thought, "gosh, that seems at least marginally useless; why wouldn't that short circuit?" and then just tried it. I tried a few variations, and about the only use I can think of is to signal as a side effect for when the dict is made.

Fun trick to learn exists. Though I would really like to know what led up to it, since it seems ... unhelpful.

Edit: Looking at other comments, I did try popping off that entry in the dict, and no, it didn't have anything in it (it doesn't make the dict anything like defaultdict).

As far as I know there is no native mechanism for short circuiting in Python. So anything that short-circuits should be considered a special case.

Binary operations will do it, like in `True or foo()`, but there's no real lazy evaluation machinery for parameters to a function. (Yet? I bet someone's working on it... at least I haven't tried any packages like lazypy for it.)

IIRC the boolean operators "and" and "or" are short-circuiting.

That's awful

What are people expecting with that?

They expect to have some form of "lazy caching":

  conf = {}

  def get_conn():
    return conf.setdefault("db_conn", expensive_creation_of_db_conn())

Probably expecting something like you get from collections.defaultdict: "dict subclass that calls a factory function to supply missing values": https://docs.python.org/2/library/collections.html#collectio...

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