Thread-safe way to add a key to a dict only if it isn't already there?

[Chris Angelico]

On Mon, Jul 9, 2018 at 5:18 AM, Marko Rauhamaa <marko at pacujo.net> wrote:
> Chris Angelico <rosuav at gmail.com>:
>> Are you assuming that Python's semantics are defined by the semantics
>> of one possible implementation language?
>
> What are Python's semantics defined by? I've been using these:
>
>    <URL: https://docs.python.org/3/reference/>
>
>    <URL: https://docs.python.org/3/library/>
>
> Unfortunately, neither spec says anything about the atomicity of
> dict.setdefault().
>
> Therefore, the application programmer must assume it is not atomic. In
> fact, as brought up in this discussion, the consultation of
> object.__hash__() and object.__eq__() almost guarantee the
> *non*-atomicity of dict.setdefault().

If by "atomic" you mean that absolutely no context switch can occur
during setdefault, then it probably isn't. But the point of an atomic
query/update operation is that there are exactly two possibilities:

1) The key did not exist in the dictionary. It now does, with the
provided default, which was returned.
2) The key did exist in the dictionary. The provided default is
ignored, and the previous value is returned.

Neither object.__hash__ nor object.__eq__ gives any way for this to be
violated (unless you mess with the concept of "the key did exist in
the dictionary" by breaking the definition of equality, but that's
nothing to do with atomicity). Here's the definition of setdefault:

setdefault(key, default=None, /) method of builtins.dict instance
    Insert key with a value of default if key is not in the dictionary.

    Return the value for key if key is in the dictionary, else default.


Simple semantics. Straight-forward. Now, maybe there's a bug in some
implementation whereby threads can violate this; but that would be a
bug to be fixed, and nothing more. You should be able to assume that
it will behave as stated.

ChrisA