Output from both 3.4.1 and 3.5.0:

>>> def foo(): pass

>>> foo.__annotations__

{}

Probably an oversight. I'm also not a C expert, but func_get_annotations (line 396 and onwards in funcobject.c) explicitely returns a new, empty dict if the function doesn't have any annotations (unlike all the other slots, like __defaults__ or __kwdefaults__, which merely return None if they're not present).

From: guido@python.org
Date: Mon, 4 Jan 2016 16:26:53 -0800
To: ncoghlan@gmail.com
Subject: Re: [Python-ideas] Bad programming style in decorators?
CC: python-ideas@python.org; surya.subbarao1@gmail.com

On Sat, Jan 2, 2016 at 10:42 PM, Nick Coghlan <ncoghlan@gmail.com> wrote:

On 3 January 2016 at 13:48, Guido van Rossum <guido@python.org> wrote:
> Whoops, Nick already did the micro-benchmarks, and showed that creating a
> function object is faster than instantiating a class. He also measured the
> size, but I think he forgot that sys.getsizeof() doesn't report the size
> (recursively) of contained objects -- a class instance references a dict
> which is another 288 bytes (though if you care you can get rid of this by
> using __slots__).

You're right I forgot to account for that (54 bytes without __slots__
did seem surprisingly small!), but functions also always allocate
f.__annotations__ at the moment.

Always allocating f.__annotations__ actually puzzled me a bit - did we
do that for a specific reason, or did we just not think of setting it
to None when it's unused to save space the way we do for other
function attributes? (__closure__, __defaults__, etc)

Where do you see that happening? The code in funcobject.c seems to indicate that it's created on demand. (And that's how I remember it always being.)

--Guido van Rossum (python.org/~guido)

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