See https://docs.python.org/3.6/library/functions.html#eval
This says the following won't happen. But it does.

Python 3.6.2 (default, Jul 29 2017, 00:00:00) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def fn(): print(3); return 4
... 
>>> eval(fn.__code__)
3
4

Compare with https://docs.python.org/3.6/library/functions.html#exec

Search for eval in title of open issues bring up related

#22057 (The doc say all globals are copied on eval(), but only __builtins__ is copied)
#25810 (Python 3 documentation for eval is incorrect)

Thank you for your report.

Quoting from https://docs.python.org/3.6/library/functions.html#eval

    This function can also be used to execute arbitrary code objects
    (such as those created by compile()). In this case pass a code
    object instead of a string.

So, your example is already documented as a legal way of using the eval() function.

I don't see anything wrong here.

Summary: There's my problem, and others. I'm willing to provide a patch, if supported.

There's a gotcha here. I fell into it. The docs for eval state
===
eval(expression, globals=None, locals=None)
The arguments are a string and optional globals and locals.
===
I read this and concluded, fairly I think, that I'm not allowed to pass in a code object [1]. So I didn't read any further. I'd already got the answer to my question.

But I knew that exec would take a code object, so I had doubt, and did my little experiment.

I'd prefer something more like exec, which says
===
This function supports dynamic execution of Python code. object must be either a string or a code object.
===

There are other problems, such as not agreeing with the help(eval) etc messages (and the still open #22057, #25810):
===
eval(source, globals=None, locals=None, /)
    Evaluate the given source in the context of globals and locals.
    
    The source may be a string representing a Python expression
    or a code object as returned by compile().
    The globals must be a dictionary and locals can be any mapping,
    defaulting to the current globals and locals.
    If only globals is given, locals defaults to it.
===
exec(source, globals=None, locals=None, /)
    Execute the given source in the context of globals and locals.
    
    The source may be a string representing one or more Python statements
    or a code object as returned by compile().
    The globals must be a dictionary and locals can be any mapping,
    defaulting to the current globals and locals.
    If only globals is given, locals defaults to it.
===

Finally, I'm willing to provide a patch, if supported. (I've not contributed to Python before.)

[1] I'd just read the docs for exec, which is up-front about 'string or code'.

Ok, I think it would be a good idea to mention that the function accepts a code object in the first sentence. I'd be happy to review such PR. The eval() documentation is located at https://github.com/python/cpython/blob/master/Doc/library/functions.rst

See https://devguide.python.org/ for more details about the contribution process.

Note that you need to sign the CLA in order to contribute to Python: https://www.python.org/psf/contrib/contrib-form/

#22057 and #25810 are out of scope for this issue, so let's not discuss them here.

OK. I'll do as you say. I've just signed the CLA.

Hi Jonathan,

Did you have any additional questions about opening a pull request for these changes?  Thanks!

This graceful reminder is most welcome. At present, it's not help I'm short of, but time. I've given myself a reminder, to spend time on this before the end of this month (January 2019).

Once again, thank you for this reminder. This is something I want to do. It would be my first Python contribution.

Hi Jonathan,
Are you still planning to work on the patch?