In recent Python the following 

>>> from ast import PyCF_ONLY_AST
>>> compile("'a'", 'whatever', 'exec', PyCF_ONLY_AST).body

In 3.6 it return

   [<_ast.Expr at 0x10b7441d0>] # that contail Str('a')

While on master:

   []

This is inconveninent for alternative repl like IPython, where basically if the user is entering a single string, the result is nothing in Python 3.7+, while it _does_ return something on earlier Python [1]. 

The documentation of `compile` says:

>  ... it can be 'exec' if source consists of a sequence of statements,

Which is not technically true any more as the first statement, if a string, will be removed. 

What's happening here is that since Python 3.7 if the _first_ statement is actually an expression containing a lonely string it is assign to the module docstring. So that's basically assuming you are parsing a module, and that the docstring make sens in this context, while in a REPL you are parsing a sucesssion of statements, in which case there is no need for a docstring that make no sens in this context.


This is _usually_ not an issue, unless this lonely statement is also the last, and what the user wants to execute in a REPL, in which case it has no side effect.

I don't have any objection to the new behavior, though I was wondering if this kind of side effect was anticipated. 

If that affect IPython, it will likely effect other alternative REPLs.

Thus, I believe it would be good to have this at least documented a tiny bit better and added in what's new, and potentially clarified in the `exec` docs.

I could argue that now the  `exec` name may be a tiny bit unsuitable for the new behavior, and would love if this could be optional.

1: https://github.com/ipython/ipython/issues/11133#issuecomment-388591332

ref #32911

And it's documented already.
https://docs.python.org/3.7/whatsnew/3.7.html#changes-in-the-python-api

Any comments?
Would you close this issue?

Fair enough that what's new include things about Module


> The first statement in their body is not considered as a docstring anymore.

Note that this sentence read backward to me. I understand what is meant because I know the new behavior. It might be good to clarify. potentially:

> The first statement in the `body` attribute of should not be considered the docstring of the module anymore, the `docstring` attribute  is reserved for that.


Though the documentation of `compile()` does not say that `compile(...,'exec')` compile a module. It says:

> The mode argument specifies what kind of code must be compiled;
> it can be 'exec' if source consists of a sequence of statements

Which now is incorrect. I was expecting `compile(..., 'exec')` to return a Module with `None` or empty string as the docstring attribute – which is also a perfectly reasonable request.

I think that `compile`  documentation should be changed to reflect what it does. 


Or (but I see why this is un-reasonable) split add the `mode='module'` that has new behavior, while `exec` does not.

Closing as the ast changes have been reverted.

messages: + msg318779
stage: patch review -> resolved