Issue 24975: Python 3.5 can't compile AST involving PEP 448 unpacking

The compile() function is not able to compile an AST created from code that uses some of the new unpacking generalizations in PEP 448.  Example:

code = '''                                                                     
a = { 'x':1, 'y':2 }
b = { **a, 'z': 3 }
'''

# Works
ccode = compile(code, '', 'exec')

# Crashes
import ast
tree = ast.parse(code)
ccode = compile(tree, '', 'exec')

# ----------------------------------

Error Traceback:

Traceback (most recent call last):
  File "bug.py", line 11, in <module>
    ccode = compile(tree, '', 'exec')
ValueError: None disallowed in expression list

Note:  This bug makes it impossible to try generalized unpacking examples interactively in IPython.

Good find, David. Thanks!

Please review the attached patch.

Good catch!  Please file a pull request via bitbucket.

Patch looks good to me. I was initially going to query the changes to the break/continue AST tests, but then realised they were needed to avoid a compile error for break/continue outside a loop now that we ensure the resulting AST can be used to generate code.

Do we not have a test that compiles the entire stdlib using the ast module? We used to have one for the old compiler package. If don't have such a test we should probably add it even if it requires a -u option.

Larry,

I created a PR for you: https://bitbucket.org/larry/cpython350/pull-requests/10/issue-24975-fix-ast-compilation-for-pep/diff

Brett,

Should we do that in a separate issue targeting 3.6 only?

Created issue #24981 to track the test case idea.

Merged.  Please forward-merge to 3.5.1 and 3.6, thanks!

New changeset e9df8543d7bc by Yury Selivanov in branch '3.5':
Issue #24975: Fix AST compilation for PEP 448 syntax.
https://hg.python.org/cpython/rev/e9df8543d7bc

New changeset ea79be5b201e by Yury Selivanov in branch '3.5':
Merge 3.5 heads (issue #24975)
https://hg.python.org/cpython/rev/ea79be5b201e

New changeset 1dcd7f257ed8 by Yury Selivanov in branch 'default':
Merge 3.5 (issue #24975)
https://hg.python.org/cpython/rev/1dcd7f257ed8

Thanks Nick and Larry for code reviews and merge!

Upon writing a proof-of-concept file for #24981, I discovered an stdlib file that gave the same error.

C:/programs/Python35/Lib\distutils/_msvccompiler.py
ValueError: None disallowed in expression list

I presume the applied fix will apply to this also.

stage: patch review -> resolved

versions: + Python 3.6
keywords: + patch
nosy: + ncoghlan, larry, benjamin.peterson, serhiy.storchaka, yselivanov

messages: + msg249448
stage: patch review