Walrus operator support

I wanted to try out the new assignment expressions aka walrus operator in Python 3.8.

It all worked fine in plain Python 3.8:

$ python
Python 3.8.0a1 (default, Feb 19 2019, 11:26:51) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> sample_data = [
...     {"userId": 1, "id": 1, "title": "delectus aut autem", "completed": False},
...     {"userId": 1, "id": 2, "title": "quis ut nam facilis", "completed": False},
...     {"userId": 1, "id": 3, "title": "fugiat veniam minus", "completed": False},
...     {"userId": 1, "id": 4, "title": "et porro tempora", "completed": True},
...     {"userId": 1, "id": 4, "title": None, "completed": True},
... ]
>>> 
>>> print("With Python 3.8 Walrus Operator:") 
With Python 3.8 Walrus Operator:
>>> for entry in sample_data: 
...     if title := entry.get("title"):
...         print(f'Found title: "{title}"')
... 
Found title: "delectus aut autem"
Found title: "quis ut nam facilis"
Found title: "fugiat veniam minus"
Found title: "et porro tempora"

But failed miserably in IPython:

$ ipython
Python 3.8.0a1 (default, Feb 19 2019, 11:26:51) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.4.0.dev -- An enhanced Interactive Python. Type '?' for help.

In [1]: sample_data = [ 
   ...:     {"userId": 1, "id": 1, "title": "delectus aut autem", "completed": False}, 
   ...:     {"userId": 1, "id": 2, "title": "quis ut nam facilis", "completed": False}, 
   ...:     {"userId": 1, "id": 3, "title": "fugiat veniam minus", "completed": False}, 
   ...:     {"userId": 1, "id": 4, "title": "et porro tempora", "completed": True}, 
   ...:     {"userId": 1, "id": 4, "title": None, "completed": True}, 
   ...: ] 
   ...:  
   ...: print("With Python 3.8 Walrus Operator:")  
   ...: for entry in sample_data:  
   ...:     if title := entry.get("title"): 
   ...:         print(f'Found title: "{title}"') 
   ...:                                                                                                  
With Python 3.8 Walrus Operator:
---------------------------------------------------------------------------
SystemError                               Traceback (most recent call last)
/usr/local/lib/python3.8/codeop.py in __call__(self, source, filename, symbol)
    131 
    132     def __call__(self, source, filename, symbol):
--> 133         codeob = compile(source, filename, symbol, self.flags, 1)
    134         for feature in _features:
    135             if codeob.co_flags & feature.compiler_flag:

SystemError: unexpected expression

I was surpised to see that happen, as IPython 7.3.0+ should have Python 3.8 support.

$ python -c "import IPython; print(IPython.sys_info())"
{'commit_hash': '19b47fa',
 'commit_source': 'repository',
 'default_encoding': 'utf-8',
 'ipython_path': '/home/piotr/Personal/ipython/IPython',
 'ipython_version': '7.4.0.dev',
 'os_name': 'posix',
 'platform': 'Linux-4.15.0-45-generic-x86_64-with-glibc2.17',
 'sys_executable': '/home/piotr/.virtualenvs/ipython-KlOhEFsK/bin/python',
 'sys_platform': 'linux',
 'sys_version': '3.8.0a1 (default, Feb 19 2019, 11:26:51) \n'
                '[GCC 5.4.0 20160609]'}

[tomyun]

Reported upstream: https://bugs.python.org/issue36332

[tirkarthi]

The upstream issue was fixed and the fix is available with Python 3.8 alpha 3. Verified the same on master. I think this can be closed.

(foo-venv) ⋊> ~/cpython on master ⨯ ipython                                                  18:59:03
Python 3.8.0a3+ (heads/master:2ea8099523, Apr  6 2019, 18:56:04)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.4.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: items = [{'name': 'foo'}, {}]

In [2]: for item in items:
   ...:     if name := item.get('name'):
   ...:         print(name)
   ...:
foo