[Bug 1650202] Re: issubclass(a_type, union_type) has stopped working

[Brian Morton]

This is expected behavior from upstream changes.

https://github.com/python/typing/commit/7baf6fe2a6d96ab09c807aac174650baa35d15dd

Optional is shorthand for Union, and Union cannot be subclassed. This
behavior "worked" before, but was undefined and returned unpredictable
results.

** Changed in: python3.5 (Ubuntu)
       Status: New => Invalid

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Title:
  issubclass(a_type, union_type) has stopped working

Status in python3.5 package in Ubuntu:
  Invalid

Bug description:
  issubclass(a_type, union_type) has stopped working:

  In zesty:

    # python3
    Python 3.5.2+ (default, Dec 13 2016, 14:16:35)
    [GCC 6.2.1 20161212] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import typing
    >>> issubclass(str, typing.Optional[str])
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/lib/python3.5/typing.py", line 770, in __subclasscheck__
        raise TypeError("Unions cannot be used with issubclass().")
    TypeError: Unions cannot be used with issubclass().

  In yakkety:

    $ python3
    Python 3.5.2+ (default, Sep 22 2016, 12:18:14)
    [GCC 6.2.0 20160927] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import typing
    >>> issubclass(str, typing.Optional[str])
    True

  ProblemType: Bug
  DistroRelease: Ubuntu 17.04
  Package: python3.5 3.5.2-9
  ProcVersionSignature: Ubuntu 4.8.0-30.32-generic 4.8.6
  Uname: Linux 4.8.0-30-generic x86_64
  ApportVersion: 2.20.3-0ubuntu8
  Architecture: amd64
  Date: Thu Dec 15 09:58:29 2016
  ProcEnviron:
   TERM=screen.xterm-256color
   PATH=(custom, no user)
  SourcePackage: python3.5
  UpgradeStatus: No upgrade log present (probably fresh install)

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