[Bug 1704473] [NEW] Traceback when using typing.Optional[Type] on xenial

[Dan Watkins]

Public bug reported:

typing in the Python 3.5.2 stdlib tracebacks when passing Type to a
Union (of which Optional is a specific case).  (This is typing bug #266:
https://github.com/python/typing/issues/266.)

(This is particularly annoying because as a stdlib library, the fix
released by upstream to PyPI can't easily be used even via pip.)

Steps to Reproduce
==================

```
lxc launch ubuntu:x reproducer
lxc exec reproducer -- python3 -c "from typing import Optional, Type; Optional[Type[BaseException]]"
```

Expected Behaviour
==================

No output, exits 0.

Actual Behaviour
================

```
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.5/typing.py", line 649, in __getitem__
    return Union[arg, type(None)]
  File "/usr/lib/python3.5/typing.py", line 552, in __getitem__
    dict(self.__dict__), parameters, _root=True)
  File "/usr/lib/python3.5/typing.py", line 512, in __new__
    for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
  File "/usr/lib/python3.5/typing.py", line 512, in <genexpr>
    for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
  File "/usr/lib/python3.5/typing.py", line 1077, in __subclasscheck__
    if super().__subclasscheck__(cls):
  File "/usr/lib/python3.5/abc.py", line 225, in __subclasscheck__
    for scls in cls.__subclasses__():
TypeError: descriptor '__subclasses__' of 'type' object needs an argument
```

** Affects: python3.5 (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: python3.5 (Ubuntu Xenial)
     Importance: Undecided
         Status: New

** Affects: python3.5 (Ubuntu Yakkety)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1704473

Title:
  Traceback when using typing.Optional[Type] on xenial

Status in python3.5 package in Ubuntu:
  New
Status in python3.5 source package in Xenial:
  New
Status in python3.5 source package in Yakkety:
  New

Bug description:
  typing in the Python 3.5.2 stdlib tracebacks when passing Type to a
  Union (of which Optional is a specific case).  (This is typing bug
  #266: https://github.com/python/typing/issues/266.)

  (This is particularly annoying because as a stdlib library, the fix
  released by upstream to PyPI can't easily be used even via pip.)

  Steps to Reproduce
  ==================

  ```
  lxc launch ubuntu:x reproducer
  lxc exec reproducer -- python3 -c "from typing import Optional, Type; Optional[Type[BaseException]]"
  ```

  Expected Behaviour
  ==================

  No output, exits 0.

  Actual Behaviour
  ================

  ```
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/usr/lib/python3.5/typing.py", line 649, in __getitem__
      return Union[arg, type(None)]
    File "/usr/lib/python3.5/typing.py", line 552, in __getitem__
      dict(self.__dict__), parameters, _root=True)
    File "/usr/lib/python3.5/typing.py", line 512, in __new__
      for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
    File "/usr/lib/python3.5/typing.py", line 512, in <genexpr>
      for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
    File "/usr/lib/python3.5/typing.py", line 1077, in __subclasscheck__
      if super().__subclasscheck__(cls):
    File "/usr/lib/python3.5/abc.py", line 225, in __subclasscheck__
      for scls in cls.__subclasses__():
  TypeError: descriptor '__subclasses__' of 'type' object needs an argument
  ```

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