Optional and other type tests in 3.6 Union

[hsolbrig]

In 3.5, I was able to test for optional and parameter types by:

from typing import Union, Optional

def is_optional(t):
   return issubclass(type(None), t)

def is_valid(e, t):
   return issubclass(type(e), t)

is_optional(Union[int, str])
> False
is_optional(Union[int, str, None])
> True
is_valid("abc", Union[int, str])
> True
is_valid(["abc"], Union[int, str])
> False

The issubclass method is no longer allowed in 3.6. How do I accomplish the equivalent in 3.6 on?

[ilevkivskyi]

In general, it is difficult to make runtime type checks reliable enough. This is why it was decided, to remove this functionality (and some other) in favour of static type checkers like mypy, pytype, etc. This also allowed to make typing module faster.

Instead, a set of tools is being developed now in #377 that will allow users to build efficiently their own runtime checks tuned to their specific needs. The set of tools will contain functions like is_union_type, is_generic_type, get_args, etc. For example, using such tools one could define:

from typing_inspect import is_union_type, get_args

def is_optional(tp):
    return is_union_type(tp) and type(None) in get_args(tp)

There was also a related discussion at http://bugs.python.org/issue29262

If you are interested in quick workarounds, then you could use the private typing API, but this is not recommended, for example something like:

import typing
def is_optional(tp):
    return type(tp) is typing._Union and type(None) in tp.__args__