Disallow List[int] etc. as expressions

[JukkaL]

The current PEP 484 draft does not allow writing code like this:

x = List[int]()

Instead, the recommended way is using type annotations:

x = [] # type: List[int]
# or
y = Undefined(List[int])
y = []

See also: python/typing#9

[gvanrossum]

(The PEP went back and forth a few times; Undefined was removed. Right now the PEP suggests using either a # type comment or defining an alias, e.g. IntList = List[int] after which you can say y = IntList().)