Consider a refinement for class variable declarations

[gvanrossum]

In mypy we are proposing to implement strict Optional checking per PEP 484, with one exception.

python/mypy#1450

The one exception will be in the case of instance variables defined in the class body. These variables may be assigned None while having a non-Optional type, provided that they are initialized to a non-None value by the end of __init__. They may be initialized in __init__ itself, or in any method on self that __init__ calls, or in __new__. Mypy will not check these variables for use before assignment to a non-None value in __init__ or called methods, though it may make sense to add that at some point in the future.

The purpose of this exception is to allow variables to be typed in a common place rather than scattered around __init__.

I think this is a good idea and we should update PEP 484 to follow this.

[gvanrossum]

Hey, did you delete your comment or is there a GitHub glitch? I can't see
it on the website.

On Thu, May 5, 2016 at 8:56 PM, Caleb Jones [email protected]
wrote:

I saw on #1450 there was some concern about writing code that relied on
control flow analysis; this seems like exactly the sort of rule that would
require control flow analysis to do correctly.

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#209 (comment)

--Guido van Rossum (python.org/~guido)

[porglezomp]

I deleted it since it wasn't really relevant to the specification of the types, it was more an implementation concern that I don't think its relevant concern here, since other typing features already require it.

[gvanrossum]

I'm closing this; it looks like the PEP already has the right words:

In non-stub code, there is a similar special case::

  from typing import IO

  stream = None  # type: IO[str]

Type checkers should not complain about this (despite the value
None not matching the given type), nor should they change the
inferred type to Optional[...] (despite the rule that does this
for annotated arguments with a default value of None). The
assumption here is that other code will ensure that the variable is
given a value of the proper type, and all uses can assume that the
variable has the given type.